Cambridge IGCSE chemistry study and revision guide

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Contents

Introduction

1Theparticulatenatureofmatter

2Elements,compoundsandexperimentaltechniques

3Atomicstructure,bondingandstructureofsolids

4Stoichiometry:chemicalcalculations

5Electricityandchemistry

6Chemicalenergetics

7Chemicalreactions

8Acids,basesandsalts

9ThePeriodicTable

10Metals

11Airandwater

12Sulfur

13Inorganiccarbonchemistry

14Organicchemistry1

15Organicchemistry2

16Experimentalchemistry

Answerstoexam-stylequestions

Index

Introduction

WelcometotheCambridgeIGCSE®ChemistryStudyandRevisionGuide.ThisbookhasbeenwrittentohelpyoureviseeverythingyouneedtoknowforyourChemistryexam.FollowingtheChemistrysyllabus,itcoversallthekeycontentaswellassamplequestionsandanswers,practicequestionsandexaminertipstohelpyoulearnhowtoanswerquestionsandtocheckyourunderstanding.

HowtousethisbookKeyobjectivesThekeyskillsandknowledgecoveredinthechapter.Youcanalsousethisasachecklisttotrackyourprogress.

KeytermsDefinitionsofkeytermsyouneedtoknowfromthesyllabus.

Sampleexam-stylequestionsExam-stylequestionsforyoutothinkabout.

Student’sanswers

Typicalstudentanswerstoseehowthequestionmighthavebeenanswered.

Examiner’scommentsFeedbackfromanexaminershowingwhatwasgood,andwhatcouldbeimproved.

Examiner’stipsAdvicetohelpyougivetheperfectanswer.

Commonerrors•Mistakesstudentsoftenmakeandhowtoavoidthem.

ExtendedContentfortheextendedsyllabusisshadedgreen.

Exam-stylequestionsPracticequestionsforyoutoanswersothatyoucanseewhatyouhavelearned.

HowtoreviseThisbookisnotintendedtogivedetailedinformationaboutthechaptersyouarerequiredtostudyfortheIGCSEChemistrycourse.Insteaditismeanttogiveconciseinformationconcerningthethingsthatyouarelikelytocomeacrossinyourexaminations.Youhaveprobablybeenusingamoredetailedtextbookoverthetwoyearsofyourcourse.Thisbookisintendedforuseoverthesixweeksjustbeforetheexaminations.Notwopeoplereviseinthesameway.Itwouldbefoolishtogiveprecise

instructionstoanyoneabouthowtheyshouldprepareforexaminations.However,Iintendtomakesomesuggestionsaboutthedifferentapproachesthatareavailable,sothatyoucanchoosethemethodsthataremostsuitableforyou.TheonlythingIwouldstronglyrecommendaboutrevisionisthatitshould

involvewritingaswellasreading.Thosewhoreadthroughnotesorbooksastheironlymeansofrevisingoftenfindthattheybecomedistractedandstartthinkingaboutotherthings.Writingthingsdownhelpsyoutofocusonwhatyouaretryingtolearn.Anotherwaytohelpyoulearnistohighlightkeywordsandphrasesthatyou

wishtodrawattentionto.Highlightingmakesyoufocusonthingsthatyoumayhavehadproblemswithuptonow.Afterhighlighting,youcouldrewritethehighlightedparts,leavingouttheless

importantparts.Itmayalsobeagoodideatoleaveoutthosepartsthatyoualreadyknow.Ifyouknowthattransitionelementsareallmetals,thereisnopointinwritingitdown,becauseyoudonotneedtoreviseit.Justfocusonthepartsthatyouarenotsofamiliarwith.Yournoteswillbemoreconciseandmorepersonalthantheinformationinthebook.Youmayprefertowritedowntheinformationinamoreeye-catchingform,suchasinadiagram.Theimportantthingisthatitispersonaltoyouandhelpsyourrevision.Whenyouhavemadenotesofthistype,trywritingthemout.Againjustfocus

ontheimportantkeywordsandphrases.Whenyoucanwritethemoutwithoutlookingatyournotes,youmaybeconfidentthatyouhavelearnedthisparticularchapter.Youcantestyourselfbyansweringthe‘Exam-stylequestions’sectionsofthebookaswellaslookingatpastpaperquestionsorquestionsyouhaveattemptedduringthecourse.Ifyoucomeacrossaproblem,alwaysgobacktoyournotesandothersourcesofinformation.Thereisnoneedtoattempttimedquestionsunderexamconditionsuntilmuchnearertheexam.

ExamtechniqueThemostimportantpiecesofadvicethatanyonecangiveabouthowtoapproachanexamarenotnew.However,itiswellworthrepeatingthem,becausethepointsmadebelowarestillmajorcausesofavoidablelossesofmarks.

•Writeclearly.Ifexaminerscannotreadwhatyouhavewritten,theycannotawardmarks,despitetheirbesteffortstodecipheryourwork.Inthistechnologicalage,whenhandwritingisnotusedasmuchasitwasinthepast,itisessentialthatyouranswerscanberead.

•Usecorrectspellings.TechnicaltermsusedinChemistrymustbewrittencorrectly.Incorrectspellingsdomorethancreateabadimpressionofacandidate’swork.Ifawordisincorrectlyspelt,itoftenchangesthewholemeaningoftheword.Anexampleisusingtheword‘alkane’insteadof‘alkene’.Thisisnotonlyaspellingmistake,butitreplacesonewordwithanentirelydifferentwordwhichhasanentirelydifferentmeaning.

•Readthequestionscarefully.Donotjustglanceataquestionandpickoutafewwords.Readthewholequestionandwhenyouhavereadit,readitagain.Aquestionmaylooklikeanotheroneyouhaveseenduringyourrevision,butifyoureaditmorecarefully,youmayrealisethattherearedifferenceswhichmeanthatadifferentapproachisrequiredtoanswerthequestion.

•Checkyouranswers.Whenyouhavefinishedeachquestion,readthroughittomakesureitmakessenseandthatitanswersthequestion.

•Donotspendtoolongonanyquestions.Ifyouspendtoolongonsomequestions,youmayfindyoudonothavethenecessaryamountoftimetoanswersomeoftheothers.Itisimportanttoanswerallthequestions.

Somecommonphrasesthatyouwillseeinquestions,andtheirmeanings,include:

•Definetheterm/whatismeantbythetermmeansgiveadefinitionofawordorphrasewhichonlyappliestothatwordorphrase.Forexample,‘Definethetermisotope’meansgiveastatementthattellssomeoneexactlywhatanisotopeis.Definitionsarefoundinthe‘Keyterms’sectionatthestartofeachchapter.

•Statemeansgiveabriefstatement.Noexplanationofthestatementisrequired.Forexample,‘Statethenameoftheacidthatisusedtomake

magnesiumsulfate’.•Explain/giveareasonorreasons.Thissometimesfollowsthecommandword‘state’,i.e.‘stateandexplain’or‘stateandgiveareason’.Thismeansyoushouldgiveapieceofinformationfollowedbyabriefexplanationofwhyyouchosethisinformation.

•Outlinemeansabriefdescriptionisrequired.•Predictmeansyouaremeanttomakeaprediction,notbasedonanyknowledgethatyouhaveremembered,butbymakingalogicalconnectionbetweenotherpiecesofinformationreferredtointhequestion.

•Deducealsomeansyouarenotrequiredtogiveananswerbasedonwhatyouhaveremembered,buttosuggestalogicalconnectionbasedoninformationinthequestion.

•Suggestmaymeanthereismorethanonepossibleanswer.ItmayalsomeanyouarerequiredtoapplyyourknowledgeofChemistrytoa‘novel’situation,e.g.anexperimentorareactionyouhavenotcomeacrossbefore.Insuchexamples,youwillbesuppliedwithsufficientinformationtomakeareasonablesuggestion.

•Calculate/determinemeanscarryoutacalculationbasedondatathatisprovided.

•Howwouldyou…?meansgiveabriefdescriptionofanexperimentthatyouwouldcarryout.Manyexamcandidatesanswerthistypeofquestionwithtoomuchtheoreticalinformationratherthanbriefexperimentaldetail.

1Theparticulatenatureofmatter

KeyobjectivesBytheendofthissection,youshouldknowthedifferentpropertiesofsolids,liquidsandgases•beabletodescribethestructureofsolids,liquidsandgasesintermsofparticleseparation,arrangementandtypesofmotion•knowwhatismeantbymelting,boiling,evaporation,freezing,condensationandsublimation•beabletodescribetheeffectoftemperatureonthemotionofgasparticles•haveanunderstandingofBrownianmotion•beabletodescribeandexplaindiffusion

•beabletoexplainchangesofstateintermsofthekinetictheory•beabletodescribeandexplainBrownianmotionintermsofrandommolecularbombardmentandstateevidenceforBrownianmotion•beabletodescribeandexplaindependenceofrateofdiffusiononrelativemolecularmass.

KeytermsMelting TheprocessthatoccurswhenasolidturnsintoaliquidMeltingpoint Thetemperatureatwhichasubstancemelts.Eachsubstancehasa

specificmeltingpointBoiling TheprocessthatoccurswhenaliquidturnsintoagasBoilingpoint Thetemperatureatwhichasubstanceboils.Eachsubstancehasa

specificboilingpointEvaporation Theprocessthatoccursatthesurfaceofaliquidasitturnsintoa

gas.Evaporationcanoccurattemperatureslowerthantheboilingpointofaliquid

Freezing TheprocessthatoccurswhenaliquidturnsintoasolidFreezingpoint

Thetemperatureatwhichasubstancefreezes.Thishasthesamevalueasthemeltingpoint

CondensationTheprocessthatoccurswhenagasturnsintoaliquidSublimation Theprocessthatoccurswhenasolidturnsintoagaswithoutfirst

turningintoaliquidBrownianmotion

Therandommotionofvisibleparticlescausedbybombardmentbymuchsmallerparticles

Diffusion Theprocessthatoccurswhenparticlesmovefromaregionofhighconcentrationtoaregionoflowconcentration

Solids,liquidsandgasesDifferencesbetweensolids,liquidsandgasesareshowninFigure1.1.

Examiner’stipWhenaskedtodrawdiagramsofthearrangementofparticlesinsolids,liquidsandgases,solidsandgasesareusuallydrawnquitewell,buttheparticlesinliquidsareusuallydrawntoofarapart.Inreality,themajorityofparticlesinaliquidaretouching.

Thedifferencesbetweenthepropertiesofsolids,liquidsandgases,alongwiththereasons(basedonkinetictheory)forthedifferences,areshowninTable1.1.

ChangesofstateFigure1.2summarisesthechangesinstatethatoccurbetweensolids,liquidsandgases.

Commonerror•Thereisoftenconfusionbetweenboilingandevaporation.Boilingonlytakesplaceattheboilingpointofaliquid,butevaporationoccursattemperaturesbelowtheboilingpoint.Puddlesofwaterevaporateonasunnyday.Thismeansthatthewaterturnsintowatervapourattemperatureswellbelowtheboilingpointofwater.Thewaterinthepuddlesdoesnotreach100°C!

HeatingandcoolingcurvesAheatingcurveshowsthechangesofstateoccurringwhenthetemperatureoficeisgraduallyincreased.Asimilar(butnotthesame)curveresultswhenagasiscooledgraduallyuntilitformsasolid.Thisisknownasacoolingcurve.

Theprocessbeginswithiceatatemperaturebelow0°C.Thetemperaturegraduallyincreasesuntilitreaches0°C,whichisthemeltingpointofice.Atthispointiceandwaterexisttogether.Thetemperaturedoesnotchangeuntilalltheicehaschangedintowaterwhichiswhythelineishorizontal.Asharpmeltingpoint(atonespecifictemperature)isanindicationthatanysolidispure.Thetemperaturethenbeginstoincreaseagainuntilitreaches100°Cwhichis

theboilingpointofwater.Thetemperaturedoesnotchangeuntilallthewaterhaschangedintowatervapourwhichiswhythelineishorizontalforasecondtime.Whenallthewaterhasboiled,thetemperaturebeginstoriseagainastheparticlesinthegaseousstategainmoreenergy.

KinetictheoryWhenheatenergyisgiventoasolid,theheatenergycausestheparticlestovibratefasterandfasteraboutafixedpositionuntiltheparticleshavesufficientenergyformeltingtooccur.Atthe

meltingpointtheenergygainedbytheparticlesissufficienttoovercometheattractionbetweenparticlesinthesolid.Theorderedarrangementofparticlesthenbreaksdownasthesolidturnsintoaliquid.Asthisisoccurring,thereisnofurtherincreaseintemperatureuntiltheorderedarrangementhascompletelybrokendownandallthesolidhasturnedintoaliquid.Theenergygiventotheparticlesthencausesthemtomovefasterfromplacetoplaceuntiltheyhavesufficientenergyforboilingtooccur.Attheboilingpointtheenergygainedbytheparticlesissufficienttocompletelyovercometheattractionbetweenthemintheliquidstate.Theparticlesthenmoveasfarawayfromeachotheraspossibleastheforcesofattractionbetweenthemarealmostcompletelyovercome.Againthereisnoincreaseintemperatureuntiltheliquidhasturnedcompletelyintoagas.Inthegaseousstate,thegasparticlesgainmoreandmoreenergyandmoveatincreasingspeeds.

BrownianmotionWhenRobertBrownusedamicroscopetoobservepollengrainsonthesurfaceofwaterin1827,henoticedthatthepollengrainsmovedinarandommanner.ThisrandommovementisknownasBrownianmotion.Thesamethingcanbeobservedifsmokeparticlesinairareobservedthroughamicroscope.

Brownianmotioniscausedbythelargerparticles(pollengrainsorsmokeparticles)beingbombardedbysmallerparticles(watermoleculesorairmolecules).Thesmallerparticlesmoveinstraightlinesuntiltheycollidewiththelargerparticles.Becausemoreofthesmallerparticlesmaycollideononesideofthelargerparticlesthantheother,themovementofthelargerparticlesisrandomandunpredictable.

DiffusionParticlesinsolidsdonotmovefromoneplacetoanother.However,particlesinliquidsmoveslowlyandparticlesingasesmovemuchfaster.Movementofparticlesfromaregionofhighconcentrationtoaregionoflow

concentrationisknownasdiffusion.Itcanbedemonstratedexperimentallyinliquidsandingases.

DiffusioninliquidsIfcrystalsofacolouredsolid,suchasnickel(II)sulfate,areplacedinaliquidsuchaswater,thecolourofthenickel(II)sulfatespreadsthroughouttheliquidinamatterofdays,producingasolutionwithauniformgreencolour.

DiffusioningasesIfbromineliquidisplacedinthebottomofagasjarwithanothergasjarontop,theliquidevaporatesandthebrowncolourofbrominegasfillsbothgasjarsafterashorttime.

Whengaseousmoleculesdiffuse,therateatwhichtheydiffuseisinverselyrelatedtotherelativemolecularmassofthegas.Therefore,moleculesingaseswithlowerrelativemolecularmasswilldiffusefasterthanmoleculesingaseswithhigherrelativemolecularmass.Thisisbecauselightermoleculesmovefasterthanheaviermolecules.

Sampleexam-stylequestions1Acompoundhasameltingpointof−30°Candaboilingpointof85°C.Whatisitsphysicalstateat25°C?Explainyouranswer.

Student’sanswer

1Liquid.Themeltingpointisbelow25°Candtheboilingpointisabove25°C.

Examiner’scommentsTherearetwocommonerrorsinaquestionofthistype.

•Somestudentsignorethenegativesignin−30°C,whichgivesthemtheimpressionthat−30°Cishigherthan25°C,whichmeanstheythinkthecompoundisasolid.

•Somestudentsknowthatthesubstanceisaliquid,butonlystatethatthemeltingpointisbelow25°Cwithoutmentioningtheboilingpoint.Suchcandidatesgetsomecreditbutnotmaximumcredit.

2WhentheapparatusshowninFigure1.7issetup,concentratedammoniareleasesammoniagas,NH3,andconcentratedhydrochloricacidreleaseshydrogenchloridegas,HCl.

Whenammoniagasreactswithhydrogenchloridegas,awhitesolidisproducedaccordingtotheequation:

aWhatisthenameofthewhitesolid?bNametheprocessbywhichthetwogasesmovethroughtheglasstube.cExplainwhythewhitesolidformsnearertheconcentratedhydrochloric

acidendoftheglasstuberatherthantheammoniasolutionend.

Student’sanswer

2aAmmoniumchloridebDiffusioncMoleculesofammoniaandhydrogenchloridediffusethroughtheglass

tube.Becauseammonia,NH3,hasalowerrelativemolecularmass(17)thanhydrogenchloride,HCl(36.5),ammoniamoleculesdiffusefasterthanhydrogenchloridemolecules.Therefore,thegasesmeetandreactnearerthehydrochloricacidend.

Examiner’scommentsaAmmoniumcompoundsareoftenmistakenlyreferredtoasammoniacompounds.Similarlyammoniaisoftenreferredtoasammonium.Studentsshouldmakesurethattheyareawareofthedifferencebetweenammonia,NH3,andtheammoniumion,NH4+,whichispartofallammoniumsaltssuchasammoniumchloride.

bThisisthecorrectanswer.cItiscommontoseestatementsaboutammoniamovingfasterthanhydrogenchloridebecauseammoniaislighterthanhydrogenchloride.Thiswouldgainverylittle(ifany)credit.Answersmustrefertoammoniaandhydrogenchloridemoleculesandalsostatethatammoniahasasmallerrelativemolecularmassthanhydrogenchloridewhichiswhyammoniamoleculesdiffusefaster.

Studentsshouldcalculaterelativemolecularmasses,usingrelativeatomicmassesinthePeriodicTable,iftheyarenotprovidedinthequestion.

Exam-stylequestions1Asubstancehasameltingpointof85°Candaboilingpointof180°C.Whatisthephysicalstateofthesubstanceat50°C?Explainyouranswer.

[Total:2marks]2UsethelettersA,B,CandDtoanswerthequestionsunderthetable.

WhichsubstanceoutofA,B,CandDisaasolid[1]

baliquid[1]

cagas[1]

dunlikelytorepresentarealsubstance?[1]

[Total:4marks]

2Elements,compoundsandexperimentaltechniques

KeyobjectivesBytheendofthissection,youshouldknowthedifferencebetweenelements,mixturesandcompounds•knowthatelementscanbeclassifiedasmetalsandnon-metals•knowandunderstandhowtousethefollowingmethodsofseparationandpurification

•chromatography(seealsoChapter15)•dissolving,filtrationandcrystallisation(seealsoChapter8)•(simple)distillation•fractionaldistillation(seealsoChapter14)

•beabletointerpretsimplechromatograms,includinguseoflocatingagents(whichdonotneedtobenamed)anduseofRfvalues(seeChapter15).

KeytermsElement Asubstancethatcannotbedecomposedintoanythingsimplerby

chemicalmeans.Itisasubstancemadeupofatomsallofwhichhavethesameatomicnumber(seealsoChapter3)

CompoundAsubstancewhichcontainstwoormoreelementschemicallycombinedinfixedproportionsbymass

Mixture Containstwoormoresubstances(elementsorcompounds)whichcanbepresentinvariableproportions

Substance Ageneraltermthatreferstoelements,mixturesandcompoundsSolution AliquidwhichcontainsasubstanceorsubstancesdissolvedinitSolvent ApureliquidSolute Thedissolvedsubstanceinasolution

ElementsTheperiodictableconsistsofelementsonly.Eachelementhasachemicalsymbol.Elementsareclassifiedasmetalsandnon-metalsasshowninTable2.1below.

Table2.1Classificationofelements

Property Metal Non-metalPhysicalstateatroomtemperature

Solid(exceptmercury)

Solid,liquid(bromineonly)orgas

Malleability Good Poor,usuallysoftorbrittleDuctility Good Poor,usuallysoftorbrittleAppearance Shiny(lustrous) UsuallydullMeltingpoint/boilingpoint Usuallyhigh UsuallylowDensity Usuallyhigh UsuallylowConductivity(electricalandthermal)

Good Poor(exceptgraphite)

Examiner’stipElementscannotbedecomposedintoanythingsimplerbychemicalmeans.Studentssometimesusethewordsmallerinsteadofsimpler,whichisanerror.Forexample,apieceofsulfurcanbebrokenwithahammerintoseveralsmallerpiecesofsulfur,butthisisnotbreakingitintoanythingsimpler.Theactofbreakingwithahammerisaphysicalprocessandnotachemicalprocess.Thussulfurisanelement.

CompoundsCompoundshaveachemicalformulawhichshowsthemtocontaintwoormoreelementswhicharechemicallycombined.Examplesofcompoundsare:

sodiumchloride,NaCl•carbondioxide,CO2copper(II)nitrate,Cu(NO3)2.

Acompound,iron(II)sulfide,hastheformulaFeS.Therelativeatomicmasses(Ar)ofiron(Fe)andsulfur(S)are56and32,respectively.Therefore,therelativemolecularmass(Mr)ofiron(II)sulfideis56+32=88.Thepercentagesofironandsulfuriniron(II)sulfideare:

Examiner’stipIfacompoundispresentinanaqueoussolution,theaqueoussolutionisamixturebecauseitcontainstwosubstanceswhicharenotchemicallycombined.Forexample,sodiumhydroxidesolution(alsoreferredtoasaqueoussodiumhydroxide)isamixture,notacompound.Itcontainssodiumhydroxideandwater,twosubstancesthatarenotchemicallycombined.

Thismeansthatallsamplesofiron(II)sulfidecontain63.6%ironand36.4%sulfurbymass.Thisiswhatismeantbythestatementthatcompoundscontainelements

chemicallycombinedinfixedproportionsbymass.

Examiner’stipStudentsmaystatethatcompoundscontaintwoormoreelements,butoftendonotmentionthattheelementsarechemicallycombined(whichmeansthattheelementsarejoinedbyionicorcovalentbonds).

MixturesMixturescontaintwoormoreelementsand/orcompoundsinvariableproportions.Mixturesdonothaveachemicalformula.Airisanexampleofamixture.Aircontainsnitrogenandoxygenwithsmaller

amountsofothergases,suchaswatervapour,carbondioxideandargon.Pollutedairmayalsocontainothergasessuchascarbonmonoxide,sulfurdioxideandnitrogendioxide.Airhasdifferentpercentagesofitsconstituentgasesindifferentplacesdueto,

forexample,theamountsofpollutantgaseswhicharelowerinthecountrysidethaninindustrialareas.However,thedifferentsamplesareallcalled‘air’,thusshowingthatthecompositionofamixturecanvary.Airdoesnothaveachemicalformulabecauseitcontainsseveralchemical

substancesasopposedtoonesubstance.

Examiner’stipManystudentsareundertheimpressionthatamixturecontainingtwosubstances,suchassaltandsand,mustcontainequalamountsofeachsubstance.Thisisnotthecase.Ifwehadamixtureofsaltandsandwhichcontainedequalamountsofeachsubstanceandweaddedmoresalttoit,itwouldstillbecalledamixtureofsaltandsand.Therefore,amixtureofsaltandsandcancontainmoresaltthansand,ormoresandthansaltorequalamountsofsaltandsand.Thisisdifferenttothecompositionofacompoundasshowninthecaseofiron(II)sulfideabove.

SeparationofmixturesChromatographyChromatographycanbeusedtoseparatethecomponentsofsolutionswhichcontainseveraldissolvedsubstances.Thesubstancesareoftencoloured,butmaybecolourless.Paperchromatographycanbeusedtoseparatethedyesinink.

•Aspotoftheinkisplacedonthechromatographypaper.•Thepaperisplacedinasuitablesolventinabeaker.Ifthesolventisvolatile(vaporiseseasily)itisnecessarytoputalidonthebeakertopreventthevapourfromescaping.

•Asthesolventrises,thedyesintheinkseparate.

Chromatographycanalsobeusedtoidentifythecomponentsofamixtureaswellasseparatethem.

•AmixtureofdyesisplacedonchromatographypaperinthepositionmarkedX.

•FourdyeswhoseidentitiesareknownareplacedinpositionsmarkedA,B,CandD,asshown(Figure2.2).Thesefourdyesarereferredtoasstandards.

•Chromatographyisthencarriedoutandthechromatographypaper(alsoknownasachromatogram)isremovedfromthebeakeranddried.

•ThepaperisthenlabelledtoshowwhatmixtureXcontains,asdescribedbelow.

Theresultsoftheexperimentshow:Xiscomposedofthreedyesbecausethemixturehasbeenseparatedintothree.ThethreedyesareA,BandD.WeknowthisbecausethethreedyesinmixtureXhavetravelledthesamedistancesasthethreestandardsA,BandDwhoseidentitiesareknown.WecanalsoconcludethatmixtureXdoesnotcontaindyeC,becausenoneofthecomponentsofXtravelledthesamedistanceasdyeC.

Chromatographycanalsobeusedtoidentifycolourlesssubstances.Theexperimentaltechniqueisthesame,butbecausethecomponentsofthemixturearecolourless,thespotsonthechromatographypaperareinvisible.Afterdrying,thepaperissprayedwithalocatingagentwhichreactswiththecomponentsofthemixturetoproducecolouredspots.InChapter15whichlooksattheseparationofaminoacidsbychromatography,thechromatographypaperissprayedwithninhydrinwhichisalocatingagentthatproducesbluecolouredspotswithaminoacids.Insteadofusingstandardsasdescribedintheaboveexperiment,components

ofamixturecanbeidentifiedbytheirRfvalues.Chromatographyiscarriedoutandafterthechromatographypaperisdried,thedistancethatthesolventhastravelledandthedistancethatthecomponentofthemixturehastravelledarebothmeasuredasshowninFigure2.4.

Examiner’stipWhenaskedwhatismeantbyRfvalue,studentsareadvisedtowritedowntheequationgivenhere,ratherthantryingtoexplainRfvalueintheformofasentence,whichismuchmoredifficult.Thecorrectequationwouldscorealltheavailablemarks.

Thisillustratesthefactthatanswersneednotalwaysbeexpressedinwords,sentencesandparagraphsalonetogetfullcredit.Diagrams,equations,sketchgraphsandformulaeareoftenmuchmoreappropriatethansentences.

WhentheRfvalueiscalculated,thecomponentofthemixturecanbeidentified

bycomparisonwithRfvaluesinadatabook.Rfvaluescanbedeterminedforallthecomponentsofthemixture.

Dissolving,filtrationandcrystallisationDissolving,filtrationandcrystallisationaremethodsusedtoseparateamixtureoftwosolids,oneofwhichissolubleinagivensolventandtheotherofwhichisinsoluble.Thismethodcanbeusedtoseparateamixtureofcommonsaltandsandand

producepuresamplesofbothsolids.

•Ifthemixtureisnotpowdereditshouldbegroundintoapowderusingamortarandpestle.Thepowdershouldbeaddedtowaterinabeaker.Thecommonsaltdissolvesandthesandremainsundissolved.

•Themixtureisthentransferredtothefiltrationapparatus.Thesand(residue)remainsinthefilterpaperandthesaltsolution(filtrate)passesthroughintotheconicalflask.Thisprocessiscalledfiltration.

•Toobtainpuresand,distilledwatershouldbepassedthroughthefilterpaper(thisisknownaswashingtheresidue)andthenthefilterpapershouldberemovedanddriedinalowovenoronawarmwindowsill.

Thefinaltwostagesareknownascrystallisation:Toobtainsaltcrystals,thesaltsolutionshouldthenbeheatedinanevaporatingdishuntilabouthalfofthewaterhasbeenremoved(alternatively,whencrystalsformonaglassrodplacedinthehotsolutionandwithdrawn,itistimetostopheating).Thehotsaturatedsaltsolutionshouldthenbeallowedtocooldownslowly.Crystalsofsaltshouldthenform.Ifthereisanyliquidleft,itshouldthenbeseparatedbyfiltration.Thesaltcrystalsshouldthenbewashedwithasmallamountofcolddistilledwaterandthendriedinalowovenoronawarmwindowsill.

CommonerrorsFiltration(oftenspeltwronglyasfilteration)canalsobecalledfiltering.Thereareothercommonerrorswhendescribingtheprocess:

Thewordsresidueandfiltrateareoftenusedthewrongwayround.Filtrateisoftenusedasanincorrectalternativetofilteredasin‘hefiltratedthesolution’.Thewordfiltrateddoesnotexist!Thefiltrateshouldnotbeheateduntilallthewaterevaporatestodryness.Thisdoesnotleadtotheproductionofgoodcrystals.Inaddition,somecrystalscontainwaterofcrystallisationwhichwouldbedrivenoffbytoomuchheat(seeChapter8).Ifthecrystalsaredriedwithtoomuchwatertheywilldissolve,whichdefeatsthepurpose.Thewatershouldideallybeicecoldtominimisetheamountthatdissolves.

(Simple)distillation(Simple)distillationisamethodofseparatingapureliquidfromasolution.

Theflaskisheated.Thewaterinthesodiumchloridesolutionevaporatesandwatervapour/steamenterstheLiebigcondenser,whereitcondensesaswater.ThewaterdripsoutoftheendoftheLiebigcondenserandcollectsinthebeaker.Thewaterispureandcanbecalleddistilledwater.Sodiumchloridedoesnotvaporiseorevenmeltbecauseithasaveryhighmeltingpoint,andthereforeitremainsinthedistillationflask.

FractionaldistillationFractionaldistillationisamethodofseparatingtwo(ormore)miscibleliquidswithdifferentboilingpoints.Itcanbecarriedoutinthelaboratoryoronanindustrialscaleasinthefractionaldistillationofliquidair(seeChapter11)orfractionaldistillationofpetroleum(seeChapter14).Inthelaboratoryethanolandwatercanbeseparatedbyfractionaldistillation

usingtheapparatusshowninFigure2.7.

Ethanolhasaboilingpointof78°Candwaterhasaboilingpointof100°C.

Ethanolhasaboilingpointof78°Candwaterhasaboilingpointof100°C.Theflaskisheatedandethanolvapourentersthefractionatingcolumn.

However,somewateralsoevaporates(belowitsboilingpoint)andentersthefractionatingcolumnaswatervapour/steam.Thewatervapour/steamcondensesinthefractionatingcolumnanddripsbackdownintothedistillationflask.Whenthetemperaturereaches78°C,theethanolvapourreachesthetopofthefractionatingcolumnandenterstheLiebigcondenserwhereitcondenses.Finally,liquidethanolcollectsasthedistillateandallthewaterremainsinthedistillationflask.

Summary:MethodsofseparationofmixturesMethodofseparation Exampleofmixture

thatisseparatedPropertythatthemethoddependson

Paperchromatography Dyesinink Adsorptionbypaper/solubilityinsolvent

Dissolving,filtrationandcrystallisation

Sandandsalt Solubility

(Simple)distillation Sodiumchloridesolution BoilingpointFractionaldistillation Ethanolandwater Boilingpoint

Sampleexam-stylequestion1Statewhetherthefollowingareelements,mixturesorcompounds.asilverbbronzecseawaterdwaterebauxitefaluminiumoxide

Student’sanswer

1aelementbmixturecmixturedcompoundemixturefcompound

Examiner’scommentaMetalscanbeelements,butalloysaremixturesofmetals.Silverisanelement.Itistheelementwithatomicnumber47inthePeriodicTable.IfyouarenotsureifasubstanceisanelementyoushouldknowthatthePeriodicTableonlycontainselements.

bBronzeisanalloy(seeChapter10)and,assuch,itisamixtureofmetals.Bronzecontainscopper,tinandothermetalsinvariableproportions.

cSeawateriswater(whichisacompound)containingmanysubstances,invariableproportions,dissolvedinit.

dWaterhastheformulaH2O.Anysubstancewithaformulathatshowsmorethanoneelementisacompound.Althoughwaterisfoundinmanyformssuchastapwater,seawateranddistilledwater,thetermwaterreferstothepurecompound.

eBauxiteisametallicorefromwhichaluminiumisextracted(seeChapter5).Thewordorereferstoanimpuresubstance.Metallicoresaremixtures.

fAluminiumoxidehastheformulaAl2O3.Bauxitecontainsthecompoundaluminiumoxidewithimpurities.

Exam-stylequestions1Statethenameoftheprocess(es)thatyouwouldusetoobtainasugarcrystalsfromamixtureofsugarandsandbpurewaterfroman

aqueoussolutionofcopper(II)sulfatecliquidoctane(boilingpoint126°C)fromamixtureofliquidoctaneandliquiddecane(boilingpoint174°C)dpuresilverchloridefromtheprecipitateformedwhenaqueoussilvernitrateisaddedtodilutehydrochloricacid.

Insomecases,onlyoneprocessisrequired,butothersmayrequiremorethanone.

[Total:8marks]2Astudentwastoldtomakepurecrystalsofcopper(II)sulfatefromanaqueoussolutionofcopper(II)sulfate.Describehowthestudentshouldcarrythisout.

[4marks]

[4marks]3Astudentisgivenamixtureoftwoaminoacids.Theaminoacidsarebothcolourlesssolidsthataresolubleinwater.Givefullexperimentaldetailsofhowyouwouldseparateandidentifytheaminoacidspresentinthemixtureusingpaperchromatography.Youareprovidedwithallthenecessaryapparatusandasuitablelocatingagent.

[5marks]

3Atomicstructure,bondingandstructureofsolids

KeyobjectivesBytheendofthissection,youshould

•knowtherelativechargesandapproximaterelativemassesofprotons,neutronsandelectrons

•knowthedefinitionofprotonnumber(atomicnumber)andnucleonnumber(massnumber)

•knowthatprotonnumberisthebasisofthePeriodicTable•knowthedefinitionofisotopesandthatisotopesareradioactiveandnon-radioactive

•knowonemedicalandoneindustrialuseofradioactiveisotopes•knowthebuildupofelectronsinshellsforthefirst20elementsinthePeriodicTable

•knowthedifferencebetweenmetalsandnon-metals•knowthationsareformedwhenatomsloseandgainelectrons•beabletodescribetheformationofionicbondsbetweenelementsfromGroupsIandVII

•beabletodescribetheformationofsinglecovalentbondsinH2,Cl2,H2O,CH4,NH3,HCl

•beabletodescribethedifferenceinvolatility,solubilityandelectricalconductivitybetweenioniccompoundsandcovalentsubstanceswithbothgiantstructuresandsimplemolecularstructures

•knowthegiantcovalentstructuresofgraphiteanddiamond•relatethestructuresofgraphiteanddiamondtotheiruses

•knowwhyisotopeshavethesamechemicalproperties

•beabletodescribetheformationofionicbondsbetweenothermetallicandnon-metallicelements

•beabletodeterminetheformulaeofioniccompoundsfromthechargesontheionspresent

•knowthatagiantioniclatticeisaregulararrangementofpositiveandnegativeions

•beabletodescribetheformationofmorecomplexcovalentmolecules,suchasN2,C2H4,CH3OHandCO2

•beabletoexplainthedifferencesinmeltingpointandboilingpointbetweenioniccompoundsandcovalentsubstanceswithbothgiantstructuresandsimplemolecularstructuresintermsofattractiveforcesbetweenparticles

•knowthestructureofsilicon(IV)oxide(silicondioxide)•beabletodescribethesimilarityinpropertiesbetweendiamondandsilicon(IV)oxide(silicondioxide)relatedtotheirstructures

•beabletodescribemetallicbondingasaforceofattractionbetweenpositiveionsandamobileseaofelectrons

•knowhowthestructureofmetalscanbeusedtoexplainmalleabilityandconductionofelectricity.

KeytermsProtonnumber(atomicnumber)

Thenumberofprotonsinoneatomofanelement

Nucleonnumber(massnumber)

Thesumofthenumberofprotonsandneutronsinoneatomofanelement

Isotopes Atomsofthesameelementcontainingthesamenumberofprotonsbutdifferentnumbersofneutrons,orAtomsofthesameelementwiththesameprotonnumber(atomicnumber)butdifferentnucleonnumber(massnumber)

Lattice Aregulararrangementofparticlespresentinasolid.Theparticles(atoms,moleculesorions)arearrangedinarepeatedpattern

AtomicstructureAtomsaremadefromsmallerparticlescalledprotons,neutronsandelectrons.

Table3.1Thepropertiesofprotons,neutronsandelectrons

Particle Relativemass/atomicmassunits RelativechargeProton 1 +1Neutron 1 0Electron 1/1837(negligible) −1

Examiner’stipMakesureyoulearntheinformationinTable3.1.Youneedtoknowthedifferencesbetweenrelativemassandrelativechargeofaproton,neutronandelectron.

Theprotonsandneutronsexistinthecentreoftheatominadenseregioncalledthenucleus.Theelectronsmovearoundthenucleusandexistinelectronshellsatincreasingdistancesfromthenucleus.

Examiner’stipInsomebooks,thetwonumbersmaybereversed.Itisagoodideatorememberthatthenucleonnumberisalwayshigherthantheprotonnumber(withtheexceptionofhydrogen,inwhichcasebothnumbersare1inthemostabundantisotope).

AtomsareoftenrepresentedasshowninFigure3.1.

Theprotonnumberisthenumberofprotonsinoneatomoftheelement.Becauseatomsdonothaveacharge,thenumberofprotonsinanatomisalwaysequaltothenumberofelectrons.

equaltothenumberofelectrons.Thenucleonnumberisthenumberofneutronsandprotonsaddedtogetherin

oneatomofanelement.Therefore

IntheexamplegiveninFigure3.1:

Numberofprotons=protonnumber

Numberofelectrons=numberofprotons

Numberofneutrons=(nucleonnumber−protonnumber)

15 15 31−15=16

Isotopesareatomsofthesameelementcontainingthesamenumberofprotonsbutdifferentnumbersofneutrons.ExamplesofisotopesofargonareshowninTable3.2.

Someisotopesareradioactiveandsomearenon-radioactive.Radioactiveisotopesdecay(whichmeansgiveoffradiation)andusuallychangeintootherelements.Radioactiveisotopesarealsoknownasradioisotopes.Radioactiveisotopes/radioisotopescanbeused

•inmedicine(e.g.cobalt-60isusedinradiotherapytreatment)•inindustry(e.g.uranium-235isusedasasourceofpowerinnuclearreactors).

Commonerror•Manystudentsthinkofisotopesasreferringonlytoradioactiveisotopes,butsomearealsonon-radioactive.

ThearrangementofelectronsinatomsElectronsarearrangedinelectronshellsatincreasingdistancesfromthenucleus.Theseshellscanholduptoamaximumnumberofelectrons,asshowninTable3.3.

Table3.3Maximumnumberofelectronspershellnumber

Shellnumber Maximumnumberofelectrons1 22 83 8*

*Shell3can,infact,holdupto18electrons,butthisdoesnotneedtobeconsideredatthislevelandonlybecomesrelevantinhigher-levelcourses.

SomeexamplesofarrangementofelectronsinshellsareshownTable3.4.

Table3.4Arrangementofelectronsinshells

Element Numberofelectronsinoneatom

Arrangementofelectronsinshells

Helium,He 2 2Carbon,C 6 2,4Phosphorus,P

15 2,8,5

Potassium,K

19 2,8,8,1

Studentsareexpectedtobeabletowritedownanddrawtheelectronarrangementofthefirst20elementsinthePeriodicTable.

Isotopesofthesameelementallhavethesamenumberofelectrons,andthereforeallhavethesamenumberofelectronsintheiroutershells.Thismeansthatisotopesofthesameelementallhavethesamechemicalproperties.Thechemicalpropertiesofelementsdependonthenumberofelectronsin

theoutershelloftheiratoms.

theoutershelloftheiratoms.

ThePeriodicTableElementsinthePeriodicTablearearrangedinorderofincreasingprotonnumber.Thismeansthataswemovefromoneelementtothenextelement,theatomshaveoneextraprotoninthenucleusandoneextraelectron.Theextraelectrongoesintotheoutershelluntiltheoutershellisfull.Thenextshellthenbeginstofillup.Elementsinthesamegroupallhavethesamenumberofelectronsintheouter

shelloftheiratoms.Thisappliesbeyondthefirst20elements.Examplesare:

•AllGroup(I)elementshave1electronintheoutershell.•AllGroup(II)elementshave2electronsintheoutershell.•AllGroup(VII)elementshave7electronsintheoutershell.•AllGroup(0)elementshaveafulloutershell.•Mostmetallicelementshave1,2or3electronsintheiroutershell.•Mostnon-metallicelementshave5,6or7electronsintheiroutershell(orafulloutershellinthecaseofNoblegases).

IonicbondingIonicbondingoccursincompoundscontainingmetallicelementscombinedwithnon-metallicelements.Metalatoms(with1,2or3electronsintheiroutershells)loseanelectronor

electronsinordertoachieveafulloutershellandformpositiveions(cations).Non-metalatoms(with5,6or7electronsintheiroutershells)gainan

electronorelectronsinordertoachieveafulloutershellandformnegativeions(anions).Anexampleoccursinsodiumchloride.Sodiumatomscontain11protonsand11electronsandchlorineatomscontain

17protonsand17electrons.Becausebothcontainequalnumbersofprotonsandelectronsbothatomsareuncharged.Anelectronmovesfromtheoutershellofasodiumatomtotheoutershellof

achlorineatomsothatbothatomsachieveafulloutershell.

Afterthetransferofelectrons,sodiumformsapositivesodiumionandchlorineformsanegativechlorideion.

Thesodiumionstillhas11protonsbutonly10electrons,thereforeithas1+chargeandiswrittenNa+.Thechlorideionstillhas17protonsbutnowhas18electrons;thereforeithas1−chargeandiswrittenCl−.Insodiumchloride,theratioofsodiumionstochlorideionsis1:1andthus

theformulaofsodiumchlorideisNaCl.InallexamplesinwhichGroup(I)elementscombinewithGroup(VII)elements,theratioofionsisalways1:1.Otherexamplesoccurinwhichatomsdonotcombineintheratio1:1.This

applieswhenthenumberofelectronslostbyonemetalatomisnotequaltothenumberofelectronsgainedbyonenon-metallicatom.

Examiner’stipRememberthatatomshaveequalnumbersofprotonsandelectrons,andarethereforeuncharged.

Positiveions(cations)havemoreprotonsthanelectronsandarethereforepositivelycharged.

Negativeions(anions)havemoreelectronsthanprotonsandarethereforenegativelycharged.

negativelycharged.

Becausetheratioofmagnesiumionstofluorideionsis1:2,theformulaofmagnesiumfluorideisMgF2.

TheformulaeofioniccompoundsTheformulaeofioniccompoundscanbededucedfromknowledgeofthe

chargesontheions.ExamplesofcommonionsareshowninTable3.5.

Boldtypedenotespolyatomicions.Theseareionswhichhavemorethanonecapitalletterintheformula(seerule4below).

Examiner’stipTable3.6showsthechargesonionsindifferentgroupsinthePeriodicTable.

Table3.6

Group ChargeonionI 1+II 2+III 3+V 3−VI 2−VII 1−

Inothercases,e.g.thetransitionelements,itisnotpossibletousethePeriodicTabletodeducethechargesonions.Insuchcases,thechargesmustbe

learnedbyheart.

ExamplesofhowtodetermineformulaeofioniccompoundsThemostimportantthingtoknowisthatallcompoundshavenooverallcharge;thereforeinthecaseofioniccompoundsthenumberofpositivechargesisequaltothenumberofnegativecharges.Toworkouttheformulaofacompoundyoushould:

1Writedowntheformulaeofthepositiveandthenegativeions.2Countthenumberofpositivechargesandthenumberofnegativecharges.3Ifthechargesarenotequal,addmorepositiveions,morenegativeionsorbothuntilthechargesareequal.

4Ifmorethanoneofapolyatomicionisrequired,thewholeformulaoftheionmustgoinabracketandthenumberofionsgoesoutsidethebracketasasubscript,e.g.(NO3)2.

Writedowntheformulaeofthefollowingcompounds.

CommonerrorsSomecommonincorrectanswersusingtheformulaofiron(III)sulfateasanexampleare

•FeSO4:Thenumberofchargeshasnotbeenmadeequal.(Thisisthecorrectformulaofiron(II)sulfate.)

•Fe2(SO)3:The4isleftoutoftheformulaofsulfate.•(Fe)2(SO4)3:AbracketisnotrequiredaroundFeasitonlyhasonecapitalletter.

•Fe2(SO)4:The4isleftoutoftheformulaofsulfateandplacedincorrectlyoutsidethebracket.

Examiner’stipYoushouldbeabletowriteformulaeofcompoundscontainingallpossiblecombinationsofpositiveandnegativeionsinTable3.5.

CovalentbondingCovalentbondingoccursinelementsandcompoundscontainingnon-metallicelementsonly.Covalentbondsareformedwhenpairsofelectronsareshared.Asharedpair

ofelectronsisknownasasingle(covalent)bond.

Doublebonds(twosharedpairsofelectrons)andtriplebonds(threesharedpairsofelectrons)alsoexist.

Atomswhichformacovalentbondjointogethertoformunchargedmolecules.Alltheatomsinvolvedachieveafulloutershellofelectrons.Examplesofcovalentmoleculescontainingsinglebondsonlyareshownin

Figure3.5.Onlytheouterelectronshellsareshown.

ExamplesofcovalentmoleculescontainingdoubleandtriplebondsareshowninFigure3.6.Onlytheouterelectronshellsareshown.

StructureofsolidsSolidshavefourdifferenttypesofstructuresasshownbelow.

GiantionicstructureSodiumchlorideisanexampleofagiantionicstructure.Itisheldtogetherbystrongforcesofattractionbetweenoppositelychargedsodiumionsandchlorideions(calledionicbonds)whicharepresentinagiantioniclattice.

Giantcovalentstructure(macromolecularstructure)Diamondisanexampleofagiantcovalentstructure.Itisheldtogetherbystrongcovalentbondsbetweencarbonatoms.Graphiteisanotherexampleofsubstancewithagiantcovalentstructure,

althoughithasmanydifferenceswhencomparedtodiamond(seepages23–24).

Silicon(IV)oxide,SiO2,isanotherexampleofagiantcovalentstructure.Ithassimilarpropertiestodiamondduetoasimilarstructure.

GiantmetallicstructuresAllmetallicelementshavegiantmetallicstructures.Theycontainpositiveionssurroundedbyamobileseaofelectrons.Metalsareheldtogetherbythestrongforcesofattractionbetweenpositiveionsandthemobileseaofelectronsknownasmetallicbonds.

Examiner’stipInexamquestionswhichaskforthemeaningofmetallicbonding,students

Inexamquestionswhichaskforthemeaningofmetallicbonding,studentsusuallydescribethegiantmetallicstructurebutdonotmentionthestrongforcesofattractionbetweenpositiveionsandthemobileseaofelectrons,knownasmetallicbonds.

SimplemolecularstructuresIodineisanexampleofasubstancewithasimplemolecularstructure.Ithasstrongcovalentbondsbetweentheatomswithinthemolecules(intramolecular)butweakintermolecularforcesofattractionbetweenthemolecules.Thepropertiesofdifferenttypesofsolid,relatedtotheirstructures,areshown

inTable3.7.

Commonerrors•Inexamquestionswhichaskwhysubstanceswithsimplemolecularstructureshavelowmeltingpointsandboilingpoints,itisverycommonlysaidthatthisisbecausecovalentbondsareweak.Thisisabaderror.Allcovalentbondsarestrongbonds.

•Thecorrectansweristhatintermolecularforcesareweakwhichiswhysubstanceswithsimplemolecularsubstancesareeithersolidswithlowmeltingpointssuchasiodine,liquidssuchaswaterorgasessuchascarbondioxide.

DiamondandgraphiteBothdiamondandgraphitehavegiantcovalent(macromolecular)structures,butbecausetherearedifferencesintheirstructureandbonding,theseleadtodifferencesinpropertiesanduses.

Table3.8Differencesinstructureandbondingbetweendiamondandgraphite

Diamond GraphiteNumberofcarbonatomscovalentlybondedtoeachcarbonatom

4 3

atomArrangementofatoms Tetrahedrally Inlayers(madeofrings

containing6carbonatoms)Bonding Allcovalent Covalentbetweenatomswithin

layersWeakvanderWaalsforcesbetweenlayers

Mobileelectrons None.Alloutershellelectronsusedinbonding

Oneelectronfromeachatomexistsinthespacesinbetweenthelayersasmobileelectrons

Hardness Hardbecauseallbondsarestronganddirectional

SoftbecauseweakvanderWaalsforcesbetweenlayersallowlayerstoslideovereachother

Conductionofelectricity

Non-conductorbecausetherearenomobileelectrons

Goodconductorsduetomobileelectronsbetweenlayers

Use Incuttingtoolsduetohighstrength

AsalubricantbecauselayerscanslideAsaconductorinmotors

Silicon(IV)oxide(silicondioxide)Silicon(IV)oxide(silicondioxide)hasagiantcovalent(macromolecular)structure.Eachsiliconatomiscovalentlybondedtofouroxygenatoms.Thebondsare

directedtetrahedrally.Eachoxygenatomiscovalentlybondedtotwosiliconatoms.Allthebondsinsilicon(IV)oxidearestrongcovalentbonds.Thereareno

mobileelectronspresent.Becauseofitsstructureandbonding,silicon(IV)oxideisstrong,hard,hasahighmeltingandboilingpointandisanon-conductorofelectricity.Thesepropertiesarelikethoseofdiamondwhichhasaverysimilarstructureandthesamebonding.

Exam-stylequestions1Completethefollowingtable

[Total:5marks]2Drawdotandcrossdiagramsshowingthearrangementofoutershellelectronsinthefollowingcovalentmolecules.

aoxygen,O2

bmethanol,

chydrogencyanide,

dphosphine,

ecarbonylchloride,

[Total:5marks]3Deducetheformulaeofthefollowingioniccompounds.amagnesiumhydroxidebcalciumchloridecammoniumphosphatedlithiumsulfideeleadnitratefcalciumcarbonategaluminiumnitratehpotassiumsulfiteizincsulfatejammoniumsulfate

[Total:10marks]4Usethetabletoanswerthequestionsthatfollow.Considerroomtemperaturetobe25°C.

aWhichsubstanceorsubstancesaresolidatroomtemperature?[1mark]

bWhichsubstanceorsubstancesareliquidatroomtemperature?[1mark]

[1mark]cWhichsubstanceorsubstancesaregaseousatroomtemperature?

[1mark]dWhichsubstancecouldhaveagiantmetallicstructure?

[1mark]eWhichsubstancehasagiantionicstructure?

[1mark]fWhichsubstancehasagiantmolecularstructure?

[1mark][Total:6marks]

4Stoichiometry:chemicalcalculations

KeyobjectivesBytheendofthissection,youshould

•beabletoconstructwordequationsandsimplebalancedequations•beabletodefinerelativeatomicmass,Ar

•beabletodefineandcalculaterelativemolecularmass,Mr

•beabletoconstructequationswithstatesymbols,includingionicequations•beabletodeduceabalancedequationforachemicalreactiongivenrelevantinformation•beabletodefinethemoleandtheAvogadroconstantandbeabletousethemolargasvolume(takenas24dm−3atroomtemperatureandpressure)•beabletocalculatereactingmasses,volumesofgasesandsolutionsandconcentrationsofsolutions•defineandcalculateempiricalformulaeandmolecularformulae•beabletousetheideaoflimitingreactants•calculatepercentageyieldandpercentagepurity.

KeytermsStoichiometry Thecalculationoftherelativequantitiesofreactantsand

productsinachemicalreactionRelativeatomicmass

Theaveragemassofoneatomofanelementonascalewhereone12Catomhas12unitsofmassexactly,Ar

Relativemolecularmass

Thesumoftherelativeatomicmasses,Mr

Empiricalformula

Thesmallestwholenumberratiooftheatomsofeachelementinacompound

Molecularformula

Thenumberofatomsofeachelementinonemoleculeofasubstance

Mole Thesamenumberofparticlesasthereareatomsin12gofthecarbon-12isotope

Avogadroconstant

Thenumberofparticlesinonemoleofasubstance.Itisequalto6.02×1023particles

StoichiometryStoichiometrymeansthecalculationoftherelativequantitiesofreactantsandproductsinachemicalreaction.

WordequationsWordequationsgivethenamesofthereactantsandproductswhichtakepartinachemicalreaction.Whenhydrogenburnsinoxygentoformwatertheword

equationis:

SymbolequationsSymbolequationsgivethecorrectformulaeofthereactantsandproductsinareaction.Symbolequationsarebalancedwhenthenumberofatomsofeachelementisthesameonbothsidesoftheequation.

Stepsforwritingbalancedequations1Writedownthewordequation(thiscanbeomittedwithexperience).2Writedownthecorrectformulaeofreactantsandproducts.3Countthenumberofatomsofeachelementonbothsides.4Ifthenumberofatomsofeachelementonbothsidesisnotthesame,putnumbersinfrontoftheformulaesothatthenumberofatomsofeachelementonbothsidesisthesame.

5Putstatesymbolsaftertheformulae,i.e.(s)=solid,(l)=liquid,(g)=gas,(aq)=aqueoussolution.Thiscanbedoneaftersteps2or3ifpreferred.

WorkedExample

Becausethenumberofatomsofoxygenisnotthesameonbothsides,thefirststepistoput2infrontofH2O.Thismultiplieseverythingthatcomesafterit.

Inbalancingtheoxygen,wehaveunbalancedthehydrogen.Thereforeweneedtoputa2infrontofH2.Theequationisthenbalanced.Statesymbolscanbeinserted.

Commonerrors•Studentsoftenuseincorrectformulae,e.g.HinsteadofH2orOinsteadofO2,orchangeformulaesuchaschangingH2OintoH2O2.Thiswouldmakethenumberofatomsofeachelementthesameonbothsides,butH2O2isnotthecorrectformulaforwater(infact,itistheformulaforhydrogenperoxide).Theonlywaytobalanceequationsistoputnumbersinfrontoftheformulae.

WorkedExample

Thealuminiumisbalanced.Tobalancethechlorine,weput2infrontofAlCl3and3infrontofCl2.

Thealuminiumisnowunbalanced,thereforewemustputa2infrontofaluminium.

IonicequationsStartingfromabalancedequationwithstatesymbols,anionicequationcanbewrittenusingthefollowingsteps.1Anythingwith(aq)asastatesymbolshouldbewrittenasionsifitis

aadiluteacid,e.g.HCl(aq)iswrittenasH+(aq)andCl–(aq)

bametalliccompound,e.g.CuSO4(aq)iswrittenCu2+(aq)andSO42–(aq)

canammoniumsalt,e.g.(NH4)2SO4(aq)iswritten2NH4+(aq)andSO42–(aq).

2Numbersinfrontofformulaeinequationsmeanthateverythingafterthenumberismultiplied,e.g.2HNO3(aq)iswrittenas2H+(aq)and2NO3−(aq).

3Theformulaeofanysubstancewithstatesymbols(s),(l),(g)arenotwrittenasions,thusarenotchangedinanionicequation.

4Anyionswhicharethesameonbothsides,knownasspectatorions,arecancelled.

Sampleexam-stylequestionWritethefollowingbalancedequationasanionicequation.

Student’sanswer

SO42-(aq)arethesameonbothsidesandarecancelledout.

ChemicalcalculationsCalculatingrelativemolecularmass,MrUsethefollowingrelativeatomicmasses,Ar,tocalculatetherelativemolecularmassesofthecompoundsshown.

H=1C=12N=14O=16Al=27S=32Pb=207

•CO2=12+(16×2)=44•N2O=(14×2)+16=44•C4H10=(12×4)+(1×10)=58•Pb(NO3)2.Therecommendedmethodistomultiplyoutthebrackets,i.e.

PbN2O6=207+(14×2)+(16×6)=331•Al2(SO4)3=Al2S3O12=(27×2)+(32×3)+(16×12)=342

Howtocalculatemoles

Molesfrommasses

Massof1molemeansrelativeatomicmassofanysubstancewhichonlycontainsatoms.Relativemolecularmassorrelativeformulamassshouldbeusedforallothersubstances.Rearrangingformass:

Rearrangingformassofonemole:

Pleasenotethatthemassmustbeingrams(g).Ifthemassisgivenin

Pleasenotethatthemassmustbeingrams(g).Ifthemassisgiveninkilograms(kg),itmustbemultipliedby1000orifitisgivenintonnes,itmustbemultipliedby1000000toconvertitintograms.

MolesfromgasvolumesThevolumeofonemoleofanygasis24dm3atroomtemperatureandpressure.

Rearrangingforvolume:

Thevolumeofthegasandthevolumeofonemoleofgasmustbeinthesameunitswhenusingtheseequations.

Molesfromvolumesandconcentrationofsolutions

Correctunitsareveryimportantintheseequations.Becausesolutionsaremeasuredoutusingburettesandpipetteswhicharegraduatedincm3,theequationsbelowmaybemoreuseful.

Examiner’stipItisagoodideatorememberthatbothoftheseexpressionshavemoles×1000onthetopline.

MolecalculationsusingequationsThefollowingcalculationsshouldbeapproachedinthefollowingorder.aCalculateanyrelativemolecularmasses,Mr,thatarerequired.

bCalculatethenumberofmolesofthesubstancewheresufficientinformationisgiventodoso.

cUsethemoleratiointheequationtocalculatethenumberofmolesoftheothersubstance.

dUseyouranswertoctocalculateeitherthe•massor•volumeofgasor•volumeofsolutionor•concentrationofsolution.

Examiner’stipItisextremelyimportanttoshowalltheworkingoutincalculations.Ifsomecorrectworkingoutisshownandthefinalanswerisincorrect,youwillstillbeawardedaconsiderableamountofcredit.

Workedexamples1Calciumcarbonatedecomposeswhenitisheatedaccordingtotheequation

Calculatethemassofcalciumoxide,CaO,thatisproducedwhen20.0gofcalciumcarbonate,CaCO3,isheateduntilthereisnofurtherchange.

[3marks]Relativeatomicmasses,Ar:C=12,O=16,Ca=40

a

bMolesofCaCO3=20÷100=0.20moles[1mark]

cMoleratiofromtheequation1moleCaCO3:1moleCaO

0.20molesCaCO3:0.20molesofCaO

[1mark]d

Examiner’stips1Becausethequestiondoesnotaskaboutcarbondioxide,CO2,thereisnoneedtocalculatetherelativemolecularmass,Mrofcarbondioxide.

2Thefinalanswershouldalwaysbeexpressedusingcorrectunits.

2Calculatethevolumeofcarbondioxideatroomtemperatureandpressurethatisproducedbyheating2.1gofsodiumhydrogencarbonate,NaHCO3,accordingtotheequation

Thevolumeofonemoleofanygasis24dm3atroomtemperatureandpressure.

[4marks]aMr:NaHCO3=23+1+12+(16×3)=84

[1mark]bMolesofNaHCO3=2.1÷84=0.025moles[1mark]

cMoleratiofromtheequation2moleNaHCO3:1moleCO20.025molesNaHCO3:0.025÷2=0.0125molesofCO2

[1mark]d

Examiner’stipsThequestionasksforthevolumeofcarbondioxide.Itisaverycommonerrortocalculatethemassinstead.Thosewhodothiscanachievethefirstthreemarksaslongastheworkingoutisclearlyshown.

Itisverycommonforstudentsnottousethemoleratiointheequationortouseitthewrongwayround,i.e.1:2insteadof2:1.Againitispossibletoscorethreemarksoutoffourunderthesecircumstancesdependingonhowmuchcorrectworkingoutisshown.

3Calculatethevolumeofaqueoussodiumhydroxide,NaOH(aq),ofconcentration0.20moldm–3whichwouldberequiredtoneutraliseexactly25.0cm3ofdilutesulfuricacid,H2SO4(aq),ofconcentration0.25moldm−3accordingtotheequation

aTherearenomassesinvolvedinthequestion,sonoMrvalueshavetobecalculated.

bMolesof

cMoleratioinequation

d

Examiner’stipsThisquestionasksyoutocalculatethevolumeofasolution.Manycandidatesusethevalueof24dm3becausetheyconfusethevolumeofasolutionwiththevolumeofagas.

Manystudentscalculaterelativemolecularmasses,althoughthereisnomentionofmassinthequestion.

Whencalculatingthenumberofmolesofasolution,manyuse

theequation

whichtheyoftenlearnas

Thisequationcanonlybeusedifthevolumeisindm3,butinthiscasethevolumeisincm3whichmeansthefactorof1000mustbeused.

4240dm3ofnitrogen,N2(g),reactswithexcesshydrogen,H2(g),accordingtotheequation

aWhatwouldbethevolumeofammonia,NH3(g),produced?

bWhatvolumeofhydrogen,H2(g),wouldreactwiththenitrogen?

Examiner’stipsThereisamuchquickerwayofdoingthiscalculation.Forgasesonly,thevolumeisdirectlyproportionaltothenumberof

moles,whichmeans

Moleratio1:3:2

Volumeratio1:3:2

Therefore,240dm3N2(g)reactswith720dm3H2(g)toproduce480dm3NH3(g).

Allvolumesaremeasuredatroomtemperatureandpressure.Thevolumeofonemoleofanygasis24dm3atroom

temperatureandpressure.aAmmonia•Therearenomassesinvolvedinthequestion,sonoMr

valueshavetobecalculated.•Molesofnitrogen=240÷24=10.0•Moleratio1moleofnitrogen:2molesofammonia10molesofnitrogen:2×10=20molesofammonia•Volumeof20molesofammonia=20×24=480dm3

bHydrogen•Moleratio1moleofnitrogen:3molesofhydrogen10molesofnitrogen:3×10=30molesofhydrogen•Volumeof30molesofhydrogen=30×24=720dm3

EmpiricalformulaeTheempiricalformulaisthesmallestwholenumberratiooftheatomsofeachelementinacompound.Theempiricalformulaofacompoundcanbecalculatedifthe

massesoftheelementsthatcombinetogetherareknown.Thesemassescanbeexpressedinunitsofmass(usuallygrams)orpercentagesbymass.

WorkedexampleAcompoundcontainsthefollowingpercentagecompositionbymass:26.7%carbon,2.2%hydrogenand71.1%oxygen.

Percentagecompositionbymassmeansthat100gofthecompoundcontains26.7gofcarbon,2.2gofhydrogenand71.1gofoxygen.

MethodCalculatethenumberofmolesofatomsofeachelement.

•Carbon,C=26.7÷12=2.225•Hydrogen,H=2.2÷1=2.2•Oxygen,O=71.1÷16=4.44375

Dividealltheabovebythesmallest

•C,2.225÷2.2=1•H,2.2÷2.2=1•O,4.44375÷2.2=2

Writedowntheempiricalformula=CHO2

Examiner’stipIfdividingbythesmallestdoesnotproduceawholenumberineachcase,multiplyallthenumbersby2.Ifthisstilldoesnotproduceawholenumberineachcase,multiplyallthenumbersby3.Continueuntilawholenumberratioisobtained.

CommonerrorsCommonerrorsindeterminingthemolecularformulaeare:

•UsingMrinsteadofAre.g.usingO2=32insteadofO=16

whencalculatingmolesofatoms.•UsingatomicnumberinsteadofArwhencalculatingmolesofatoms.

•Overapproximatione.g.ifacompoundcontainsmanganese,Mn,andoxygen,O,andthenumberofmolesofatomsis

Dividingbothbythesmallest

•Somecandidatesdecidethat1.5isapproximately1andwritetheempiricalformulaasMnO.Thisisincorrect.

•Somecandidatesdecidethat1.5isapproximately2andwritetheempiricalformulaasMnO2.Thisisincorrect.

•Thecorrectmethodistomultiplyboth×2,i.e.1×2=2and1.5×2=3andtheempiricalformulaisMn2O3.

•Anumberwouldhavetobeveryclosetoawholenumber(say0.1away)ifsuchanapproximationistobemade.

MolecularformulaeThemolecularformulaisthenumberofatomsofeachelementinonemoleculeofasubstance.ExamplesofmolecularandempiricalformulaeareshowninTable

4.1.

Table4.1Examplesofmolecularandempiricalformulae

Name Molecularformula EmpiricalformulaButane C4H10 C2H5Hydrogenperoxide H2O2 HOGlucose C6H12O6 CH2OBenzene C6H6 CHMethane CH4 CH4

DeterminationofmolecularformulaefromempiricalformulaeItispossibletodeterminethemolecularformulaofasubstancefromitsempiricalformulaalone,butonlyiftheMrofthesubstanceisalsoknown.IftheempiricalformulaofacompoundisCH2,themolecular

formulaofthecompoundcanbeexpressedas(CH2)n,wherenisawholenumber.IftheMrofthecompoundis70,theMrofCH2=12+(1×2)=14

Therefore,n=70÷14=5andthemolecularformulaisCH2×5=C5H10.

LimitingreactantsWhentwosubstancesaremixed,studentsusuallyassumethatbothsubstanceswillreactcompletelyandthatneitherisleftover.Thisispossible,butitisalsopossiblethattoomuchofeithersubstanceisused,inwhichcaseoneofthetwosubstanceswillbeleftoverattheendofthereaction.Thesubstancethatisallusedupiscalledthelimitingreactantandtheothersubstanceissaidtobeinexcess.

Workedexample5.6gofiron,Fe,and4.0gofsulfur,S,aremixedtogetherandheated.

TheequationisDeducewhichsubstanceisthelimitingreactant.

Therefore,0.10moleofFereactswith0.10moleofS.However,thereare0.125molesofS.0.125isgreaterthan0.10,

thereforesomeSisleftover.SisinexcessandFeisthelimitingreactant.

PercentageyieldIfthereactantsshowninanequationareconvertedcompletelyintotheproducts,wesaythatthepercentageyieldis100%.However,insomecircumstances,yieldsarelessthan100%.

Workedexample0.60gofmagnesiumribbon,Mg,wereburnedinexcessoxygen,O2,accordingtotheequation

Themassofmagnesiumoxide,MgO,thatwasproducedwasfoundtobe0.80g.Calculatethepercentageyield.

Therefore,0.025moleofMg:0.025moleofMgOMassofMgO=0.025×40=1.00gThus,iftheyieldwas1.00g,thepercentageyieldwouldbe100%.

However,theyieldisonly0.80g.

PercentagepurityNaturallyoccurringsubstancesareimpureandcontainlessthan100%ofacompound.Anexampleislimestonewhichcontainslessthan100%ofcalciumcarbonate,CaCO3(s).Thepercentagebymassofcalciumcarbonateinlimestoneisknownasthepercentagepurity.

Workedexample1.00goflimestoneisaddedto100cm3of0.200moldm–3hydrochloricacid(anexcess)(seeequation1).

Equation1

Theleftoveracidwastitratedandfoundtobeneutralisedby24.8cm3,0.100moldm–3ofsodiumhydroxidesolution,NaOH(seeequation2).

Equation2

Moleratioinequation21moleNaOHreactswith1moleHCl

2.48×10–3molesofNaOHreactwith2.48×10–3molesof

HClMolesofHClthatreactedwithcalciumcarbonate,CaCO3MolesofHCladded−molesHClleftover

Moleratioinequation22molesHClreactwith1moleCaCO3

0.01752molesHClreactwith

Exam-stylequestionsThevolumeofonemoleofanygasis24dm3atroomtemperatureandpressure.

1Whatmassofhydrogengasisproducedwhen8.1gofaluminiumpowderreactswithexcessdilutehydrochloricacidaccordingtotheequation

[Total:3marks]2Whatvolumeofoxygengas,O2(g),isproducedatroomtemperatureandpressurewhen0.142gofpotassiumsuperoxide,KO2(s),reactswithexcesscarbondioxide,CO2(g),accordingtotheequation

[Total:3marks]

3Whatmassofcalciumcarbide,CaC2(s),isrequiredtoproduce120cm3ofethynegas,C2H2(g),byreactionwithexcesswateraccordingtotheequation

[Total:3marks]

420.0cm3ofaqueousKOHneutralised35.0cm3ofdilutesulfuricacid,H2SO4(aq),whoseconcentrationwas0.20moldm−3.Theequationis

Calculatetheconcentrationoftheaqueouspotassiumhydroxide,KOH(aq),in

amoldm−3

bgdm−3

[Total:4marks]5Acompoundhascompositionbymasswhichis54.5%carbon,9.1%hydrogenand36.4%oxygen.

TheMrofthecompound=44.Calculatethe

aempiricalformulabmolecularformulaofthecompound.[Total:4marks]

6When0.38goftitanium(IV)chloride,TiCl4(s),reactedwithexcesssodium,thereactionproduced0.024goftitanium,Ti(s).Theequationis

Calculatethepercentageyieldoftitanium.[Total:4marks]

5Electricityandchemistry

KeyobjectivesBytheendofthissection,youshould

•beabletodefineelectrolysis,electrolyteandelectrode•beabletodescribetheproductsofelectrolysisandstatetheobservationsmadewhenvariouselectrolytesconductelectricity•beabletodescribetheelectroplatingofmetalsandoutlinetheusesofelectroplating•beabletodescribeandexplainthereasonswhycopperand(steel-cored)aluminiumareusedincablesandwhyplasticsareusedasinsulators

•beabletoconstructionichalf-equationsforreactionsatthecathode•understandthereasonswhydifferentsubstancesareconductors,insulatorsandelectrolytes•beabletodescribeproductionofenergyfromsimplecells•beabletodescribeinoutlinethemanufactureofaluminiumfrompurealuminiumoxideinmoltencryolite(seeChapter10)•beabletodescribeinoutlinetheproductionofchlorine,hydrogenandsodiumhydroxidefromconcentratedaqueoussodiumchloride•knowwhattheproductsofelectrolysisofaqueouscopper(II)sulfatearewithbothcarbonelectrodesandcopperelectrodes,andrelatethistotherefiningofcopper(seeChapter10).

KeytermsElectrolysisTheprocessbywhichanioniccompound,whenmoltenorin

aqueoussolution,ischemicallychangedbythepassageofanelectriccurrent

Electrolyte AliquidwhichischemicallychangedbyanelectriccurrentElectrodes Theconductingrodsbywhichtheelectriccurrententersand

leavestheelectrolyteAnode Thepositive(+)electrodeCathode Thenegative(−)electrode

Substancesthatconductelectricitycanbesubdividedintoconductorsandelectrolytes(Table5.1).

Table5.1Differencesbetweenconductorsandelectrolytes

Conductors ElectrolytesPhysicalstate

Solid Liquid

Differences Conductelectricity,butarenotchemicallychangedbytheelectriccurrent.Theonlychangeundergonebyconductorsisthattheybecomehot,whichisaphysicalchange

Conductelectricityandarechemicallychangedbytheelectriccurrent.Theproductsofchemicalchangeareformedattheelectrodes

Examples AllmetallicelementsandalloysGraphiteandgraphene

MoltenioniccompoundsAqueoussolutionscontainingions

Particlesresponsibleforconduction

Moving(mobile)electrons Movingions

Examiner’stipThetermselectrolysis,electrolyteandelectrodeallbeginwiththesameletter.Makesureyouknowthedifferencebetweentheseterms.

ElectrolytesElectrolytesmustbeintheliquidstate.Solidioniccompounds,e.g.sodiumchloride,donotconductelectricityinthesolidstatebecausealthoughtheycontainions,theionsareheldtogetherbystrongforcesofattractioninthegiantioniclattice.Becausetheionsarenotmoving,solidsodiumchloridedoesnotconductelectricity.Therearetwowaystomakeionicsolidsintoelectrolytes:

Heatuntiltheionicsolidmelts.Thisrequiresalargeamountofheatenergy,becauseioniccompoundshavehighmeltingpoints(seeChapter3).Moltenioniccompoundsareelectrolytesbecausetheionsaremovingintheliquidstate.

Dissolvetheionicsolidinwater.Anaqueoussolutionofanioniccompoundalsocontainsmovingions.

Whenmoltenioniccompoundsandaqueoussolutionsofioniccompoundsconductelectricity,thepositiveions(cations)movetothecathode(−)andthenegativeions(anions)movetotheanode(+).Atthecathode,positiveionsgainelectronsandarereduced(seeChapter7),

forexampleAttheanode,negativeionsloseelectronsandareoxidised(seeChapter7),for

exampleThus,theproductsofelectrolysisareformedattheelectrodes.Whenionslosetheirchargetoformatomsormoleculestheyaresaidtobe

discharged.

PracticalelectrolysisElectrolysiscanbecarriedoutinschoollaboratoriesusingtheapparatusshowninFigure5.1.

Theelectrolyteisplacedinacrucible,ifitisasolidthathastobeheateduntilitsmeltingpoint,orabeaker,ifitisaliquidatroomtemperature.Figure5.2showshowgaseousproductscanbecollectedduringelectrolysis,

asintheelectrolysisofdilutesulfuricacid.TheproductsthatformduringelectrolysisaresummarisedinTable5.2.

Table5.2Summaryofproductsformedduringelectrolysis

Typeofelectrolyte

Productsatanode(+) Productsatcathode(−)

Moltenioniccompound

Non-metallicelement Metallicelement

Aqueoussolutionscontainingions

Eitheroxygengasorhalogen(chlorine,bromineoriodine)fromanyconcentratedaqueoussolutionofahalide(chloride,bromideoriodide)

Eitherhydrogengasormetallicelementbelowhydrogeninthe

ofahalide(chloride,bromideoriodide)reactivityseries,e.g.copper

Examiner’stipRememberthatmoltenioniccompoundsproduceanon-metallicelementattheanodeandametallicelementatthecathode.

Aqueoussolutionsproduceoxygenorahalogenattheanodeandhydrogenorametalatthecathode.Thehydrogenandoxygencomefromthewaterthatiscontainedintheaqueoussolution.

Someexamplesofproductsoftheelectrolysisofdifferentelectrolytes,usingcarbonorplatinum(inertelectrodes),areshowninTable5.3.

Examiner’stipMakesureyourememberthat

•Veryreactivemetalsthatreactwithcoldwater(suchaspotassium,sodiumandcalcium)cannotbeproducedbyelectrolysisofaqueoussolutions.

•Duringtheelectrolysisofallaqueoussolutionscontainingpositiveionsofametalabovehydrogeninthereactivityseries,hydrogenisproducedatthecathodeasopposedtothemetallicelement.Theextractionofthesemetalsbyelectrolysiscanonlybecarriedoutusingamoltenelectrolyte(seeextractionofaluminium,p.73,inChapter10).

•Aqueoussolutionsofacidsalwaysproducehydrogenatthecathode(−).This

isbecausethepositiveH+ioniscommontoboththeacidicsubstanceandwater.

Electrolysisofcopper(II)sulfateIfaqueouscopper(II)sulfateiselectrolysedusingcarbonorplatinumelectrodes(inertelectrodes),theproductsarecopperatthecathodeandoxygenattheanode(seeTable5.3).However,iftheanodeismadeofcopper,oxygenisnotproducedatthe

anode.Instead,thecopperanodegoesintosolutionaspositiveions:

Eventually,theCu2+(aq)ionsreachthecathodewherethereversereaction

occursandcoppermetalisformed:Thiselectrolyticprocessisusedintherefining(purification)ofcopper

whichiscarriedoutonalargescale(seeChapter10).

•Theanodeisimpurecopper•Theelectrolyteisaqueouscopper(II)sulfate•Thecathodeispurecopper.

ThecopperattheanodegoesintosolutionaspositiveCu2+ions.Impuritieseithergointosolutionaspositiveionsorfallofftheanodeandaredepositedatthebottomofthecontainer.Purecopperformsatthecathode.Noneoftheothermetallicionsaredischargedatthecathode.Purecopperisessentialwhencopperisbeingusedasanelectricalconductor,

asinelectricalwiring.Impuritiesinthecopperdecreaseitselectricalconductivityconsiderably.

ElectrolysisofconcentratedaqueoussodiumchlorideSodiumchloride(commonsalt)isfirstminedandthenelectrolysedasaconcentratedaqueoussolution.Electrolysisofconcentratedaqueoussodiumchloridecanbecarriedoutinthelaboratoryoronanindustrialscale.Theionsthatarepresentinthesolutionare

Attheanode,hydrogengasisproduced2H++2e−→H2.TheNa+(aq)remaininthesolution.Atthecathode,chlorinegasisproduced2Cl−→Cl2+2e.TheOH−(aq)remaininthesolution.BecausetheelectrolytecontainsNa+(aq)andOH−(aq),theelectrolytehas

changedintoaqueoussodiumhydroxide,NaOH(aq).Thethreeproductsofelectrolysishaveseveralindustrialuses.

ElectroplatingElectroplatingisanotherelectrolyticprocessthatcanbecarriedoutinaschoollaboratoryoronalargescale.Electroplatingmeanscoatinganobjectwithathinlayerofametal.Thepurposeis

•toimproveappearance•topreventcorrosion,e.g.rusting.

Electroplatingiscarriedoutusing

•theplatingmetalastheanode•theobjecttobeplatedasthecathode•anaqueoussolutioncontainingionsoftheplatingmetalastheelectrolyte.

IntheexampleofsilverplatingshowninFigure5.3,thesilveranodegoesintosolutionassilverions.

Thesilverionsintheelectrolytearedischargedatthecathode.

Thesilverionsthatarereleasedattheanodereplacethosethataredischarged.Thesilverproducedatthecathodeelectroplatesthespoon.

ExtractionofmetalsReactivemetalsareextractedbyelectrolysisofmoltenioniccompounds(seeextractionofaluminium,pp.73–4,Chapter10).Unreactivemetalsareextractedbyelectrolysisofaqueoussolutions(see

refiningofcopper,pp.75–6,Chapter10).

Usesofmetals,plasticsandceramicsAluminiumisusedinsteel-coredcables,becauseit

•isunreactiveduetoacoatingofaluminiumoxide•isagoodconductorofelectricity•hasalowdensity.Copperisusedinelectriccablesandwiresduetoveryhighelectrical

conductivity.Plasticsandceramicsdonotconductelectricityandthereforeareusedas

insulatorsinthepowersupplyindustry.

CellsElectrolysisuseselectricalenergytocarryoutchemicalreactions.Incells,chemicalreactionsareusedtoproduceelectricalenergy.Iftwo

dissimilarmetalsareplacedinanelectrolyte,electricalenergyisproduced.Thisistheprincipleofthebattery.Suchcellscanbesetupinordertoputmetalsinorderofreactivity(seeChapter10).Fuelcellshaveafuel,suchashydrogenorethanol.Thefuelreactswith

oxygeninordertogenerateelectricity(seeChapter6).

Exam-stylequestions1Completethefollowingtabletoshowtheproductsofelectrolysisusingcarbonelectrodes.

[Total:10marks]2Astudentwantedtoelectroplateaknifewithnickel.Whatshouldthestudentuseas

atheanodebtheelectrolytecthecathode?

[Total:3marks]3Astudentcarriesoutelectrolysisofconcentratedaqueouspotassiumiodideinabeakerusingcarbonelectrodes.

aNametheproductattheanode.[1mark]

bWriteanionichalf-equationforthereactionoccurringatthecathode.[1mark]

cStatethetypeofreactionoccurringattheanode.[1mark]

dStatethenameofthesolutionleftinthebeakerwhentheelectrolysishasfinished.

[1mark]eNamethetypeofparticlesthatareresponsiblefortheconductionof

electricityintheconductingwire.

[1mark]fNamethetypeofparticlesthatareresponsiblefortheconductionof

electricityintheelectrolyte.[1mark]

[Total:6marks]

6Chemicalenergetics

KeyobjectivesBytheendofthissection,youshould

•beabletodescribewhatismeantbyexothermicandendothermicreactions•beabletointerpretenergyleveldiagramsshowingexothermicandendothermicreactions•describethereleaseofheatenergybyburningfuels•knowthathydrogenisusedasafuel•knowthatradioactiveisotopes,suchas235U,canbeusedasasourceofenergy

•knowthatbondbreakingisanendothermicprocessandbondformationisanexothermicprocess•beabletodrawandlabelenergyleveldiagramsforexothermicandendothermicreactionsusingdataprovided•beabletocalculatetheenergychangeinareactionusingbondenergies•beabletodescribetheuseofhydrogenasafuelinfuelcells.

KeytermsExothermicreaction

Areactioninwhichthereisanoveralltransferofenergytothesurroundings

Endothermicreaction

Areactioninwhichthereisanoverallgainofenergyfromthesurroundings

Bondenergy Theamountofenergyrequiredtobreakonemoleofcovalentbondsingaseousmolecules

Examplesofexothermicreactionsare

•combustionoffuelssuchasalkanesandhydrogengas;fuelsreleaseheatenergywhentheyburninairoroxygen.

•respiration(seeChapter13).

Examplesofendothermicreactionsare

•photosynthesis(seeChapter13)•thermaldecomposition(seeChapters10and13).

EnergyleveldiagramsExothermicandendothermicreactionscanberepresentedbyenergyleveldiagrams.Thesediagramsshowtheenergyofthereactantsandproductsasareactionprogresses.

ExothermicreactionsThecompletecombustionoffuels,suchasmethane,isanexampleofanexothermicreaction(Figure6.1).Inanyexothermicreaction,theproductshavelessenergythanthereactantsbecauseenergyhasbeengivenouttothesurroundings.

EndothermicreactionsThethermaldecompositionofcalciumcarbonateisanexampleofanendothermicreaction(Figure6.2).Inanyendothermicreaction,theproductshavemoreenergythanthereactantsbecauseenergyhasbeentakeninfromthesurroundings.

FuelsFossilfuels(petroleum,naturalgasandcoal)allreleaseenergywhentheyburn.Uranium235(235U)isasourceofnuclearenergy.Ithastheadvantageofnot

causingglobalwarmingasnogreenhousegasesarereleasedwhenitisusedasafuel,buttherearesafetyissuesconcerningtheadverseeffectsofradiation.Hydrogenalsoreleasesenergywhenitburns.Anadvantageofusinghydrogen

asafuelasanalternativetofossilfuelsisthatwateristheonlyproduct;carbondioxide(agreenhousegas)isnotproduced.Hydrogencanalsobeused,alongwithoxygen,inafuelcelltorelease

electricalenergy.

FuelcellsInhydrogenfuelcells,theoverallreactionisthesameaswhenhydrogenisburnedinairoroxygen.

Fuelcellsoperateinacidicoralkalineconditions.AnalkalinehydrogenfuelcellisshowninFigure6.3.Oneadvantageofcarryingoutthisreactioninafuelcellisthatfuelcellsare

muchmoreefficientthaninternalcombustionengines,whichmeansthereismuchlessenergyloss.

BondenergiesMostchemicalreactionsinvolvebreakingofcovalentbondsinthereactantsandformationofnewcovalentbondsintheproducts.

•Breakingofbondsisanendothermicprocess(energyistakenin).•Formationofbondsisanexothermicprocess(energyisgivenout).

Becausetheamountofenergyputintobreakbondsisveryunlikelytobeequal

Becausetheamountofenergyputintobreakbondsisveryunlikelytobeequaltotheamountofenergygivenoutwhennewbondsareformed,mostreactionsareeitherendothermicorexothermic.Anexampleisthereactionbetweengaseoushydrogenandgaseouschlorine

toformgaseoushydrogenchloride.Theequationis

Bondenergyistheamountofenergyrequiredtobreak1moleofcovalentbondsingaseousmolecules.Itisnumericallyequaltotheamountofenergygivenoutwhennewbondsformingaseousmolecules.Whencovalentbondsinmoleculesarebroken,themoleculeschangeinto

atoms.Theatomsthenjointogethertoformnewmolecules.BondenergiesareshowninTable6.1.

Table6.1Bondenergies(kJ/mole)

Bond Bondenergy(kJ/mole)H–H 435Cl–Cl 242H–Cl 432

Theequationcanbewrittentoshowthestructureofthemolecules:

Energytakenintobreakbonds EnergygivenoutwhenbondsformH–H=435kJ 2×H–Cl=2×432=864Cl–Cl=242kJ Totalenergyputin:435+242=677kJ Totalenergygivenout=864kJ

Because864isalargernumberthan677,thismeansthatmoreenergyisgivenoutwhenthebondsformintheproductsthantheenergythathastobeputintobreakthebondsinthereactants.Therefore,thereactionisexothermicandthe

overallenergychangeiswhichmeansthatwhen1moleofgaseousH2moleculesreactwith1moleof

gaseousCl2moleculestoform2molesofgaseousHClmolecules,187kJofenergyaregivenouttothesurroundings.Thiscanalsobeshownonanenergyleveldiagram.

Exam-stylequestions1Propaneburnsinexcessoxygentoformcarbondioxideandwateraccordingtoequationbelow.(Notethatthetableofbondenergieswillberequiredtoanswerthequestion.)

BondBondenergykJ/molC–C 347C–H 435O=O 497C=O 803O–H 464

Calculatetheoverallenergychangeoccurringwhen1moleofC3H8(g)reactswith5molesofO2(g)toform3molesofCO2(g)and4molesofH2O(g)byusingthefollowingsteps:

aDrawthestructuresofallthemoleculesonbothsidesoftheequation.Showalltheatomsandallthebonds.

[2marks]

bWritedownthenumberofmolesofeachtypeofbondthathavetobebrokeninthereactants.(Remembertoconsiderthenumberofmolesofbothreactantsintheequation.)[2marks]

cCalculatethetotalamountofenergythathastobeputintobreakallthebondsinthereactantsin(b).

[1mark]dWritedownthenumberofmolesofeachtypeofbondthathavetobe

formedintheproducts.(Remembertoconsiderthenumberofmolesofbothproductsintheequation.)[2marks]

eCalculatethetotalamountofenergythatisgivenoutwhenallthebondsintheproductsin(d)areformed.

[1mark]fUseyouranswersto(c)and(e)tocalculatetheoverallenergychangein

thereaction.Youmustshowthecorrectunitsinyouranswerandsaywhetherthereactionisexothermicorendothermic.

[3marks]

7Chemicalreactions

KeyobjectivesBytheendofthissection,youshould

•beabletoidentifyphysicalandchemicalchangesandknowthedifferencesbetweenthem

•describeandexplaintheeffectsofchangingtheconcentrationofaqueoussolutions,particlesize/surfaceareaofsolids,pressureofgases,catalysts(includingenzymes)andtemperatureonratesofreaction

•describetheapplicationoftheabovefactorstothedangerofexplosivecombustionwithfinepowders(e.g.flourmills)andgases(e.g.methaneinmines)

•demonstrateknowledgeandunderstandingofapracticalmethodforinvestigatingtherateofareactioninvolvinggasevolution

•interpretdataobtainedfromexperimentsconcernedwithrateofreaction

•deviseandevaluateasuitablemethodforinvestigatingtheeffectofagivenvariableontherateofareaction

•describeandexplaintheeffectsofchangingtemperatureandconcentrationintermsofcollisionsbetweenparticlesusingtheconceptofactivationenergy

•describeandexplaintheroleoflightinphotochemicalreactionsandtheeffectoflightontherateofthesereactions

•describetheuseofsilversaltsinphotography•describephotosynthesisasareactionbetweencarbondioxideandwaterinthepresenceofchlorophyllandsunlighttoproduceglucoseandoxygen

•understandthatsomechemicalreactionscanbereversedbychangingthereactionconditions,e.g.theeffectofheatonhydratedcopper(II)sulfateandhydratedcobalt(II)chloride

•beabletodemonstrateknowledgeandunderstandingoftheconceptofequilibrium

•beabletopredicttheeffectofchangingtheconditions(concentration,temperatureandpressure)onotherreversiblereactions

•defineoxidationandreductionintermsofoxygenloss/gain

•defineredoxintermsofelectrontransfer•identifyredoxreactionsbychangesinoxidationstateandbythecolourchangesoccurringwhenusingacidifiedpotassiummanganate(VII)andpotassiumiodide

•defineoxidisingagentandreducingagent•identifyoxidisingagentsandreducingagentsfromsimpleequations.

KeytermsCatalyst Asubstancewhichincreasestherateofachemicalreaction.The

catalystischemicallyunchangedattheendofthereactionEnzyme Abiologicalcatalyst.EnzymesareproteinmoleculesOxidationGainofoxygenorlossofhydrogenReductionLossofoxygenorgainofhydrogenOxidisingagent

Asubstancethatoxidisesanothersubstanceinaredoxreaction.Anelectronacceptor

Reducingagent

Asubstancethatreducesanothersubstanceinaredoxreaction.Anelectrondonor

Oxidation LossofelectronsReductionGainofelectrons

Physicalchangesarechangesinwhichnewchemicalsubstancesarenotproduced.Changesinstate,thatismelting,boiling,evaporation,condensation,sublimationandfreezing(seeChapter1),andseparationofmixtures,e.g.filtration,distillation,fractionaldistillation,chromatographyandcrystallisation(seeChapter2),areexamplesofphysicalchanges.

Chemicalchangesorchemicalreactionsarechangesinwhichnewchemicalsubstancesareproduced.Decomposition,electrolysis,respiration,photosynthesis,redox,neutralisation,cracking,addition,substitution,polymerisationandcombustionareexamplesofchemicalchanges.Chemicalequationscanalwaysbeusedtorepresentchemicalchanges.Thetermphysicalpropertiesofasubstancereferstopropertiesofa

substancethatcanbemeasuredandthatinvolvephysicalchanges.Examplesaremeltingpoint,boilingpointanddensity.Thetermchemicalpropertiesofasubstancereferstopropertiesofa

substancewhichinvolvechemicalchanges.Examplesarethethingsthatsubstancesreactwithanddetailsofsuchreactions.Itcanbesaidthataphysicalpropertyofmetalsisthattheyallconduct

electricity,whereasachemicalpropertyofmetalsisthat(someofthem)reactwithacidstoproduceasaltandhydrogen.

Examiner’stip

Studentsshouldensurethattheyareawareofthedifferencesbetweenphysicalpropertiesandchemicalpropertiesandthattheyknowexamplesofbothfordifferenttypesofsubstance.

RateofreactionTherateofachemicalreactioncanbedeterminedbymeasuringeitherhowtheamountofoneofthereactantsdecreaseswithtimeorhowtheamountofoneoftheproductsincreaseswithtime.

Commonerrors•Studentscommonlyrefertocatalystsbeingabletoalterthespeedofachemicalreaction.Thisdoesnotspecificallymeanthatacatalystincreasestherate,becausealteringthespeedsuggeststhatcatalystsmaydecreasetherate,whichisnotthecase.

•Anothercommonerroristosuggestthatcatalystsdonottakepartinareaction.Thisisnotthecase,becauseincreasingtheratesuggeststhatcatalystshaveaconsiderableparttoplay.

•Theterm‘biocatalyst’isnotrecognisedashavingthesamemeaningasbiologicalcatalyst.

ExperimentalinvestigationsReactionsinwhichsolidsreactwithliquidstoproducegasesamongotherproductsarecommonlyinvestigatedinthischapter.

Experiment1Anexampleis

Thereactionbetweenaknownexcessofzincgranulesand50.0cm3of0.10moldm−3dilutesulfuricacidwasinvestigatedbyastudent.TheapparatusshowninFigure7.1wasused.

Thetemperatureiskeptat25°Cbyusingathermostaticallycontrolledwaterbath.Thevolumeofhydrogenproducedcanbemeasuredatregulartimeintervals.Agraphisthenplotted(Figure7.2).

Thegraphissteepestatthestartwhichmeansthatthisiswhentherateofreactionisfastest.Itthenbecomeslesssteepwhichmeansthatthereactionbecomesslower.Eventuallythegraphlevelsout,whichmeansthatnomorehydrogengasisreleasedandtherateofreactioniszero.

CollisiontheoryInanyreactionofthetype

particlesofAandBmustcollidewitheachotheriftheyaretoproduceproductC.Therearetwotypesofcollision:successfulandunsuccessful.Inanunsuccessfulcollision,particlesofAandBmerelybounceoffeach

otherandremainasAandB.However,inasuccessfulcollision,particlesofAandBcollideandchange

intoC.

Collisionsareonlysuccessfulifthereactingparticlescontainatleastaminimumamountofenergycalledtheactivationenergy.Therateofachemicalreactiondependsonthenumberofsuccessful

collisionsoccurringinanygiventime.Ifachangeismadetoincreasethenumberofcollisions,thisautomatically

leadstoanincreaseinthenumberofsuccessfulcollisionsbecauseacertainproportionofallcollisionsarealwayssuccessful.Inthereactioninvestigated

theionicequationis

Thisshowsthatcollisionsmusttakeplacebetweenzincatomsandhydrogenionsforthereactiontooccur.InExperiment1,therateofreactionisfastestatthestartbecausethisiswheretheconcentrationofhydrogenionsishighest,whichmeansthatthenumberofcollisionsbetweenhydrogenionsandzincatomsinanygiventimeismostfrequentatthestart.Therateofreactionthendecreasesbecauseastheconcentrationofhydrogen

ionsdecreases,collisionsoccurlessfrequently.Whenallthesulfuricacidisusedup,theconcentrationofhydrogenions

becomeszero,thereforetherearenomorecollisionsandtheratebecomeszero.

Examiner’stipInreactionsbetweengases,itispossibletomakesimilarstatementsabouttheconcentrationofagas.However,itismorelikelythatgasesarereferredtointermsofpressureratherthanconcentration.Thehigherthepressureexertedbyagas,theclosertogetherarethegaseousmoleculesandthemorefrequentwillbetheircollisionswitheachother.

Thestudentthencarriedoutexperimentsonthesamereaction,changingonlyonevariableineachcase.Thechangedvariableishighlighted.

Experiment2TheconcentrationofsulfuricacidisdoubledinExperiment2butthevolumeofsulfuricacidishalvedwhichmeansthatthenumberofmolesofsulfuricacidisthesame.Figure7.4showsthegraphthatwasplottedinExperiment2withthegraph

forExperiment1.

Becausethegraphissteeperatthestart,thisiswheretherateofreactionisfastest.Therateofreactionincreasesastheconcentrationincreases.Thisisbecause

theconcentrationofhydrogenionsishigher,whichmeansthatthenumberofcollisionsbetweenhydrogenionsandzincatomsisalsohigherinanygiventime.ThegraphsinExperiments1and2(Figure7.4)leveloffatthesamevolume

ofhydrogen,becausethevolumeofhydrogengivenoffisonlydependentonthenumberofmolesofsulfuricacid,whichisthesameinbothexperiments(asitisinallfiveexperiments).InExperiments3,4and5,thegraphwouldbethesameshapeasin

Experiment2,thatis,itwouldbesteeperatthestartandleveloffatthesamevolumeofhydrogen.

Experiment3Usingzincpowderwearedecreasingparticlesize/increasingsurfacearea.Collisionscanonlyoccuronthesurfaceofthezinc.Withasmallerparticlesize,therearemorezincatomsavailabletocollidewiththehydrogenions.Morecollisionsoccurringinanygiventimemeansthatmoresuccessfulcollisionsoccurinanygiventimeandthereforeafasterrateofreactionoccurs.

Decreasedparticlesizeandincreasedrateofreactioncanleadtohazards,forexample,inflourmillsandcoalmines.Flourdustandcoaldust(bothhavingextremelylargesurfaceareas)havebeenknowntoreactexplosivelywithoxygenintheairwhenasparkhasbeencreatedbymachinery.

Experiment4Athighertemperature,thereactingparticleshavemoreenergy.Thismeansthattheparticlesmovefasterandcollidemoreofteninanygiventime.

However,therewillbemoreparticleswithenergygreaterthanorequaltotheactivationenergy.Therefore,morecollisionswillbesuccessfulcollisionsinanygiventime.Thisisthemainreasonwhyratesofreactionarefasterathighertemperatures.

Experiment5Aqueouscopper(II)sulfateactsasacatalystinthisreaction.

Catalystslowertheactivationenergyofareaction.Thismeansthattherearemoreparticleswithenergygreaterthanorequaltotheactivationenergy.Therefore,morecollisionsaresuccessfulcollisionsinanygiventimeandtherateofreactionisfaster.

Activationenergyistheamountofenergythathastobesuppliedtoreactantstomakeareactionoccur.Theloweringofactivationenergyinacatalysedreactioncanbeshownintheenergyprofile(Figure7.5).Ascanbeseen,usingacatalysthasnoeffectontheoverallenergychangeofareactionbutlowerstheactivationenergy,thusincreasingtherateofreaction.

PhotochemicalreactionsPhotochemicalreactionsrequirelightinordertooccur.Furthermore,therateofsuchreactionsincreaseswhenlightintensityincreases.Examplesofphotochemicalreactionsare:

•photosynthesisinwhichcarbondioxideandwaterreactinthepresenceofsunlightandchlorophylltoproduceglucoseandoxygen(seeChapter13)

•substitutionreactionofalkaneswithchlorine(seeChapter14)whichrequiresUVlight

•photochemicaldecompositionofsilversaltsinphotography.Whenaqueoussilvernitrateisaddedtoanaqueoussolutioncontaininghalideions(Cl−,Br−orI−),aprecipitateofasilverhalideoccurs,e.g.

Whenthesilverbromideisexposedtolightitdecomposesintoitselementsandtheprecipitatedarkensasitgraduallydecomposesintosilverandbromine.Silverbromideisusedinphotographicfilmsbecauseitdarkenswhenexposedtolight.

Therateofthisreactionincreasesaslightintensityincreases.

ReversiblereactionsSomereactionscanbereversedbychangingtheconditions.Ifcrystalsofhydratedcopper(II)sulfateandhydratedcobalt(II)chlorideareheated,theychangecolourastheylosetheirwaterofcrystallisationandbecomeanhydroussalts.

However,inbothcases,thereactionscanbemadetoproceedinthereversedirectionsbyaddingwatertotheanhydroussaltsinwhichcasethecrystalsformagain,ascanbeseenbythereversecolourchange.

Thesereactionsarecalledreversiblereactions.Theycanbemadetoproceedinthereversedirectionbychangingtheconditions.

EquilibriumIfreversiblereactionsareallowedtoproceedinaclosedcontainer,theyreachastatethatisknownaschemicalequilibrium.Ifamixtureofhydrogenandiodinegasesisheatedinaclosedcontainer,the

hydrogenreactswiththeiodinetoproducehydrogeniodide:

Thisiscalledtheforwardreaction.Assoonashydrogeniodidemoleculesareformed,theystarttodecompose

intohydrogenandiodine:

Thisiscalledthereverseorbackwardreaction.Therefore,tworeactionsareoccurringinthesamecontaineratthesame

time.Furthermore,onereactionisthereverseoftheother.Thiscanbeshownbythefollowingexpression:

Theforwardreactionstartsoffquicklyandtheratedecreasesastheconcentrationsofhydrogenandiodinedecrease.Thebackwardreactionstartsoffslowlyandtherateincreasesasthe

concentrationofhydrogeniodideincreases.Eventually,bothratesbecomeequal.Thesystemistheninastateof

chemicalequilibrium.Whenthisoccurs,reactantsandproductsareallbeingusedupandproducedatthesamerateandthereforetheirconcentrationsbecomeconstant.

CharacteristicsofequilibriumsystemsEquilibriumcanonlyoccurinaclosedsystem(closedcontainer),inwhichnosubstancescanescapetotheoutsideorenterfromtheoutside.

•Therateoftheforwardreactionisequaltotherateofthereversereaction.•Theconcentrationsofallreactantsandproductsbecomeconstant.

Examiner’stipWhenaskedtodescribethecharacteristicsofanequilibriumsystem,studentsmaketwoverycommonerrors.Theyoftenstate:1Theforwardreactionisequaltothereversereaction.Withoutusingtheword‘rate’,thisisameaninglessstatement.

2aTheamountsofreactantsandproductsbecomeconstant.Inthiscase,theword‘amounts’mustbereplacedby‘concentrations’.

bTheconcentrationsofproductsandreactantsbecomeequal.Thisisincorrect.‘Theconcentrationsofproductsandreactantsbecomeconstant,

whichmeansthattheystopchanging’,isthecorrectstatement.

EffectsofchangingtheconditionsofanequilibriumsystemThefollowingequationrepresentstheequilibriumwhichoccursintheHaberprocess:

Theforwardreactionisexothermic.ThismeansthatN2(g)+3H2(g)→2NH3(g)isanexothermicreaction.Therefore,2NH3(g)→N2(g)+3H2(g)isanendothermicreaction.Theequationshowsthattherearefourgaseousmoleculesontheleft-hand

sideofthe signandtwogaseousmoleculesontheright-handsideofthesign.

Table7.1Theconditionsofthisequilibriumsystemandtheirresults

Change Effectonequilibriumposition Resultinthisexample

Increasetheconcentrationofreactants

Shiftstotheright Concentrationofproductsincreases

Increasetheconcentrationofproducts

Shiftstotheleft Concentrationofreactantsincreases

Increasethetotalpressure

Shiftsinthedirectionoffewermolecules Concentrationofproductsincreases

Increasetemperature

Shiftsinendothermicdirection Concentrationofreactantsincreases

Addacatalyst Increasestherateofbothforwardandreversereactions,butdoesnotshifttheequilibrium

Nochange

Decreasesinconcentrations,pressureandtemperaturehavetheoppositeeffecttoincreases.ThisissummarisedinFigure7.6.

Examiner’stipsWhenstudentsareaskedaboutchangingtemperatureandpressureonanequilibriumsystem,theyoftendemonstrateconfusionbetweenrateofreactionandequilibrium.Thisisshowninthesamplequestionbelow.

Sampleexam-stylequestionWhathappenstothepositionofequilibriumintheHaberprocessreaction,i.e.

N2(g)+3H2(g) 2NH3(g),whenthetemperatureincreases?Theforwardreactionisexothermic.

Student’sanswer

Therateofthereversereactionincreasesbecausetheforwardreactionisexothermic.

Examiner’scommentThestudentshouldnothaveusedtheword‘rate’.Ifthetemperatureofanequilibriumsystemisincreased,therateofbothforwardandreversereactionsareincreased,becauseincreaseintemperatureincreasestherateofallreactions(exceptenzymecatalysedreactions).

Thecorrectanswerisastatementthatsays

‘Theequilibriumshiftstotheleftintheendothermicdirection’

or

‘Theequilibriumshiftstotheleftbecausetheforwardreactionisexothermic’

Studentsareadvisedtotreatequilibriumandrateastwocompletelyseparatethingswhicharenotrelatedtoeachother.

RedoxOxidationwasoriginallydefinedasgainofoxygen.Inthefollowingreaction

magnesiumisoxidisedbecauseitgainsoxygen.Becauseoxygencausestheoxidation,oxygenistheoxidisingagent.Reductionistheoppositeofoxidationandwasoriginallydefinedaslossof

oxygen.Inthefollowingreaction

copper(II)oxideisreducedbecauseitlosesoxygen.Hydrogenisthereducingagent.Inthisreaction,hydrogengainsoxygen,thereforehydrogenisoxidised.Itfollowsthatoxidationandreductionalwaysoccuratthesametime.Areactioninwhichoxidationandreductionbothoccurisknownasaredox

reaction.

Anotherexampleofaredoxreactioninvolvingoxygenis

inwhichiron(II)oxideisoxidisedtoiron(III)oxideusingoxygenasanoxidisingagent.Thisgivesrisetoanothertypeofreactioninwhichanelementisoxidisedfromaloweroxidationstatetoahigheroxidationstateasinironbeingoxidisedfrom+2to+3.However,gainofoxygenand/orlossofhydrogenareverylimiteddefinitions

ofoxidationandreduction,becausemanyredoxreactionsdonotinvolveoxygenorhydrogen.Thefollowingreaction

inwhichiron(II)chlorideisoxidisedtoiron(III)chlorideusingchlorineastheoxidisingagentisanexample.Theionicequationforthisreactionis

Thiscanbebrokendownintotwoionichalf-equations:

•Fe2+isoxidisedtoFe3+bylossofelectrons.Cl2istheoxidisingagent.•Thereforeoxidationiselectronloss.•Oxidisingagentsareelectronacceptors.•Cl2isreducedto2Cl−bygainofelectrons.Fe2+isthereducingagent.•Thereforereductioniselectrongain.•Reducingagentsareelectrondonors.

Examiner’stipInionichalf-equations

•electronsappearontherightforoxidation•electronsappearontheleftforreduction.

Inanyredoxreaction,electronsaretransferredfromthereducingagenttotheoxidisingagent.Thereducingagentisoxidisedandtheoxidisingagentisreduced.

TestingforoxidisingandreducingagentsAqueouspotassiummanganate(VII)isanoxidisingagentwhichcanbeusedtotestforthepresenceofreducingagents.Whenareducingagentisadded,theaqueouspotassiummanganate(VII)changescolourfrompurpletocolourless.

Aqueouspotassiumiodideisareducingagentwhichcanbeusedtotestforthepresenceofoxidisingagents.Whenanoxidisingagentisadded,theaqueouspotassiumiodidechangescolourfromcolourlesstobrown.

Exam-stylequestions1Statewhetherthefollowingchangesarephysicalchangesorchemicalchanges.

aDissolvingsodiumchlorideinwater.[1mark]

bTheelectrolysisofaqueoussodiumchloride.[1mark]

cExposingaprecipitateofsilverchloridetosunlight.[1mark]

dFractionaldistillationofliquidair.[1mark]

eSeparatingthedyesininkbychromatography.[1mark]

[Total:5marks]

2Whenanexcessofmarblechips(calciumcarbonate)isaddedto50cm3of0.10moldm−3hydrochloricacidat25°C,thefollowingreactionoccurs:

Thevolumeofcarbondioxidegaswascollectedinagassyringeandmeasuredatregulartimeintervals.Thisisexperiment1.

Theexperimentwasrepeatedasshowninthetablebelow.Graphswereplottedineachcase(Figure7.7).

Thecalciumcarbonateisinexcessinallfiveexperiments.Copyandcompletethetablebelowtoshowwhichgraphcorrespondstoeachdifferentexperiment.Eachlettermaybeusedonce,morethanonceornotat

all.

[Total:4marks]3Stateinwhichdirections(ifany)thefollowingequilibriummixtureswouldshiftifthepressureonthesystemwasincreased.Explainyouranswerineachcase.

a

[1mark]b

[1mark]

c

[1mark][Total:3marks]

4Stateinwhichdirections(ifany)thefollowingequilibriummixtureswouldshiftifthetemperatureonthesystemwasdecreased.Explainyouranswerineachcase.

a exothermicintheforwarddirection

[1mark]

b endothermicintheforwarddirection

[1mark][Total:2marks]

5Theequationforthereactionbetweenmagnesiumandcopper(II)sulfatesolutionisshown.

aWriteanionicequationforthereaction.[1mark]

bWritetwoionichalf-equationsrepresentingoxidationandreductioninthereaction.

[2marks]cStatetheformulaofthespecieswhichactsasanoxidisingagentinthe

reaction.Explainyouranswer.[2marks]

dStatetheformulaofthespecieswhichactsasareducingagentinthereaction.Explainyouranswer.

[2marks][Total:7marks]

8Acids,basesandsalts

KeyobjectivesBytheendofthissection,youshouldbeableto

•describethecharacteristicpropertiesofacidsintheirreactionswithmetals,basesandcarbonates,andtheireffectonlitmusandmethylorange•describethecharacteristicpropertiesofbasesintheirreactionswithacidsandwithammoniumsaltsandtheireffectonlitmusandmethylorange•describeneutrality,relativeacidityandalkalinityintermsofpHmeasuredusingUniversalindicatorpaper•describeandexplaintheimportanceofcontrollingacidityinsoil•classifyoxidesaseitheracidicorbasicrelatedtometallicandnon-metalliccharacter•suggestmethodsofpreparationofsalts

•defineacidsandbasesintermsofprotontransfer•describethemeaningofweakandstrongacidsandbases•classifyotheroxidesasneutraloramphoteric.

KeytermsAcid AprotondonorBase AprotonacceptorStrongacid

Existscompletelyasionsinaqueoussolution

Weakacid

Onlypartiallyionisedinaqueoussolution

Strongbase

Existscompletelyasionsinaqueoussolution

Salt Anionicsubstanceformedwhenthepositivehydrogenionsinanacidarereplacedbypositivemetallicionsorammoniumions

AcidsAcidsareproton(H+)donors.Thecommonlaboratorystrongacidsaredilutehydrochloric,nitricand

sulfuricacids.Theirformulaearegivenbelow.

•Hydrochloricacid:HCl•Nitricacid:HNO3•Sulfuricacid:H2SO4

Inaqueoussolutionstrongacidsdonotcontainanyparticleswiththeseformulaebecausetheyexistcompletelyasions,i.e.

Weakacids,suchasethanoicacid,CH3COOH(aq),existmainlyascovalentmoleculeswiththeformulaCH3COOH(aq),asmallnumberofwhichdissociateintoions,i.e.

Reactionsofacids

WithmetalsAcidsreactwithmetalsabovehydrogeninthereactivityseries(althoughitwouldbedangeroustouseaGroupImetaloranythingbelowcalciuminGroupIIinareactionwithacids).Thegeneralequationis

Themetaldissolves,bubblesareseenandasolutionofthesaltformswhosecolourdependsonthemetalused.Anexampleis

WithcarbonatesAcidsreactwithbothsolubleandinsolublecarbonates.Thegeneralequationis

Solidcarbonatesdissolve,bubblesareseenandanaqueoussolution(whosecolourdependsonthecarbonateused)ofthesaltforms.Anexampleis

Thistypeofreactionoccurswithcarbonateseitherassolidsorasaqueoussolutions.

WithbasesAcidsreactwithallbasestoformsaltsandwater(inthecaseofammonia,anammoniumsaltistheonlyproduct).Thegeneralequationis

Withinsolublebasesthesoliddissolvesandasolutionforms.Nobubblesareseenbecausenogasisproduced.Anexampleis

Withalkalistherearenoobservationsasacolourlesssolutionisproducedfromtwocolourlesssolutions.Anexampleis

WithammoniaThegeneralequationis

Anexampleis

StrongandweakacidsStrongandweakacidscanbedistinguishedexperimentallybyanyofthefollowingexperimentalmethods.

Strongacid WeakacidAddUniversalindicatorpaper Red

orpH0–2

Orangeyellow

orpHlessthan7andmorethan2

Addmagnesiumribbon Bubblesquickly

Bubblesslowly

Addaninsolublecarbonate,e.g.calciumcarbonate

Bubblesquickly

Bubblesslowly

Setupacircuitwithabulb Bulblightsbrightly

Bulblightsdimly

Examiner’stip•Manystudentshavetheimpressionthatifanacidisweak,moreoftheweakacidisrequiredtoneutralisethesameamountofalkalicomparedtoastrongacid.Theamountofanyacidthatisrequiredtoneutraliseagivenamountofalkalionlydependsonthenumberofmolesoftheacidandnotwhethertheacidisstrongorweak.

•Somequestionsbeginwith,‘Howwouldyoudistinguish…?’Forexample,‘Howwouldyoudistinguishbetweenastrongacidandaweakacid?’Theintentionisthatthestudentgivesbriefexperimentaldetailswithresults,forexample,addmagnesiumribbontobothandbubblesoccurmuchfasterwiththestrongacidthantheweakacid.Insteadstudentsoftenanswerwiththeory,forexample,strongacidsionisecompletelyandweakacidsionisepartially.Althoughthesestatementsarecorrecttheydonotaddressthequestion,‘Howwouldyoudistinguish…?’Itisalsonecessarytostatewhatwouldhappenwithbothsubstancesortogiveacomparison,ratherthansaythestrongacidproducesbubblesrapidlywithoutreferencetotheweakacid.

BasesBasesaremetallicoxidesorhydroxides(orammonia)whichneutraliseacidstoformasaltandwater.

•Basesthataresolubleinwaterarecalledalkalis.•Basesthatdonotdissolveinwaterareknownasinsolublebases.

Alkalisarehydroxidesoroxidesofmetals(orammonia)thatproduceOH−(aq)whendissolvedinwater.Thetwolaboratorystrongalkalisareaqueoussodiumhydroxideand

potassiumhydroxide.Theybothexistcompletelyasionsinaqueoussolution.

Ammoniasolutionisaweakbase.AnaqueoussolutionofammoniaexistsmainlyasNH3molecules,asmallnumberofwhichreactwithwatermoleculestoproduceions.

NH3(aq)acceptsH+fromH2O(l)formingNH4+(aq)whichshowsthatbasesareprotonacceptorsbydefinition.

ReactionsofbasesAsdescribedabove,basesneutraliseacids.

Application:PlantsneedsoiltobeataspecificpHtogrowwell.Soilaciditycanbeneutralisedbytheadditionofasuitablebase,suchascalciumhydroxide,Ca(OH)2(s),alsoknownasslakedlime.

WithammoniumsaltsInsolublebasesandalkalisreactwhenheatedwithammoniumsalts.Ammoniagasisgivenoff.Thegeneralequationis

Anexampleis

Examiner’stipManystudentsdonotapplytherulesforwritingformulaeofioniccompoundswhenwritingequationsofthistype.Inthisexample,theformulaofcalciumchlorideisoftenwrittenasCaCl,withoutconsiderationofthechargesontheionspresent(seeChapter3fordetails).

IndicatorsMethylorangeandlitmuscanbeusedtoindicatewhethersubstancesareacidsoralkalis,butgivenoinformationaboutacidstrength.

Methylorange LitmusColourinacidicsolution Red RedColourinneutralsolution Orange PurpleColourinalkalinesolution Yellow Blue

ThepHscale(Figure8.1)usesnumberstodistinguishbetweenacidsandalkalisofdifferentstrengths.

AqueoussolutionsofacidshavepHlessthan7.AqueoussolutionsofalkalishavepHmorethan7.NeutralsolutionshavepHof7.ThelowerthepHnumbers,thestrongertheacid.ThehigherthepHnumbers,

thestrongerthealkali.StrongacidsareregardedashavingapHof0–2.Strongalkalisareregarded

ashavingapHof12–14.UniversalindicatorhasdifferentcolourstoshowapproximatepHnumbers

asshown.

ApproximatepH ColourofUniversalindicatorLessthan3 Red3–6 Orange–yellow7 Green8–11 BlueMorethan11 Purple

OxidesOxidescanbeputintofourcategories.1Acidicoxidesarenon-metallicoxidesthatneutralisealkalisandformsalts.Examplesarecarbondioxide,CO2,nitrogendioxide,NO2,andsulfurdioxide,SO2.Theseoxidesalldissolveinwaterandreactwiththewatertoformacids.

2Basicoxidesaremetallicoxidesthatneutraliseacidsandformsalts.Examplesaremagnesiumoxide,MgO,calciumoxide,CaO,andcopper(II)oxide,CuO.Somebasicoxidesdissolveinwatertoformalkalinehydroxides,whereasothersareinsolubleinwater.

3Somenon-metallicoxidesareneutraloxideswhichmeansthattheydonotreactwitheitheracidsoralkalis.Anexampleiscarbonmonoxide,CO.

4Somemetallicoxidesareamphotericoxideswhichmeansthattheyreactwithbothacidsandalkalistoformsalts.Examplesarezincoxide,ZnO,andaluminiumoxide,Al2O3.

SaltsSaltsareionicsubstancesformedwhenthepositivehydrogenionsinanacidarereplacedbypositivemetallicionsorammoniumions.Saltscanbemadebydifferentexperimentalmethods,dependingontheir

solubilityinwater.

Solubilityofsalts

PreparationofsaltsTherearethreegeneralmethodsofpreparationofsolidsalts.Inallcases,detailsofcrystallisation,washinganddryingcanbefoundin

Chapter2.Method1:Addinganexcessofaninsolublebaseorinsolublecarbonateor

metaltoadiluteacid.Method2:Titrationusinganacidandanalkaliorasolublecarbonate.Methods1and2canonlybeusedforsaltsthataresolubleinwater.Method3:Mixingtwosolutionstoobtainasaltthatisinsolubleinwaterby

precipitation.

Examiner’stipUsingacidstopreparesalts:

•hydrochloricacid,HCl,isusedtopreparechlorides•nitricacid,HNO3,isusedtopreparenitrates•sulfuricacid,H2SO4,isusedtopreparesulfates(orhydrogensulfates)•thepositiveioninthesaltcomesfromtheinsolublebaseorinsolublecarbonateormetaloralkali.

Examples

Method1Copper(II)sulfatecrystalscanbemadebythismethod.

•Addsolidcopper(II)oxideorcopper(II)hydroxideorcopper(II)carbonatetodilutesulfuricacidinabeaker.

•Stirand/orheatthemixture.•Addthesoliduntilitwillnolongerdissolvewhichmeansalltheacidhasreactedandthesolidisinexcess(ifcopper(II)carbonateisused,therewillbenofurtherbubblingwhenalltheacidhasreacted).

•Filterofftheexcesssolid.•Usingcopper(II)oxidetheequationis

•Makepurecrystalsofcopper(II)sulfatebycrystallisation,washinganddrying(seeChapter2).

Method2Sodiumsulfatecrystalscanbemadebythismethod.

•Carryoutsufficienttitrationstofindouttheexactvolumeofdilutesulfuricacidinaburettethatisrequiredtoneutraliseagivenpipettevolumeofaqueoussodiumhydroxide.Usemethylorangeasasuitableindicator.

•Repeattheprocesswithoutusingtheindicator,butusingthesamevolumeofacidandalkaliasusedinthetitration.

•Makepurecrystalsofsodiumsulfatebycrystallisation,washinganddrying(seeChapter2).

Iftwicethevolumeofthesamedilutesulfuricacidisused,orhalfthevolumeofthesameaqueoussodiumhydroxide,sodiumhydrogensulfate(anacidsalt)isthesaltproduced,accordingtotheequation

Crystalsofsodiumhydrogensulfatecanbemadefromthissolutioninthesameway.

Method3Leadsulfatecanbemadebythismethod.

•Becauseleadnitrateistheonlysolubleleadsalt,leadnitratesolutionmustbeusedandmixedwithanysolublesulfate,suchasaqueoussodiumsulfate(dilutesulfuricacidcouldalsobeusedbecauseitcontainsaqueoussulfateions).

•Theprecipitateofleadsulfatemustberemovedbyfiltrationandthenwashedwithdistilledwateranddriedinalowovenoronawarmwindowsill.

Theequationis

Anionicequationforanyprecipitationreactionshowsthetwoaqueousionsontheleftandthesolidprecipitateontherightinallcases.Inthiscase

Exam-stylequestions1Therearethreegeneralmethodsofpreparationofsolidsalts.Method1:Addinganexcessofaninsolublebaseorinsolublecarbonateormetaltoadiluteacid.

Method2:Titrationusinganacidandanalkaliorasolublecarbonate.Methods1and2canonlybeusedforsaltsthataresolubleinwater.Method3:Mixingtwosolutionstoobtainasaltthatisinsolubleinwaterbyprecipitation.

Foreachofthefollowingsaltpreparations,choosemethod1,2or3,nameanyadditionalreagentwhichisrequiredandwritetheequation.

aCobalt(II)chloridestartingwiththeinsolublecompoundcobalt(II)carbonate.

[3marks]bTheinsolublesaltleadiodide,fromaqueousleadnitrate.

[3marks]cPotassiumnitratefromaqueouspotassiumhydroxide.

[3marks][Total:9marks]

2Givefullexperimentaldetailsofhowyouwouldmakepuredrycrystalsofmagnesiumsulfatestartingwithmagnesiumcarbonate.Youshouldincludeanequationinyouranswer.

[Total:10marks]3Youareprovidedwithamixtureofscandiumoxideandcopper(II)oxidewhicharebothsolids.Scandiumoxideisanamphotericoxideandcopper(II)oxideisabasicoxide.Describehowyoucouldobtainasampleofpure

copper(II)oxidefromthemixture.Bothsolidsareinsolubleinwater.[Total:6marks]

9ThePeriodicTable

KeyobjectivesBytheendofthissection,youshouldbeabletodescribethePeriodicTableasameansofclassifyingelementsanditsusetopredictpropertiesofelements•describethechangefrommetallictonon-metalliccharacteracrossaperiod•describesomepropertiesoftheGroupIelementslithium,sodiumandpotassiumandpredictthepropertiesofotherGroupIelements•describesomepropertiesoftheGroupVIIelementschlorine,bromineandiodineandpredictthepropertiesofotherGroupVIIelements•describethenoblegasesinGroupVIIIor0asbeingunreactive,monatomicgasesandexplainthisintermsofelectronicstructure•beabletodescribesomepropertiesoftransitionelements

•describeandexplaintherelationshipbetweengroupnumber,numberofoutershellelectronsandmetallic/non-metalliccharacter•identifytrendsingroupsgiveninformationabouttheelementsconcerned•knowthattransitionelementshavevariableoxidationstates.

KeytermsPeriodicTable

Containsalltheelementsarrangedinorderofprotonnumber

Groups TheverticalcolumnsinthePeriodicTablePeriods ThehorizontalrowsinthePeriodicTableAlkalimetals

GroupI.Metalswhichreactwithwatertoproducealkalinesolutions(suchaslithium,sodiumandpotassium)

Halogens GroupVII.Non-metallic,diatomicmoleculesNoblegases

Group0.Unreactive,monatomic,colourlessgases

Transitionelements

MetalsfoundintheelongatedsectionofthePeriodicTablebetweenGroupsIIandIII(suchascopper,ironandnickel)

PeriodicTableThePeriodicTablecontainstheelementsarrangedinorderofprotonnumber(atomicnumber).

•Theverticalcolumnsofelementsarecalledgroups.•Thehorizontalrowsofelementsarecalledperiods.

AcrossPeriods2and3,thereisagradualchangefrommetalsontheleft-handsidetonon-metalsontheright-handside.

Commonerror•Studentsoftenthinkthattheelementsarearrangedinorderofmassnumberorrelativeatomicmass.Mostoftherelativeatomicmassesoftheelementsdoincreaseasprotonnumbersincrease,butinsomeplacestherelativeatomicmassdecreases,e.g.argontopotassium.

Atomsofelementsinthesamegrouphavethesamenumberofelectronsintheoutershell.Thenumberofelectronsintheoutershelldeterminesthechemicalpropertiesoftheelement.Thenumberofshellspresentinanatomofanelementisthesameasthe

periodnumberinwhichtheelementisfoundinthePeriodicTable.Potassiumhasprotonnumber19andthereforeitselectronconfigurationis

2,8,8,1.ThereisoneelectronintheoutershellwhichmeanspotassiumisinGroupIofthePeriodicTable.Potassiumhaselectronsinfourshells,whichmeansitisinPeriod4.

GrouppropertiesGroupITheGroupIelementsareknownasthealkalimetalsbecausetheyreactwithwatertoproducealkalinesolutions.TheGroupIelementsareveryreactivemetals.TheGroupIelements,inorderofincreasingprotonnumber,arelithium,

sodium,potassium,rubidium,caesiumandfrancium.Onlylithium,sodiumandpotassiumarefoundinschoollaboratories,becauserubidium,caesiumandfranciumaredangerouslyreactiveandfranciumisalsoradioactive.

Examiner’stipExaminationquestionsoftenaskforobservations,orask,‘Whatwouldyousee…?’inaparticularchemicalorphysicalchange.WhenGroupImetalsreactwithwater,suitableobservationsarethemetaldisappears•themetalmelts•bubbles/fizzing/effervescence(thesealleffectivelymeanthesamething)•themetalfloatsandmovesaroundonthesurface•potassiumandthoseGroupImetalsbelowpotassiumburstintoflames.

However,thefollowingarenotobservations:namesoftheproducts•agasisgivenoff(itisnotpossibletoseeacolourlessgas)•analkalinesolutionforms(itisnotpossibletoseethatasolutionisalkalinebyobservationalone)•colourchangeofanindicator(unlessanindicatorismentionedinthequestion).

TheGroupIelements

•arestoredunderoilbecausetheyreactrapidlywithoxygenintheair•aregoodconductorsofheatandelectricity•canbecutwithaknifebecausetheyaresoft•areshinywhencut,buttarnishrapidlyduetoreactionwithoxygenintheair•havelowdensitiesandlowmeltingpointsandboilingpointscomparedtotransitionmetals.

ThemeltingpointsandboilingpointsofGroupIelementsdecreasedownthegroup.Densitieschangeinanirregularmanner.

ReactionwithwaterAllGroupIelementsreactvigorouslywithwateratroomtemperature.Thereactionsareusuallycarriedoutinaglasstrough.Observationsarethemetalmovesaroundandfloatsonthesurfaceofthewater•thereactionproducesheatwhichcausesthemetaltomeltasitreacts•bubblesofhydrogengasaregivenoff•themetalrapidlydisappears,formingacolourlesssolutionofthealkalinemetalhydroxide.

Theequationforthereactionwithsodiumis

TheequationswithalltheotherGroupImetalswouldbeexactlythesame(includingbalancingnumbers)ifthesymbolsfortheothermetalsreplacedthatofNaintheaboveequation.ReactivityoftheGroupImetalsincreasesdownthegroup.Sodiummoves

aroundthesurfacefasterthanlithiumandalsodisappearsmorerapidly.Potassiumburstsintoalilacflame.Ifrubidiumandcaesiumareaddedtowateranexplosivereactionoccurs,whichiswhytheyarenotkeptinschoollaboratories.

GroupVIITheGroupVIIelementsareknownasthehalogens.TheGroupVIIelementsinorderofincreasingprotonnumberarefluorine,

chlorine,bromine,iodineandastatine.Onlychlorine,bromineandiodinearefoundinschoollaboratories.Fluorineistooreactivetobeusedinschoolsandastatineisradioactive.Theelementsareallnon-metallicandexistasdiatomicmolecules(molecules

containingtwoatoms).TheappearanceofthosefoundinschoolsisshowninTable9.1.

Table9.1Physicalappearanceofchlorine,bromineandiodine

Element AppearanceatroomtemperatureChlorine PalegreengasBromine Red-brownliquidIodine Grey-blacksolid

Thecoloursbecomedarkerasthegroupisdescended.Thechangeinphysicalstatefromgastoliquidtosoliddownthegroupindicatesanincreaseinmeltingpointandboilingpointanddensitydownthegroup(duetoanincreaseinthestrengthofintermolecularforces).

HalogendisplacementreactionsIfhalogens(orsolutionsofthehalogensinwater)areaddedtocolourlessaqueoussolutionsofpotassiumhalides(chlorides,bromidesandiodides),displacementreactionsoccurintheexampleshighlightedinTable9.2.

Anexampleofadisplacementreactioniswhenchlorinedisplacesbrominefromanaqueoussolutionofpotassiumbromide.Theequationis

AscanbeseenfromTable9.2,chlorinedisplacesbromineandiodine•brominedisplacesiodine,butdoesnotdisplacechlorine•iodinedoesnotdisplacechlorineorbromine.

Halogenshigherupthegroupcandisplacethoselowerdown.Alternatively,wecansaythatreactivityincreasesupGroupVII.ThisisoppositetothetrendinreactivityshowninGroupI.Wecanusethisinformationtomakepredictionsabouttheotherhalogensand

halides(seetheExam-stylequestionsattheendofthechapter).

Group0TheGroup0elementsareknownasthenoblegases.TheGroup0elements,inorderofincreasingprotonnumber,arehelium,neon,argon,krypton,xenonandradon.

TheGroup0elements

•areallcolourlessgases•areallmonatomic;thismeanstheyexistasindividualatomsbecausetheiratomsallhaveafulloutershellofelectronsanddonotformcovalentbondswithotherGroup0atomstoformdiatomicmolecules•areveryunreactivebecausetheiratomsallhaveafulloutershellofelectrons,thereforetheydonotneedtoshare,loseorgainelectronstoachieveafulloutershellofelectrons.

UsesofGroup0elements•Heliumisusedinfillingballoonsbecauseofitslowdensity.Itsunreactivenaturemeansthathazardsassociatedwithhydrogenareabsentifheliumisused.

•Argonisusedinlightbulbstopreventthetungstenfilamentfromburning.Thisisbecauseargondoesnotsupportburningduetoitsunreactivity.

TransitionelementsTransitionelementsareallmetals(alsoknownastransitionmetals).TheyarefoundintheelongatedsectionofthePeriodicTablebetweenGroupsIIandIII.Commonexamplesarecopper,ironandnickel.

PhysicalpropertiesTransitionelementsareallmetalsandshowtheusualphysicalpropertiesofmetals(seeChapter10).Inaddition,transitionmetalsareharderandstrongerthantheelementsinGroupI•havehigherdensitiesthantheelementsinGroupI•havehighermeltingpointsthantheelementsinGroupI.

Chemicalproperties•Transitionelementsformcolouredcompounds,e.g.copper(II)sulfatecrystalsareblueandpotassiummanganate(VII)ispurple.

•Theelementsandtheircompoundsshowcatalyticactivity,e.g.ironintheHaberprocessandvanadium(V)oxideintheContactprocess.

•Transitionelementshavevariableoxidationstates,e.g.ironcanformFe2+andFe3+ions.

Exam-stylequestions1ThediagrambelowshowspartofthePeriodicTable.UsethelettersAtoHinclusivetoanswerthequestionsthatfollow.Eachlettermaybeusedonce,morethanonce,ornotatall.Givetheletterthatrepresents

atheGroupIelementthatismostreactive[1mark]

btheGroupVIIelementthatismostreactive[1mark]

catransitionelement[1mark]

danelementinPeriod3[1mark]

eanelementwhoseatomshavefourelectronsintheiroutershell.[1mark]

[Total:5marks]

2Lithiumisaddedtocoldwaterinaglasstrough.aGivethreeobservationsyouwouldexpecttomake.

[3marks]bWriteachemicalequationforthereactionthatoccurs.Includestate

symbols.[3marks]

cIfmethylorangeisaddedtotheliquidinthetroughafterthereaction,whatcolourwoulditturnto?

[1mark][Total:7marks]

3Usethetableofhalogendisplacementreactionstoanswerthefollowingquestions.Writechemicalequations(withstatesymbols)andionicequationsforthereactionsthatoccurbetween

achlorineandaqueouspotassiumiodide[4marks]

bbromineandaqueouspotassiumiodide.[4marks]

[Total:8marks]4UsethePeriodicTabletopredictreactionsthatwouldoccurbetween

afluorineandaqueouspotassiumchloridebastatineandaqueouspotassiumfluoridecbromineandaqueouspotassiumastatidediodineandaqueouspotassiumfluoride.

Ifyoupredictthatareactionwouldoccur,writeachemicalequationforthereaction.Ifyoupredictthatareactionwouldnotoccurwritenoreaction.

[Total:6marks]5Copperandironhavevariableoxidationstates.Statetheformulaeof

acopper(I)oxide[1mark]

bcopper(II)nitrate[1mark]

ciron(II)chloride[1mark]

diron(III)sulfate.[1mark]

[Total:4marks]

10Metals

KeyobjectivesBytheendofthissection,youshouldbeableto

•describethephysicalpropertiesofmetals•describethechemicalpropertiesofmetals•identifyrepresentationsofalloysfromdiagramsofstructureandexplainintermsofpropertieswhyalloysareusedinsteadofpuremetals•placethefollowinginorderofreactivity:potassium,sodium,calcium,magnesium,zinc,iron,(hydrogen)andcopperbyreferencetothereactionswith•waterorsteam

•dilutehydrochloricacidand•thereductionoftheiroxideswithcarbon•deduceanorderofreactivityfromagivensetofexperimentalresults•describetheeaseofobtainingmetalsfromtheiroresbyreferencetothepositionofelementsinthereactivityseries•describeandstatetheessentialreactionsintheextractionofironfromhematite•describetheconversionofironintosteelusingbasicoxidesandoxygen•knowthataluminiumisextractedfromitsorebauxitebyelectrolysis•statetheadvantagesanddisadvantagesofrecyclingiron/steelandaluminium•namesomeusesofaluminium•namesomeusesofcopper

•namesomeusesofmildsteelandstainlesssteel

•describethereactivityseriesasrelatedtothetendencyofametaltoformitspositiveion,illustratedbyreactionwith

•theaqueousions•theoxidesoftheotherlistedmetals

•accountfortheapparentunreactivityofaluminiumintermsoftheoxidelayerwhichadherestothemetal•describeandexplaintheactionofheatonthehydroxides,carbonatesandnitratesofthelistedmetals•describeinoutlinetheextractionofzincfromzincblende•describeinoutlinetheextractionofaluminiumfrombauxite,includingtheroleofcryoliteandthereactionsattheelectrodes•explaintheusesofzincingalvanisingandfor

makingbrass.

PropertiesofmetalsPhysicalpropertiesThephysicalpropertiesofmetalsareshowninTable10.1(seealsoChapter9).

Table10.1Physicalpropertiesofmetals

Property MetallicpropertyPhysicalstateatroomtemperature Solid(exceptmercury)Malleability GoodDuctility GoodAppearance Shiny(lustrous)Meltingpointandboilingpoint UsuallyhighDensity UsuallyhighConductivity(thermalandelectrical) Good

Metalsaremalleable(canbehammeredintodifferentshapes)andductile(canbedrawnintowires).Althoughmetallicbondsarestrong,theyarenotrigid,

whichmeansthattherowsofionsinmetalscanslideovereachotherwhenaforceisapplied(Figure10.1).Thisisbecausetheionsinametallicelementareallthesamesize.Whenametallicobjectisrequiredtobeparticularlystrong,analloyisoften

usedinsteadofametallicelement.Inalloyssuchasbrass,bronzeandsteel,themetallicelementismixedwithsmallamountsofanotherelementorelements.Theionsoratomsoftheotherelementsareadifferentsizetothoseofthemainelement.Thispreventstherowsofmetallicionsfromslidingovereachother.Therefore,analloyretainsitsshapemuchbetterthanapuremetalwhenaforceisapplied.

ReactivityseriesMetalscanbeplacedinorderoftheirreactivitywithotherelements.Thisisknownasthereactivityseries(seeright).

Thepositionsofcarbonandhydrogenareinsertedforreferencetodisplacementreactions.Potassium,sodiumandcalciumreactwithcoldwatertoproduceanaqueoussolutionofanalkalinehydroxideandhydrogengas(seeChapter9),forexample

Magnesium,zincandironreactextremelyslowlywithcoldwater.Theydo

reactmorerapidlywhenheatedwithsteam,forexample

Metalsabovehydrogeninthereactivityseriesreactwithdiluteacidstoproduceasaltandhydrogen(thereactionofpotassiumandsodiumwithacidswouldbetoodangeroustocarryoutinschoollaboratories).

Metalsbelowhydrogeninthereactivityseries,e.g.copper,donotreactwithcoldwater,steamordiluteacids.Oxidesofmetalsbelowcarboninthereactivityseriesarereducedtothemetal

whenheatedwithcarbon,forexample

DisplacementreactionsMetalswilldisplaceothermetalsfromaqueoussolutionsoftheirions,e.g.magnesiumwilldisplacecopperfromanaqueoussolutioncontainingitsionssuchascopper(II)sulfatesolution.

Theionicequationis

Thereactionoccursbecausemagnesiumisamorereactivemetalthancopper.Thismeansthatmagnesiumhasagreatertendencytoformpositiveionsthancopper.Ifcopperwasaddedtoasolutioncontainingmagnesiumions,suchas

aqueousmagnesiumsulfate,noreactionoccurs.Metalshigherupinthereactivityserieswilldisplacethoselowerdown.Asimilarreactionoccursifametaloxideisheatedwithamorereactive

metal.Forexample,ifzincpowderisheatedwithcopper(II)oxide,thefollowingreactionoccurs:

However,ifzincoxideisheatedwithcopper,noreactionoccurs.

However,ifzincoxideisheatedwithcopper,noreactionoccurs.

DeducingorderofreactivityofmetalsToputmetalsinorderofreactivity,reactionscanbeattemptedasin,forexample,addingametaltoanaqueoussolutioncontainingionsofanothermetalorheatingametalwiththeoxideofanothermetal.Anothermethodinvolvestheuseofelectrochemicalcells(seeChapter5)

containingtwodissimilarmetalsinanelectrolyte(Figure10.3).

Inelectrochemicalcellsofthistype,themorereactivemetalisthenegativeterminal,becausethemorereactivemetalistheonewiththegreatertendencytoreleaseelectronsandformpositiveions.Thereadingonthevoltmeter(knownasthecellEMF(electromotiveforce))representsthedifferenceinreactivitybetweenthetwometals.

Examiner’stipIfacellwassetupwithAandBinanelectrolyte

•Awouldbethenegativeelectrode•Bwouldbethepositiveelectrode•thecellEMFwouldbe1.10V+0.46V=1.56V.

Cell1tellsusthatmetalA(−)ishigherinthereactivityseriesthanmetalC(+)and1.10Visameasurementofthedifferenceinreactivity.Cell2tellsusthatmetalC(−)ishigherinthereactivityseriesthanmetalB

Cell2tellsusthatmetalC(−)ishigherinthereactivityseriesthanmetalB(+)and0.46Visameasurementofthedifferenceinreactivity.Therefore,theorderofthethreemetalsinthereactivityseriesis

UnexpectedbehaviourofaluminiumAluminiumappearsbetweenmagnesiumandzincinthereactivityseries.However,aluminiumoftenappearstobemuchlessreactivethanitspositioninthereactivityseriessuggests.Forexample,ifaluminiumisplacedinanaqueoussolutionofcopper(II)

sulfate,thereishardlyanyreactionuntilthelayerofaluminiumoxideisremoved.Thisisbecausealuminiumissoreactivethatitreactswiththeoxygenintheairformingalayerofaluminiumoxidewhichadherestothealuminiumunderneathandprotectsthemetal.Suchalayercanbedeliberatelyplacedontothesurfaceofaluminiummetalbyaprocesscalledanodising.Thismeansthataluminiumcanbeusedforthingswhichwouldnotnormallybeassociatedwithreactivemetals,suchasaeroplanebodies,cookingfoilandpotsandpans.

ThermaldecompositionofmetalliccompoundsTheeasewithwhichmetallichydroxides,nitratesandcarbonatesdecomposewhenheatedisrelatedtothepositionofthemetallicelementinthereactivityseries.Compoundsofmetalsatthetopofthereactivityserieseitherrequirealarge

amountofheatenergytodecomposeortheydonotdecomposeatall.Wesaythattheyarestabletoheat.Asthereactivityofthemetallicelementsdecreases,theircompoundsarelessstabletoheatandaremoreeasilydecomposedbyheat.Inthecaseofnitrates,GroupInitratesdecomposepartially,whereasthe

nitratesoftheotherlistedmetalsdecomposemorecompletely.

HydroxidesHydroxidesofveryreactivemetalsdonotdecomposewhenheated.Thoseoflessreactivemetalsdecomposeintotheiroxidesandgiveoffsteam,for

example,

NitratesAllnitratesdecomposewhenheated,butnitratesdonotalldecomposetoproducesimilarproducts.GroupInitrates(exceptlithiumnitrate)decomposepartiallytoformthe

metallicnitriteandoxygengas,forexample,

Othermetallicnitratesoflessreactivemetalsdecomposemorecompletely,producingthemetaloxideandgivingoffnitrogendioxide,abrowngas,andoxygengas.

CarbonatesGroupIcarbonates(exceptlithiumcarbonate)donotdecomposewhenheated.Thecarbonatesofalltheotherlistedmetalsdecomposeintotheoxideandcarbondioxidegas.Theamountofheatrequiredfordecompositionisgreaterforcarbonatesofveryreactivemetals,suchascalciumcarbonate,

butcarbonatesoflessreactivemetalslikecopper(II)carbonatedecomposeatmuchlowertemperatures

Commonerror•Studentsoftenstatethatmetalshigherupinthereactivityseriesdecomposewithmuchmoredifficultythanthoselowerdown.Itisnotthemetalsthatdecompose;itistheircompounds,namelythehydroxides,nitratesand

carbonates.Metals,beingelements,cannotdecomposebecauseanelementissomethingthatcannotdecomposeintoanythingsimpler.

ExtractionofmetalsMetalscanbeextractedfromtheiroresmoreeasilyaswegodownthereactivityseries.Therearethreegeneralmethodsofextractingmetalsfromtheirores:

1Metalsofaveragereactivity,e.g.ironandzinc,areextractedbychemicalreductionusingcarbon/carbonmonoxideasreducingagents.

2Metalsoflowreactivity,e.g.copper,areextractedbyachemicalreductionusingcarbon/carbonmonoxideasreducingagentsor

belectrolysisofaqueoussolutionscontainingtheirions.3Veryreactivemetals,e.g.potassium,sodium,calcium,magnesiumandaluminium

acannotbeextractedbychemicalreductionbecausetheoresarenotreducedbychemicalreducingagents,suchascarbon,carbonmonoxideorhydrogenbcannotbeextractedbyelectrolysisofaqueoussolutions,becausehydrogenisformedatthecathodeinsteadofthemetal(seeChapter5).

Therefore,thesemetalsareextractedbyelectrolysisofmoltenioniccompounds.

AluminiumAluminiumisextractedfrombauxitewhichisimpurealuminiumoxide,Al2O3.Bauxiteisfirstpurifiedandthenelectrolysisiscarriedout.Aluminiumoxideisnotreducedbycarbonmonoxideoranyothercommonreducingagent,whichmeanselectrolysishastobeused.Thisisexpensiveduetothehighcostofelectricity.Aluminiumoxidehasameltingpointof2017°Cwhichwouldrequirea

largeamountofheatenergytoachieveandthereforewouldfurtherincreasecosts.Instead,thealuminiumoxideisdissolvedinanotheraluminiumcompound,moltencryolite,Na3AlF6.Theadvantagesofdissolvingaluminiumoxideinmoltencryolitearetheelectrolytecanbemaintainedintheliquidstatebetween800°Cand1000°C,atemperatureconsiderablylowerthan2017°C,whichgreatlyreducesenergycosts•cryoliteimprovestheconductivityoftheelectrolyte.

Aluminiumoxideinmoltencryolitebehavesinthesamewayasmoltenaluminiumoxideasfarastheproductsofelectrolysisareconcerned.Electrolysisiscarriedoutinasteeltankusingcarbon(graphite)electrodes.

Theanodesarecarbon(graphite)blocksthatareloweredintotheelectrolyte.Thecathodeisthecarbon(graphite)liningofthetank.Theelectrodereactionsare

Moltenaluminiumcollectsatthebottomofthetankandissiphonedoff.Theoxygenthatisproducedattheanodereacts,atthehightemperatureofthe

cell,withthegraphiteanodesproducingcarbondioxidegaswhichescapes.Thustheanodesburnawayand

havetobereplacedregularly.Thehighcostofelectricityisthelargestexpenseforthisprocess,whichis

carriedoutinregionswherecheapelectricity,e.g.fromhydro-electricpower,isavailable.

IronIronisextractedfromhematite,impureiron(III)oxide,Fe2O3,inablastfurnace.

Hematite,coke,C,andlimestone,CaCO3,arefedintothetopoftheblastfurnace.Ablastofhotairentersnearthebottom.Cokereactswiththeoxygenintheairformingcarbondioxide.Thereactionis

highlyexothermicandprovidesthehightemperaturerequiredfortheotherreactions.

Thecarbondioxidereactswithmorecokehigheruptoproducecarbonmonoxideinanendothermicreaction.

Thecarbonmonoxidereducesthehematitetomolteniron

andthemoltenirontricklestothebottomandistappedoff.Thefunctionofthelimestoneistoremovethemainimpurityintheironore

whichissilicondioxide(silicon(IV)oxide).

Examiner’stipYoushouldknowthatthefunctionofthecokeis

•toactasafuel,becausethehighlyexothermicreactionwithoxygenprovidesahighenoughtemperatureforthereductionofthehematite•toproducecarbonmonoxidewhichreducesthehematitetoiron.

Thelimestonedecomposesatthehightemperatureinsidetheblastfurnace.

Calciumoxidethenreactswithsilicon(IV)oxidetoformcalciumsilicatewhichformsamoltenslagasaseparatelayerabovethemolteniron(itislessdensethaniron).

Slagisusedbybuildersandroadmakersforfoundations.Theironproducedintheblastfurnaceiscalledpigironorcastiron.It

containsabout4%carbonanditsuseislimitedbecauseitisbrittle.Themajorityofpigironisconvertedintosteel.

ConversionofironintosteelPigironcontainsabout4%carbonandothernon-metallicimpurities,suchasphosphorus,siliconandsulfur.Theproductionofsteelinvolvesremovingmostofthenon-metallicimpurities(exceptsmallamountsofcarbon)•addingsmall,controlledamountsoftransitionelements(additives).Thetransitionelementsthatareusedandtheproportionsinwhichtheyareaddeddeterminethepropertiesofthesteelsthatareformed.

Theimpuritiesareremovedbythebasicoxygenprocess:

•Oxygenathighpressureisblownontothesurfaceofthemoltenmetal.•Thiscausesoxidationofsomeofthecarbontocarbondioxide,andsulfurtosulfurdioxide,bothofwhichescapeasgases.

•Siliconisoxidisedtosilicon(IV)oxideandphosphorusisoxidisedtophosphorus(V)oxide,whicharebothsolids.

•Calciumoxide(quicklime)isadded.Thisreactswiththesolidoxidestoproducecalciumsilicateandcalciumphosphatewhichareremovedasmoltenslag.

•Theamountofcarbonremainingcanbecontrolleddependingonthetypeofsteelrequired.Mildsteelcontainsapproximately0.5%carbonand99.5%iron.

•Transitionelementsarethenaddedinexactquantitiestoproducedifferenttypesofsteel.Stainlesssteelcontains18%chromium,8%nickel,aswellas74%iron.

ZincZincisextractedfromzincblende(impurezincsulfide,ZnS)byaprocesssimilartotheblastfurnaceprocess,usingcarbonasthereducingagent.Zincblendeispurified.Zincsulfideisthenheatedverystrongly(roasted)in

acurrentofairtoconvertitintozincoxide.

Thezincoxideisheatedverystronglywithpowderedcokeinafurnace.

Thezincvapourthencoolsandcondenses.Itisremovedasmoltenzinc.

CopperTherefiningofcopperisbasedontheelectrolysisofaqueouscopper(II)sulfateusingcopperelectrodes(seeChapter5).Refiningmeanspurification.Smallamountsofimpuritiesincoppercutdown

itselectricalconductivitynoticeably.Theelectrolysisiscarriedoutusingimpurecopperastheanode(+),pure

copperasthecathode(−)andaqueouscopper(II)sulfateastheelectrolyte.Thecopperfromtheanode(+)goesintosolutionaspositiveions.

Theimpuritieseithergointosolutionaspositiveionsorfallofftheanodeanddepositatthebottomofthecontainer.Theyareremovedfromtimetotime.Coppermetalformsatthecathode(−)

Thuspurecopperisformedonthecathode(whichwasoriginallymadeofpurecopper).Thecathodeisremovedandreplacedfromtimetotime.

Recyclingaluminiumandiron/steelMuchaluminiumisrecycledfromdrinkscans.Ironandsteelarerecycledfromhouseholdgoods.Theadvantagesofrecyclingare

•Naturalresourcesofbauxiteandhematitewilllastlonger.•Drinkscansandotherhouseholdobjectswillnottakeuplargeamountsofroominlandfillsites,wheretheyreactwithoxygenfromtheair(thusremovingit)astheyareoxidised.

•Thecostofrecyclingismuchlowerthanextractingthemetalsfromtheirores,largelyduetoreducedenergycosts.

Thedisadvantagesofrecyclingare

•Thecollectionandsortingofdomesticmaterialstoberecycledcanbeexpensive,timeconsumingandrequireenergy.

•Thepurityofmetalsobtainedbyrecyclingmaynotbeashighasthatobtainedbyextractionfrommetalores.

UsesofmetalsUsesofsomemetals,relatedtotheirproperties,areshowninTable10.2.

Table10.2Usesofsomemetals

Metal Use PropertiesAluminium Manufactureof

aircraftStrengthandlowdensity

Foodcontainers ResistancetocorrosionMildsteel Carbodies Highmalleability,hightensilestrength

Machinery Stainlesssteel

Cutlery Tough,doesnotcorrodeChemicalplants

Copper Electricalwiring HighelectricalconductivityCookingutensils Appearance,highthermalconductivity,high

meltingpointZinc Galvanising Protectsironfromrusting

Makingbrass Brassismoreductileandstrongerthancopper

Exam-stylequestions1Theresultsofsomeexperimentscarriedoutbyaddingametaltoaqueoussolutionscontainingionsofanothermetalareshowninthetablebelow,where✓isdisplacementreactionoccursand✗isnoreactionoccurs.

aPutthefourmetalsinorderofreactivity,startingwiththemostreactivefirst.

[1mark]

[1mark]bWriteachemicalequationforthereactionoccurringwhenmetalBis

addedtoA(NO3)2(aq).

[1mark]cWriteanionicequationforthereactionoccurringwhenmetalCisadded

toD(NO3)2(aq).[1mark]

dIfmetalsAandBarebothplacedinanaqueoussolutionofanelectrolyte,whichmetalwouldbethenegativeelectrode?Explainyouranswer.

[1mark]eWriteanequationforthereactionoccurringwhenmetalBisaddedtothe

oxideofmetalDandthemixtureisheated.[1mark]

[Total:5marks]2Youareprovidedwithamixtureofpowderedcopperandpowderedzinc.Describehowyouwouldobtainasampleofpurecopperfromthemixture.Youshouldgiveallobservationsforanyreactionsthatyoudescribe.

Note:neithermetaldissolvesinwater.[Total:4marks]

3Impurenickelcanberefinedusingamethodsimilartothemethodusedtorefinecopper.Drawadiagramoftheapparatusthatyouwouldusefortherefiningofnickel.Youshouldfullylabelallthechemicalsubstancesinthediagram.

[Total:4marks]4Copper(II)oxide→adddilutenitricacid→bluesolutionB→crystallisation→bluecrystalsB→heatcrystalsstrongly→blacksolidC+browngasD+colourlessgasE

aNameB,C,DandE.[4marks]

bWriteequationsforthereactionsthatoccurwhen

icopper(II)oxideisaddedtodilutenitricacidiibluecrystalsBareheatedstrongly.

[4marks]

[4marks]cWhencopper(II)oxideisreactedwithdilutenitricacid,bluesolutionBis

produced.Givethenamesoftwoothersubstancesthatcouldbeusedinsteadofcopper(II)oxidetoproducebluesolutionBwhenreactedwithdilutenitricacid.

[2marks][Total:10marks]

11Airandwater

KeyobjectivesBytheendofthissection,youshouldbeabletodescribethechemicaltestsforwaterusingcobalt(II)chlorideandcopper(II)sulfate•describeinoutlinethetreatmentofthedrinkingwatersupplyintermsoffiltrationandchlorination•namesomeusesofwaterinindustryandinthehome•statethecompositionofcleandryairasbeingapproximately78%nitrogen,21%oxygenandtheremainderbeingamixtureofnoblegasesandcarbondioxide•namethecommonpollutantsintheairasbeingcarbonmonoxide,sulfurdioxide,oxidesofnitrogenandleadcompoundsandtostatethesourcesofthesepollutants•statetheadverseeffectsofthesecommonpollutantsanddiscusswhythesepollutantsareofglobalconcern•statetheconditionsrequiredfortherustingofiron•describeandexplaincoatingmethodsofrustprevention•describetheneedfornitrogen,phosphorusandpotassiuminfertilisers•describethedisplacementofammoniafromammoniumsalts(seeChapter8)

•describetheseparationofoxygenandnitrogenfromliquidairbyfractionaldistillation•describethecatalyticremovalofoxidesofnitrogenfromcarexhaustgases•describeandexplaingalvanisingandsacrificialprotectionintermsofthereactivityseries(seeChapter10)asamethodofrustprevention•describeandexplaintheessentialconditionsfortheHaberprocessforthemanufactureofammoniaincludingthesourcesofhydrogen(fromhydrocarbonsorsteam)andnitrogen(fromair)(seealsoChapter7onratesandequilibriumandChapter14oncracking).

KeytermsFiltration Atreatmentfordrinkingwaterthatinvolvespassingimpurewater

throughscreenstofilteroutfloatingdebrisChlorinationAtreatmentfordrinkingwaterinwhichsmallamountsofchlorine

areaddedtokillbacteriaFractionaldistillation

Amethodforseparatingthecomponentsofair

Catalyticconverters

Presentincarexhaustsinsomecountriestoremovepollutantgasesandconvertthemintonon-pollutantgases

Rust Ironformsrustwhenitisexposedtooxygenandwater.Rustishydratediron(III)oxide

Fertilisers Substancesaddedtothesoiltosupplynutrientsthatareessentialforthegrowthofplants

WaterTestforwaterTestsforthepresenceofwatercanbecarriedoutusinganhydrouscobaltchlorideoranhydrouscopper(II)sulfate.ThecolourchangesareshowninTable11.1.

Table11.1

Originalcolour FinalcolourAnhydrouscobaltchloride Blue PinkAnhydrouscopper(II)sulfate White Blue

Cobaltchloridepaperiscommonlyusedinsteadoftheanhydroussolid.

Examiner’stipThesesubstancesarenotusedasatestforpurewater.Thecolourchangesshownoccurifwateroranythingcontainingwater(includingallaqueoussolutions)isused.

Purityofwatercanbedeterminedbymeasuringtheboilingpoint,whichis100°Cat1atmospherepressure.

TreatmentofdrinkingwaterImpurewaterismadefitfordrinkingbythefollowingmethods:Filtration:Thisinvolvespassingimpurewaterthroughscreenstofilteroutfloatingdebris.Chlorination:Smallamountsofchlorinegasareaddedtokillbacteria.

Commonerror•Studentsoftenstatethatchlorineisaddedtopurifywater.Watercontainingsmallamountsofaddedchlorineshouldnotbedescribedaspure.

UsesofwaterWaterisusedinindustry

•asasolvent•asacoolant•forcleaning•asachemicalreactant(e.g.inthehydrationofethene;seeChapter15).

Waterisusedinthehome

•incooking•incleaning•fordrinking.

AirAirisamixture,andincommonwithallmixturesitscompositioncanvary,particularlyinindustrialandruralareas.Theapproximatecompositionofcleandryairis

78%nitrogen21%oxygen0.03%carbondioxide•1%argon.

Verysmallamountsofothernoblegasesarealsopresent.

Examiner’stipHydrogeniscommonlyandincorrectlyregardedbystudentsasaconstituentofair.

FractionaldistillationofliquidairInordertoseparatethecomponentsofair,•itiscooledtoremovecarbondioxideandwatervapourassolids•itiscompressedandexpandedcontinuallytoliquefyitat−200°C

•theliquidairisthenseparatedbyfractionaldistillationintoliquidoxygen,liquidnitrogenaswellasliquidGroup0elements.

AirpollutionCommongaseouspollutants,theirsourcesandrelatedpollutionproblemsareshowninTable11.2.

Table11.2

Pollutant Source PollutionproblemsCarbonmonoxide,CO

Incompletecombustionoffossilfuels

Toxic

Sulfurdioxide,SO2 Combustionoffossilfuelscontainingsmallamountsofsulfur

Irritationoftherespiratorysystem

Dissolvesinrainwaterformingacidrainwhichcausesdamagetobuildingsmadeof,e.g.limestoneandmarble

Oxidesofnitrogen(oftenrepresentedasNOxtosignifymorethanoneoxideofnitrogen)includingnitrogendioxide,NO2

Nitrogenandoxygen(bothfromtheair)reacttogetheratveryhightemperaturesincarengines

Productionoflow-levelozone(respiratorysystemirritant)Nitrogendioxide,NO2,dissolvesinrainwaterformingacidrainwhichcausesdamagetobuildingsmadeof,forexample,limestoneandmarblePhotochemicalsmog

Leadcompounds Leadcompoundsinpetrol(onlyincountrieswhereleadedpetrolisstillused)

Toxic

CatalyticconvertersCatalyticconvertersarepresentincarexhaustsinsomecountries.Theirpurposeistoremovepollutantgasesandconvertthemintonon-

pollutantgases.Thecatalystspresentincatalyticconvertersincludealloyscontaining

transitionelements,suchasplatinum,rhodiumandpalladium.Thereareseveralreactionsoccurringinsidecatalyticconvertersincluding

and

Thuspollutantgasesareconvertedintonon-pollutantgases.

CommonerrorsThereareprobablymoremisunderstandingsconcerningatmosphericpollutionthananyotherChapter.Manystudentsthinkthatallatmosphericpollutantsareresponsibleforall

environmentalproblems,particularlyglobalwarming.ItisveryimportantthatstudentsstudyTable11.2carefullyandlearnaboutthesourcesandpollutionproblemscausedbyeachindividualpollutant.Thefollowingpointsneedtoinbenoted:

•Oxidesofnitrogenareproducedbyreactionbetweennitrogenandoxygen,bothofwhichcomefromtheair.Thenitrogenisnotpresentinthefuel.

•Oxidesofnitrogenareoftenmistakenlythoughttobeproducedincarexhaustsasopposedtoincarengines.

•Pollutantsareremovedbycatalyticconvertersincarexhausts.•Ifcarsdonothaveefficientcatalyticconverters,thegaseouspollutantsentertheatmospherethroughthecarexhaust.

•Sulfurdioxideisoftenregardedasbeingproducedbythedeliberateburningofsulfur,asopposedtothesmallamountsofsulfurimpuritiesinfossilfuels.

RustingofironRustcanbedescribedashydratediron(III)oxidewithaformulathatcanberepresentedasFe2O3.xH2O(xisusedbecausetheamountofwaterofcrystallisationvariesfromonesampleofrusttoanother).Irononlyformsrustwhenitisexposedtooxygen(e.g.fromtheair)and

water.

PreventionofrustingRustingcanbepreventedbycoatingtheironwithpaintoilorgreaseplasticothermetals,suchaszinc(whichisknownasgalvanising).

Thesemethodsallworkbypreventingoxygenandwatercomingintocontactwiththeironandthuspreventingareactiontakingplace.

SacrificialprotectionSomemetalswillcontinuetopreventironfromrustingevenifthesurfaceisscratched.Suchmetalsmustbeaboveironinthereactivityseries,butmustnotbesoreactivethattheywillreactrapidlywithwaterthemselves.Zincandmagnesiumarebothusedinthisway.Tin,whichisbelowironinthereactivityseries,willonlyprotectironifitisnotscratched(Figure11.1).Inaddition,barsofzincmaybeattachedtothehullsofshipswithout

attemptingtocoverthesurfaceoftheiron.Rustingwillnotoccurinthesecircumstances(Figure11.2).

Thefirststageofrustingistheoxidationofirontoiron(II)ionsbyoxygeninthepresenceofwater:

Ifzincispresent,evenifthezincisscratched,thezincwillbeoxidisedinpreferencetotheiron.

Thisoccursbecausezincisamorereactivemetalthanironandthereforezincformspositiveionsmorereadilythanirondoes.Theelectronstravelfromthezinctotheiron.Theirondoesnotloseelectrons,

whichmeansthatoxidationofiron(whichisthefirststageofrusting)doesnotoccur.Iftinisusedinsteadofzinc,whenthetinisscratchedtheironwillbeoxidised

inpreferencetothetinbecauseironisamorereactivemetalthantin.Thustinonlypreventsrustingwhenitisnotscratched.

Commonerror•Itisacommonerrorforstudentstostatethatifgalvanisedironorsteelisscratchedandexposedtoairandwater,thezincformsrustinsteadofiron.Ironistheonlymetalthatcanformrust.

NitrogenandfertilisersFertilisersaresubstancesthatareaddedtothesoiltosupplynutrientsthatareessentialforthegrowthofplants.Fertiliserscontainnitrogen,phosphorusandpotassium(NPK).Veryfewplantscanutilisenitrogenfromtheair.Thenitrogenmustbe

suppliedtotheplantsintheformoffertiliserscontainingammoniumsaltsornitrates.

TheHaberprocessNitrogenobtainedfromthefractionaldistillationofliquidair(seeearlierinthisChapter)andhydrogenfromhydrocarbonsbycracking(seeChapter14)orfromsteam,arefirstconvertedintoammoniaintheHaberprocess.Nitrogenandhydrogenreacttogetherinareversiblereactiontoproduce

ammonia.

Thegasesare

•passedoveracatalystofiron•atatemperatureof450°C•atapressureof200atmospheres.

Themixtureemergingfromthecatalystchambercontainsabout15%ammonia.Theammoniaisseparatedfromtheunreactednitrogenandhydrogenbyliquefyingit,andtheunreactednitrogenandhydrogenarepassedoverthecatalystagain.Eventuallyallthenitrogenandhydrogenareconvertedintoammonia.

Theammoniaisusedfor

•themanufactureoffertilisers,suchasammoniumsulfateandammoniumphosphate•themanufactureofnitricacidwhichisconvertedintofertilisers,suchasammoniumnitrate.

Exam-stylequestions1Thepercentageofoxygenintheaircanbedeterminedbypassingairbackwardsandforwardsoverheatedcopper,usingtheapparatusshowninFigure11.3.Thecopperwasinexcess.

Thevolumeofairatthestartwas100cm3.Astheairwaspassedbackwardsandforwards,thevolumeofairdecreased.Thefinalvolume,measuredatroomtemperatureandpressure,was79cm3.

aWhatcolourchangeinvolvingthecopperwouldyouexpecttosee?[2marks]

bWriteachemicalequationforthereactionintheheatedtube.[2marks]

cWhydoesthevolumeofairdecrease?[1mark]

dWhatismeantbythephrase‘thecopperwasinexcess’?[1mark]

eIdentifythemaingaspresentinthe79cm3remainingattheend.[1mark]

[Total:7marks]

2TheequationfortheequilibriumoccurringintheHaberprocessis

Theforwardreactionisexothermic.Thereactioniscarriedoutbypassingnitrogenandhydrogenoveracatalystofironatatemperatureof450°Candapressureof200atmospheres.

aWhatismeantbythetermcatalyst?[2marks]

[2marks]bCopyandcompletethetable.

cAmmoniaisconvertedintofertiliserssuchasammoniumsulfate.

iWhichsubstancewouldbeaddedtoammoniatoproduceammoniumsulfate?

[1mark]iiWriteachemicalequationforthereactionoccurringinquestion(c)(i)

above.[2marks]

dAmmoniaisalsoconvertedintothefertiliserammoniumphosphate.Completetheequationfortheformationofammoniumphosphatefromammonia.(Youmayleaveoutthestatesymbols).

[2marks][Total:13marks]

12Sulfur

KeyobjectivesBytheendofthissection,youshouldbeabletonamesomesourcesofsulfur•knowthatsulfurisusedinthemanufactureofsulfuricacid•knowthatsulfurdioxideisusedtobleachwoodpulpinthemanufactureofpaper,andasafoodpreservative(bykillingbacteria)

•describethemanufactureofsulfuricacidbytheContactprocess,includingessentialconditionsandreactions•describethepropertiesandusesofdiluteandconcentratedsulfuricacid.

KeytermsSulfuricacid

Usedformakingdetergentsandfertilisers,usedastheacidincarbatteriesandusedformakingpaints,dyesandsyntheticfibres

Dilutesulfuricacid

Atypicalstrongacid

Sulfurdioxide

Usedforbleachingofwoodpulpinthemanufactureofpaperandinthepreservationoffoodbykillingbacteria

Sulfurisfoundastheimpureelementinsulfurbedsbelowtheground.Itisalsofoundasmetallicores,mainlysulfides,suchaszincblende(seeChapter10).

ManufactureofsulfuricacidSulfurisusedinthemanufactureofsulfuricacidintheContactprocess.Thesulfurisfirstconvertedintosulfurdioxide,SO2(g),byheatingitstronglyinair.

Thesulfurdioxideisthenmixedwithoxygenandthetwogasesarepassedoveracatalystofvanadium(V)oxide,V2O5(s)atatemperatureof450°Catapressureof1–2atmospheres

producinganequilibriummixturecontainingthetworeactantgasesandsulfurtrioxide,SO3(g).

Sulfurtrioxideisthendissolvedin98%concentratedsulfuricacidtoproduceoleum,H2S2O7(l).

Theoleumisthenaddedtothecorrectamountofwatertoproducesulfuricacidoftherequiredconcentration.

Examiner’stipIfsulfurtrioxideisaddedtowater,areactionoccursandsulfuricacidistheproduct.

However,thisreactionisveryexothermicandtheheatgivenoffissufficienttovaporisethesulfuricacidandathickmistisproducedthatisverydifficulttocollect.Thustheprocessisnotcarriedoutinthisway.

UsesofsulfuricacidSulfuricacidisused

•formakingdetergents•formakingfertilisers(seeChapter11)•astheacidincarbatteries•formakingpaints,dyesandsyntheticfibres.

PropertiesofdilutesulfuricacidDilutesulfuricacidisatypicalstrongacid.Itisadiproticacid,whichmeansthatonemoleculeofsulfuricacidreleases

twoprotonswhenitformsionsinaqueoussolution.

Itcanalsoreleaseoneproton.

Therefore,sulfuricacidcanformsaltscalledsulfatescontainingSO4andacidsaltscalledhydrogensulfates,containingHSO4−(seeChapter8).Itreacts

•withmetalsabovehydrogeninthereactivityseriestoproduceasaltandhydrogen(seeChapters8and10)•withbases(includingalkalis)toformsalts(sulfates)andacidsalts(hydrogensulfates),andwater(seeChapter8)•withcarbonatestoformasalt,waterandcarbondioxide(seeChapter8).

UsesofsulfurdioxideSulfurdioxideisused

•forthebleachingofwoodpulpinthemanufactureofpaper•inthepreservationoffoodbykillingbacteria.

Exam-stylequestions

1SulfuricacidismadeintheContactprocess.Thecatalysedreaction

takesplaceatatemperatureof450°Candapressureof1–2atmospheres.Theforwardreactionisexothermic.aNamethecatalystintheabovereaction.

[1mark]bSuggestwhyatemperaturebelow450°Cisnotused.

[1mark]cWhatwouldhappentotherateatwhichequilibriumwouldbereachedifa

higherpressurewasused?Explainyouranswer.[2marks]

dSuggesttworeasonswhyapressureabove1–2atmospheresisnotused.[2marks]

eHowisthesulfurtrioxideproducedintheabovereactionconvertedintosulfuricacid?Giveequationsforanyreactionsthatyoureferto.

[2marks][Total:8marks]

2Writechemicalequationsforthereactionsbetweendilutesulfuricacidandacopper(II)carbonate[2marks]

baqueoussodiumhydroxideinwhichsodiumsulfateisproduced[2marks]

caqueouspotassiumhydroxideinwhichpotassiumhydrogensulfateis

produced[1mark]

dzinc.[1mark]

[Total:6marks]

13Inorganiccarbonchemistry

KeyobjectivesBytheendofthissection,youshouldbeabletostatethatcarbondioxideandmethanearegreenhousegasesandexplainhowtheymaycontributetoclimatechange•statetheformationofcarbondioxideinavarietyofreactions•statethesourcesofmethane,includingdecompositionofvegetationandwastegasesfromdigestioninanimals•describethemanufactureofquicklime(calciumoxide)fromcalciumcarbonate(limestone)intermsofthermaldecomposition•namesomeusesofquicklimeandslakedlime•nametheusesofcalciumcarbonate

•describethecarboncycleinoutlinetoincludetheprocessesofcombustion,respirationandphotosynthesis.

KeytermsGreenhousegases

GaseswhichabsorbinfraredradiationproducedbythesolarwarmingoftheEarth’ssurface

Greenhouseeffect

GreenhousegasespreventingheatenergyfromescapingfromtheEarth

CarbondioxideandmethaneCarbondioxideandmethanearebothgreenhousegases.ThismeansthattheirbondsabsorbinfraredradiationproducedbythesolarwarmingoftheEarth’ssurface.ThispreventsheatenergyfromescapingfromtheEarth.Thiseffect,knownasthegreenhouseeffect,isthoughttoleadtoglobalwarming.Carbondioxideisformedinlaboratoryreactionsandinindustrialand

environmentalprocessesbycompletecombustionofcarbon-containingsubstancessuchasfossilfuels(seeChapter14)•asaproductofthereactionbetweenanacidandacarbonate(seeChapter8)•fromthethermaldecompositionofcarbonates(seeChapter10)•byrespirationinwhichglucoseisoxidisedtocarbondioxideandwaterinlivingthings.

Carbondioxideisremovedfromtheatmospherebyreactionwithwaterintheleavesofgreenplantscontainingchlorophyll,inthepresenceofsunlight.Thereactionisknownasphotosynthesis.Theproductsareglucoseandoxygen.

Methaneisformedbydecompositionofvegetation•Wastegasesfromdigestioninanimals.

Examiner’stipTheequationsforrespirationandphotosynthesisarethereverseofoneanother.

Thecarboncycleshowshowthepercentageofcarbondioxideintheatmosphereremainsapproximatelyconstantat0.03%duetothevariousprocessesbywhichitisreleasedintoandabsorbedfromtheatmosphere.

CarbonatesQuicklime(calciumoxide)ismanufacturedbythethermaldecompositionofcalciumcarbonate(limestone).Thisisareversiblereaction.

Theprocessiscarriedoutinalimekiln.Adraftofaircarriesoutthecarbondioxideandcausestheequilibriumtoshifttotheright(seeChapter7).Eventuallyallthecalciumcarbonateisconvertedintocalciumoxide.Calciumoxidereactswithwatertoproducecalciumhydroxide(slakedlime).

UsesofquicklimeandslakedlimeBothquicklime,CaO,andslakedlime,Ca(OH)2,canbeusedtoneutraliseacidicsoil(seeChapter8)•neutraliseacidicindustrialwasteproducts,e.g.fluegasdesulfurisation,inwhichsulfurdioxidegasinfactoryemissionsisneutralisedbyquicklimeorslakedlime.

UsesoflimestoneLimestone,CaCO3,isusedinthemanufactureofiron(seeChapter10)•manufactureofcement.

Exam-stylequestions1Statewhyitisnecessarytoneutralisesoilacidityandgivethechemicalnameofacompoundwhichcanbeaddedtosoilinordertoneutraliseit.

[Total:2marks]2aCarbondioxidecanbeproducedbythe

icompletecombustionofoctaneiithermaldecompositionofcalciumcarbonateiiireductionofiron(III)oxidebycarbonmonoxideivreactionbetweencalciumcarbonateanddilutehydrochloricacid.

Writeachemicalequationforeachofthesereactions.Youmayomitstatesymbols.

[Total:7marks]bCarbondioxidecanberemovedfromtheEarth’satmosphereby

photosynthesis.

iNametheotherreactantinphotosynthesis.[1mark]

iiNamethetwoproductsofphotosynthesis.[2marks]

iiiStatetwoconditionsthatarerequiredforphotosynthesis.[2marks]

[Total:12marks]

14Organicchemistry1

KeyobjectivesBytheendofthissection,youshouldbeableto

•nameanddrawthestructuresofmethane,ethane,ethene,ethanol,ethanoicacidandtheproductsofthereactionsstatedinthisChapter

•describetheconceptofahomologousseriesasa‘family’ofsimilarcompoundswithsimilarchemicalpropertiesduetothepresenceofthesamefunctionalgroup

•describethepropertiesofalkanesasbeinggenerallyunreactiveexceptintermsofburning

•describethebondinginalkanes•distinguishbetweensaturatedandunsaturatedhydrocarbons•frommolecularstructures•byreactionwithaqueousbromine•namethefossilfuels:coal,naturalgasandpetroleum•namemethaneasthemainconstituentofnaturalgas•describepetroleumasamixtureofhydrocarbonsanditsseparationbyfractionaldistillation(seeChapter2)

•nametheusesofthefractions•describethepropertiesofmoleculeswithinafraction•describethemanufactureofalkenesandhydrogenbycracking

•nameanddrawthestructuresofunbranchedalkanesandalkenescontaininguptofourcarbonatomspermolecule

•describethegeneralcharacteristicsofhomologousseries•recallthatcompoundsinahomologousserieshavethesamegeneralformula

•describeandidentifystructuralisomerism•describesubstitutionreactionsofalkaneswithchlorine

•describethepropertiesofalkenesintermsofadditionreactionswithbromine,hydrogenandsteam.

KeytermsEmpiricalformula

Thesmallestwholenumberratiooftheatomsofeachelementinacompound

Molecularformula

Thisisthenumberofatomsofeachelementinonemoleculeofasubstance

Displayedformula

Alltheatomsandallthebondsinonemoleculeofthecompound

Structuralformula

Thisshowshowatomsarearrangedingroupsofatoms

Structuralisomerism

Theexistenceofcompoundswiththesamemolecularformulabutdifferentstructuralformulae

Organiccompoundsarecovalentcompoundscontainingcarbonatomsbondedtohydrogen(always),aswellasoxygen,thehalogensandnitrogenatoms.

HomologousseriesOrganiccompoundsbelongto‘families’ofsimilarcompoundsknownashomologousseries,examplesofwhicharealkanes,alkenes,alcohols,carboxylicacidsandesters.

Homologousseriesarecompoundswhichhave

•thesamegeneralformula:thisisbecauseeachmemberdiffersfromthepreviousmemberbya-CH2–groupofatoms

•thesamechemicalproperties:thisisbecauseeachmemberhasthesamefunctionalgroup

•physicalpropertiesthatshowaconstantgradation,e.g.meltingpointsandboilingpointsthatshowalmostconstantincreases.

Afunctionalgroupisagroupofatomsthatallmembersofahomologousserieshaveincommonandwhichisresponsibleforallmembersofahomologousserieshavingthesamechemicalproperties.

FormulaeoforganiccompoundsOrganiccompoundshaveseveraldifferenttypesofformulae(Table14.1).Theseare

•Empiricalformula.Thisisthesmallestwholenumberratiooftheatomsofeachelementinacompound(seeChapter4).

•Molecularformula.Thisisthenumberofatomsofeachelementinonemoleculeofasubstance(seeChapter4).Thisgivesnoinformationabouthowtheatomsarejoinedtogether.

•Displayedformula.Thisshowsalltheatomsandallthebondsinonemoleculeofthecompound.

•Structuralformula.Thisshowshowatomsarearrangedingroupsofatoms.

Examiner’stipTheterm‘displayedformula’isnotalwaysusedinexaminationquestions.Insteadstudentsaremorelikelytobeaskedtodrawthestructureofamoleculeshowingallatomsandallbonds.

StructuralisomerismStructuralisomerismistheexistenceofcompoundswiththesamemolecularformulabutdifferentstructuralformulae(andthereforedifferentdisplayedformulae).

ExampleTherearetwostructuralisomerswiththemolecularformulaC4H10.ThesehavedifferentstructuralanddisplayedformulaeasshowninTable14.2.Becausetheyaretwodifferentcompoundstheyhavetwodifferentnames.Butaneisoftenreferredtoasastraightchainoranunbranchedmolecule,

becausethecarbonatomsarearrangedoneafteranother.2-methylpropaneisoftenreferredtoasabranchedchainmolecule.

CommonerrorsStudentsoftenconfusethetwowordsisotopesandisomers:

•Isotopes(seeChapter3)areatomsofthesameelementwiththesameprotonnumberbutdifferentnucleonnumber.

•Structuralisomersarecompoundswiththesamemolecularformulabutdifferentstructuralformulae.

Allotropeisanothersimilarwordwhichyoumayhavecomeacross.Thisreferstodifferentcrystallineformsofthesameelement,e.g.diamondandgraphite.

•Whendrawingdisplayedformulae,allcarbonatomsmusthavefourbonds(sticks)andallhydrogenatomsmusthaveonebond.Oxygenatomsmusthavetwobonds,halogenatomsonebondandnitrogenatomsthreebonds.

HydrocarbonsHydrocarbonsarecompoundscontainingcarbonandhydrogenonly.Alkanesandalkenesareimportantexamplesofhydrocarbons.

Examiner’stipWhenasked‘Whatismeantbythetermhydrocarbon?’itisimportanttousetheword‘only’,aswellasreferringtocompoundscontainingcarbonandhydrogen.Compoundssuchasethanol,C2H5OH,containcarbonandhydrogen,butbecausetheyalsocontainoxygen,theyarenothydrocarbons.

Alkanes•AlkanesaremembersofahomologousserieswiththegeneralformulaCnH2n+2.

•Thenamesofalkanesallendin-ane.•Alkanesaresaturatedhydrocarbons;thismeansthatalltheirbondsaresinglebonds(eitherC–CorC–H).

•Alkanesdonotcontainafunctionalgroup,becausetheycontainonlyC–CandC–Hbondswhichcanbesaidofallotherhomologousseriesoforganiccompounds.

Examiner’stipThenamesofalkanesareimportantbecauseunbranchedmembersofallotherhomologousseriesarenamedafterthealkanewiththesamenumberofcarbonatoms.Thereforeallorganicmoleculeswith

•1carbonatombeginwithmeth-•2carbonatomsbeginwitheth-•3carbonatomsbeginwithprop-•4carbonatomsbeginwithbut-.

Someexamplesoforganiccompoundswithtwocarbonatomsare

PropertiesofalkanesAlkanesareunreactivecomparedtoalkenes.Thisisbecausethesinglebondsinalkanesneedalotofenergytobreak,butthedoublebondsinalkenesneedlessenergytobeconvertedintosinglebonds(whichiswhathappenstoalkenesinadditionreactions).

CombustionAlkanesundergocombustioninairoroxygenproducingenergy,whichiswhyalkanesareusedasfuels.Completecombustionoccursinexcessoxygen.Theproductsarecarbon

dioxideandwater,forexample

Incompletecombustionofalkanesinalimitedsupplyofairoroxygenleadstotheproductionof(toxic)carbonmonoxideaswellaswater(seeChapter11).

ReactionwithchlorineItisnotpossibletoaddatomstoalkanemoleculeswithoutfirstremovingatoms.Thereforealkanesundergosubstitutionreactionsasopposedtoadditionreactions.Asubstitutionreactionisareactioninwhichoneatomorgroupofatomsis

replacedbyanotheratomorgroupofatoms.Whenmethaneisreactedwithchlorineinthepresenceofultravioletlight,a

substitutionreactionoccursinwhichonechlorineatomreplacesonehydrogenatominmethane.Theorganicproductischloromethane.

Unlessthechlorinesupplyislimited,thereactionshouldnotbeusedasamethodofpreparationofchloromethanebecausethechloromethanereactswithmorechlorine.Allthehydrogenatomsaresubstitutedbychlorineatoms,oneatatime,untilallthehydrogenatomshavebeenreplacedbychlorineatoms.Hydrogenchloridegasisproducedateachstage.

Similarreactionsoccurwithotheralkanesandchlorine.

Alkenes•AlkenesaremembersofahomologousserieswiththegeneralformulaCnH2n.

•AlkenescontainafunctionalgroupwhichisaC==C.•BecauseaC==Cmustbepresentinallalkenes,thereisnoalkenewithonecarbonatom.

•Thenamesofalkenesallendin–ene.•Alkenesareunsaturatedhydrocarbons.•Notallthebondsaresinglebondsinunsaturatedhydrocarbons.Unsaturatedmoleculescontainatleastonecarbon–carbondoublebondorcarbon–carbontriplebond.

TestingforunsaturationAqueousbromine(brominewater)canbeusedtodistinguishbetweensaturatedandunsaturatedsubstances(Table14.4).

Table14.4Usingbrominewatertotestforsaturatedandunsaturatedsubstances

Typeofmolecule Saturated UnsaturatedEffectofaddingaqueousbromine

Nochange(aqueousbromineremainspalebrown)

Aqueousbrominechangesfrompalebrowntocolourless

StructuralisomerisminalkenesThereisonlyonestructureforthealkenescontainingtwoandthreecarbonatoms:etheneandpropene.

Withfourcarbonatoms(C4H8)therearetwounbranchedalkenes,becausethedoublebondcanbeintwodifferentpositionsinthecarbonchain(Table14.5).

Thenumber1inbut-1-enemeansthatthedoublebondisbetweencarbonatoms1and2.Thenumber2inbut-2-enemeansthatthedoublebondisbetweencarbon

atoms2and3.Structuresofunbranchedalkenescontaininguptofourcarbonatomsare

showninTable14.6.

Examiner’stipWhenstudentsareaskedtogivethestructuresoftwoalkeneswithmolecularformulaC4H8,theyoftendraw

Thesearebothbut-1-enedrawndifferently.Oneisthesameastheother,onlyit

Thesearebothbut-1-enedrawndifferently.Oneisthesameastheother,onlyitisdrawnbacktofront.Thedoublebondisbetweencarbonatoms1and2inbothcases.

Isomersmustbedifferentmolecules,notthesamemoleculedrawndifferently.

ReactionsofalkenesItispossibletoaddatomstoalkenemoleculeswithoutfirstremovingatoms.Thereforealkanesundergoadditionreactionsasopposedtosubstitutionreactions.

Additionmeanstwomoleculesjointogethertomakeonemolecule.Intheadditionreactionsofalkenes,thedoublebondbecomesasinglebond

andanatomoragroupofatomsjoinsontobothcarbonatomsthatformedthedoublebond.

WithbromineIfbrominetheelement(Br2(l))oraqueousbromine(Br2(aq))isaddedtoanyalkene,anadditionreactionoccurs.Ifthealkeneisethene,theproductis1,2-dibromoethane.

Withhydrogen(industrial)Ifetheneandhydrogenarepassedoveranickelcatalystat200°C,theproductisethane.Thisadditionreactioniscalledhydrogenation.

Hydrogenationisusedinindustrytoconvert(unsaturated)vegetableoilsto(saturated)margarinebyreactionwithhydrogeninthepresenceofanickelcatalyst.

Withsteam(industrial)

Ethenereactswithsteamusingacatalystofphosphoricacid(H3PO4),at300°Cand60atmospherespressure.Theproductisethanol.

Thisreaction,knownashydrationofethene,isusedtomanufactureethanol(seeChapter15).ThereactionsofethenearesummarisedinFigure14.3.

FuelsandthepetroleumindustryFossilfuelsarefuelsformedbynaturalprocessesovermillionsofyearsasaresultofthedecayofburieddeadorganisms.Examplesarecoal,naturalgasandpetroleum(crudeoil).Fossilfuelsareknownasnon-renewablefuelsbecauseoncetheyrunouttheycannotbereplaced.Theyarealsoknownasafiniteresource.Methane,CH4,isthemainconstituentofnaturalgas.Petroleum(crudeoil)isamixtureofhydrocarbonswhichisseparatedby

fractionaldistillation.Theprocessdoesnotproduceindividualhydrocarbons,butinsteadproducesmixturesofhydrocarbonsknownasfractions.Thesefractions(Figure14.4)aremixturesofhydrocarbonswhichhaveanarrowrangeofboilingpoints.Astheboilingpointrangesincrease,thehydrocarbonscontainanincreasingnumberofcarbonatoms.

ManufactureofalkenesbycrackingFractionaldistillationofpetroleumproduces

•higherboilingpointfractionsthatareinexcessofrequirement•insufficientamountsofthelowerboilingpointfractions,whichareparticularlyindemandas

•fuelsforpetrolengines(alkanesbetweenC5andC10)•monomers(short-chainalkenes)fortheproductionofpolymers•sourcesoforganicchemicals.

Alkenesaremanufacturedbycrackinglong-chainalkanesobtainedfrompetroleum.Crackingreferstodecompositionofalkaneswhichspecificallyinvolvesbreakingcarbon–carbonbondstoformsmallermolecules.Therearetwotypesofcracking:catalyticandthermal.

CatalyticcrackingThealkanemoleculesarepassedovercatalystsknownaszeolitesattemperaturesof500°C.

ThermalcrackingThisusesahighertemperature(800°C)thancatalyticcracking.

Crackingproduces

•short-chainalkenesforproductionofpolymersandorganicchemicals•alkanescontainingbetween5and10carbonatomsasfuelsforpetrolengines•hydrogenwhichisusedtomanufactureammonia.

Ifalong-chainalkaneiscracked,differentmoleculesofthealkanemaybreakindifferentplacestogiveavarietyofproductswhichcanalsobeseparatedby

differentplacestogiveavarietyofproductswhichcanalsobeseparatedbyfractionaldistillation.Forexample,C14H30moleculescouldcrackintooctaneandpropene

orethene,propeneandhydrogen.

Examiner’stipStudentswillnotbeaskedtopredictwhatthedefiniteproductsofcrackingarewithoutbeingprovidedwithfurtherinformation.

Sampleexam-stylequestionWriteanequationforthecrackingofhexaneintoanalkaneandanalkene,bothhavingthesamenumberofcarbonatoms.

AnswerBoth

and

areacceptableanswers,becausebothproduceanalkaneandanalkenewiththesamenumberofcarbonatoms.Neitheranswerismorecorrectthantheother.Ifthequestionhadspecifieda1:1moleratiooftheproducts,onlyC6H14→

C3H8+C3H6wouldbecorrect.

Exam-stylequestions1Drawthestructures,showingalltheatomsandallthebonds,oftwodifferentunbranchedalkeneswiththemolecularformulaC5H10.Youarenotexpectedtonamethealkenes.

[2marks]2Ethanereactswithchlorineinasubstitutionreaction.aUnderwhatconditiondoesthereactiontakeplace?

[1mark]bNametheorganicproductformedwhenethaneandchlorinereactina1:1

moleratio.[1mark]

cIfanexcessofchlorineisused,givethemolecularformulaofoneotherorganicproductthatcouldform.

[1mark][Total:3marks]

3Propenereactswith

iaqueousbromineiihydrogeniiisteam.

aWhatnameisgiventothetypeofreactionthatoccursinallthreecases?[1mark]

bStatetheobservationyouwouldexpecttoseeinreaction(i)ifexcesspropeneisused.

[2marks]cWritedownthemolecularformulaeoftheproductsthatforminreactions

(i),(ii)and(iii).[3marks]

dWhatisthenameofthecatalystusedin(ii)?[1mark]

[Total:7marks]4Alkanesareconvertedintoalkenesbycracking.aWhatisthemolecularformulaofthealkanethatcontainsninecarbon

atoms?[1mark]

bDrawthestructureandgivethenameofanalkenewithfourcarbonatoms.[2marks]

cWriteanequationforthecrackingofoctane,C8H18,into

ianalkaneandanalkeneformedinthemoleratio1:2

iihydrogenandtwootherproducts.[2marks]

[Total:5marks]

15Organicchemistry2

KeyobjectivesBytheendofthissection,youshouldbeableto

•describethemanufactureofethanolbyfermentationandbythecatalyticadditionofsteamtoethene

•outlinetheadvantagesanddisadvantagesofthesetwomethodsofmanufacturingethanol

•describethepropertiesofethanolintermsofburning•nametheusesofethanolasasolventandasafuel

•nameanddrawthestructuresofunbranchedalcoholsandcarboxylicacidscontaininguptofourcarbonatomspermolecule•nameanddrawthestructuresoftheesterswhichcanbemadefromunbranchedalcoholsandcarboxylicacids,eachcontaininguptofourcarbonatoms

•describethepropertiesofaqueousethanoicacid

•describetheformationofethanoicacidbytheoxidationofethanolwithacidifiedpotassiummanganate(VII)•describeethanoicacidasatypicalweakacid•describethereactionofacarboxylicacidwithanalcoholinthepresenceofacatalysttogiveanester

•definepolymersaslargemoleculesbuiltupfromsmallmoleculescalledmonomers

•explainthedifferencesbetweenadditionandcondensationpolymerisationandunderstandthatdifferentpolymershavedifferentunitsand/ordifferentlinkages

•describetheformationofpolyetheneasanexampleofaddition

polymerisation•namesomeusesofplasticsandofman-madefibressuchasnylonandTerylene•describethepollutionproblemscausedbynon-biodegradableplastics

•deducethestructureoftheadditionpolymerfromagivenalkeneandviceversa•describetheformationofnylon(apolyamide)andTerylene(apolyester)bycondensationpolymerisation

•nameproteinsandcarbohydratesasconstituentsoffood

•describethestructureofproteinsaspossessingthesameamidelinkagesasnylon,butwithdifferentunits•describethehydrolysisofproteinstoaminoacids•describecomplexcarbohydratesintermsofalargenumberofsugarunitsjoinedtogetherbycondensationpolymerisation•describehydrolysisofcomplexcarbohydrates(e.g.starch)byacidsorenzymestogivesimplesugars•describeinoutlinetheusefulnessofchromatographyinseparatingandidentifyingtheproductsofhydrolysisofcarbohydratesandofproteins.

KeytermsEthanol Analcoholusedasafuel(e.g.inspiritcampingstovesandin

petrol)Polymers Largemoleculesmadeupbythereactionsofsmallmolecules

calledmonomersPolymerisationTheformationofpolymersfrommonomers

AlcoholsManufactureofethanolEthanolismanufacturedonalargescalebyfermentationofcarbohydratesandhydrationofethene.

FermentationofcarbohydratesCarbohydrates,suchassugar,arebrokendownbyenzymesinyeasttoproduceglucose,C6H12O6.Theenzymesinyeastalsocatalysethebreakdownofglucosetoformethanolandcarbondioxide.Thereactionoccursatatemperatureof37°Candiscarriedoutunderanaerobicconditions(intheabsenceofoxygen).

Whentheconcentrationofethanolreaches14%,itkillstheyeast.Theyeastcellsareremovedbyfiltrationandtheethanolispurifiedbyfractionaldistillation.

HydrationofetheneEtheneisproducedfrompetroleumbyfractionaldistillationfollowedbycrackingoflong-chainalkanes.Ethenereactswithsteamusingacatalystofphosphoricacid,H3PO4,at300

°Cand60atmospherespressure.

TheadvantagesanddisadvantagesofthetwoprocessesareshowninTable15.1.

Table15.1Advantagesanddisadvantagesoffermentationandhydrationinthemanufactureofethanol

Fermentation HydrationAdvantages Usescarbohydratesfromplants

whicharearenewableresourceThereisonlyoneproductinthereactionwhichmeans

whicharearenewableresource thereactionwhichmeansthereisnowaste

Requiresatemperatureof37°Cwhichmeansenergycostsarelow

Acontinuousflowprocessisusedwhichisefficient

Disadvantages Abatchprocessisusedwhichisinefficient

Usesethenefrompetroleumwhichisanon-renewableresource

Landwhichcouldbeusedtogrowplantsforfoodisusedforethanolproduction

Requiresatemperatureof300°Cwhichmeansenergycostsarehigh

PropertiesofethanolEthanolisusedasafuelinspiritcampingstovesanditisalsoaddedtopetrolandusedinethanolfuelcells.Itundergoescompletecombustiontoproducecarbondioxideandwater.

Ethanolisalsousedonalargescaleasasolvent.

ConversionofalcoholstocarboxylicacidsCarboxylicacidsareformedinthelaboratorybyoxidationofalcoholsusingaqueouspotassiummanganate(VII)whichactsasanoxidisingagentwhenacidifiedwithsulfuricacid.Ethanolisoxidisedtoethanoicacidbythismethod.Asimplifiedversionoftheequation,whichrepresentsoxygenfromthe

oxidisingagentas[O],is

Formulaeandnamesofalcohols•AlcoholsaremembersofahomologousserieswiththegeneralformulaCnH2n+1OH.

•Thenamesofalcoholsallendin-ol.•Alcoholscontainthefunctionalgroupwhichisthe-O–Hgroup.•AlcoholswithmorethantwocarbonatomshaveunbranchedstructuralisomersinwhichtheO–Hgroupcanbeindifferentpositionsonthecarbonchain.AnumberisusedtoindicatethepositionoftheO–Hgroup(Table15.2).

UnbranchedalcoholswithuptofourcarbonatomsareshowninTable15.2.

Carboxylicacids•Carboxylicacidsaremembersofahomologousseries.•Thenamesofcarboxylicacidsallendin-oicacid.•Thefunctionalgroupincarboxylicacidsis–COOHwhichcanalsobewrittenas–CO2H.

•ThisisdisplayedasshowninFigure15.1

UnbranchedcarboxylicacidswithuptofourcarbonatomsareshowninTable15.3.

PropertiesofaqueousethanoicacidEthanoicacid,CH3COOH,isatypicalweakacid.Itreactswithmetals,basesandcarbonatestoproducesalts(seeChapter8).Thesaltsarecalledethanoatesandcontaintheethanoateion,CH3COO−.

Esters•Estersaresweet-smellingliquids.•Thenamesofestersallendin-oate.•EstershaveageneralformulaofCnH2nO2.•Thefunctionalgroupinestersis–COORwhichcanalsobewrittenas–CO2R.Rrepresentsagroupcontainingcarbonandhydrogenatoms.

•ThefunctionalgroupinestersisdisplayedasshowninFigure15.2.•Estersaremadebythereactionbetweenacarboxylicacidandanalcohol.•Thetypeofreactioniscalledesterification.Thealcoholandcarboxylicacidareheatedwithacatalystofconcentratedsulfuricacid.

Thegeneralequationinwordsis:

Themoleculescanberepresentedas

Thereactionoccursas

Anexampleis

Namingestersisunlikenaminganyotherorganicmoleculeswehavemetsofar.TheformulaisdividedintotwoandeachpartisnamedaccordingtothenumberofcarbonatomsitcontainsasshowninFigure15.4.ThenamesandformulaeofunbranchedestersareshownTable15.4.

ThenamesandformulaeofunbranchedestersareshowninTable15.4.Allnamesshouldbetwocombinedwords.Thefirstofthesewordsismethyl,ethylorpropyl.

Examiner’stipsRememberthatmostformulaeoforganiccompoundsbeginwithaCatom.However,methanoicacidiswrittenHCOOHandbecauseitbeginswithanH,itsstructuralformulaisconstantlywrittenincorrectlybystudents.Thesamethingappliestomethanoateesters,suchasmethylmethanoatewhichshouldbewrittenHCOOCH3.

•Therearenoesterswithonecarbonatom.•Therearetwoisomericesterswiththreecarbonatoms.•Therearethreeisomericunbranchedesterswithfourcarbonatoms.

PolymersPolymersarelargemolecules(ofnodefinitesize)madeupbythereactionsofsmallmoleculescalledmonomers.Theformationofpolymersfrommonomersiscalledpolymerisation.Therearetwotypesofpolymerisationknownasadditionpolymerisation

andcondensationpolymerisation.

AdditionpolymerisationIfalkenessuchasethenearetreatedtoconditionsofhightemperatureandhighpressureinthepresenceofasuitablecatalyst,thedoublebondsbecomesinglebonds,makingmoreelectronsavailableforthecarbonatomstojointogetherandformlongchains.Thishappenstothousandsofethenemoleculeswhichjointogethertoform

onelong-chainmolecule.Thechemicalnameoftheproductispolyethene.Itscommercialnameispolythene.Thisreactionisknownasadditionpolymerisation,becausethemonomers

jointogetherwithouttheremovalofanyatoms.Thereisonlyoneproductjustasinotheradditionreactionsofalkenes(seeChapter14).TheequationforpolymerisationofetheneisshowninFigure15.5wheren

representsanumberlargerthan10000.

OtherexamplesofadditionpolymerisationTheoreticallyanymoleculewithacarbon–carbondoublebondcanformanadditionpolymer.Thechemicalnameofthepolymerisalwaysthesameasthatofthe

Thechemicalnameofthepolymerisalwaysthesameasthatofthemonomerwithpoly-infront.Propene,CH3CH==CH2,undergoesadditionpolymerisationtoform

polypropene.ThestructureofthepolymercanonlyberepresentedbydrawingthemonomerasinFigure15.6,changingthedoublebondtoasinglebondanddrawingtwoextensionbondsoneithersidewhichshowthatthepolymerextendsinbothdirections.

UsesofadditionpolymersAdditionpolymershaveseveraluseswhicharedependentonstrength,hardness,poorelectricalandthermalconductivity.ExamplesarePolytheneisusedforcarrierbags,bucketsandbowls.PVCisusedforwindowframes,gutteringandinsulatingelectricalwires.Polystyreneisusedforinsulationandpackaging.

Commonerror•Whenaskedtowriteanequationforthepolymerisationofpropene,acommonresponseis

Thisisincorrect.ThecarbonatomintheCH3grouphasfivebondsandthecarbonatomintheCH2grouphasthreebonds.Themonomermustbedrawnasadisplayed(orpartiallydisplayed)structure.

ThecorrectanswerisshowninFigure15.7.

CondensationpolymerisationAcondensationreactionisareactioninwhichtwomoleculesjointogetherandasimplemolecule,suchaswater,isremovedatthesametime.Esterificationisanexampleofacondensationreaction.Condensationpolymersareformedfrommonomerswithtwofunctional

groupseach.Asimplemolecule,suchaswater,iseliminatedasthemonomersjointogether.Examplesofsuchfunctionalgroupsare—OH,—COOHand—NH2.Polyestersandpolyamidesareexamplesofcondensationpolymers.

PolyestersPolyesterscanbemadefromdicarboxylicacids(moleculeswithtwo—COOH

groups)anddiols(moleculeswithtwo—OHgroups).Thesemonomerscanbe

representedas

Thepolymerisationoccursbytheremovalofamoleculeofwaterwhena—COOHgroupandan—OHgroupreact.Themonomersjointogetheras

Becausethereare—COOHgroupsand—OHgroupsatbothendsofthemonomers,morelinkagescanformandthepolymerchaincangrowinbothdirections.

UseofTeryleneTeryleneisanexampleofpolyestermadefromadicarboxylicacidandadiol.

Itisasyntheticfibreusedinclothingmanufacture.

Polyesterscanalsobemadeusingonemonomerwithbothan—OHanda—COOHgroup,forexample,

Inthiscase,therepeatunitshowstheresidueofthesinglemonomermolecule.

PolyamidesPolyamidescanbemadefromdicarboxylicacids(moleculeswithtwo—COOHgroups)anddiamines(moleculeswithtwo—NH2groups).Thesemonomers

canberepresentedas

Thepolymerisationoccursbytheremovalofamoleculeofwaterwhena—COOHgroupanda—NH2groupreact.Themonomersjointogetheras

Becausethereare—COOHgroupsand—NH2groupsatbothendsofthemonomers,morelinkagescanformandthepolymerchaincangrowinbothdirections.

UseofnylonNylonisanexampleofapolyamidemadefromadicarboxylicacidanda

diamine.Nylonisasyntheticfibreusedinclothingmanufacture.Itisalsousedin

ropes,parachutesandstringsfortennisrackets.

Polyamidescanalsobemadeusingonemonomerwithbothan—NH2anda—COOHgroup.

Table15.5Additionandcondensationpolymers

Additionpolymers CondensationpolymersMonomers ContainaC=Cdoublebond Containtworeactivefunctional

groups,e.g.—NH2,—COOH,—OH

PolymerisationOccurswithoutanylossofatomsproducingonlyoneproduct(thepolymer)

Occurswithremovalofsimplemolecule,e.g.water,producingtwoproducts

Polymers Havesameempiricalformulaasthemonomer

Havedifferentempiricalformulafromthemonomers

DisposalofpolymersHouseholdwastecontainslargequantitiesofpolymers(plasticobjects).Thesecanbedisposedofbyburyinginlandfillsitesincineration(burning).

Bothmethodsofdisposalcontributesignificantlytoenvironmentalpollution.

•Buryinginlandfillsitesmeansthatplasticswillremainintheenvironmentandtakeuplargeamountsofspace,especiallyiftheyarenon-biodegradable(whichadditionpolymersare).

•Incinerationcanleadtoproductionoftoxicgases,suchascarbonmonoxide,andacidicgases,suchashydrogenchloride,whichcontributetoacidrain.

Attemptstoovercometheseproblemsinclude•developmentofbiodegradableplastics(thosethatbreakdownintheenvironmentasaresultofbacterialactivity)•developmentofphotodegradableplastics(breakdowninsunlight)•sortingandrecyclingschemes.

NaturalpolymersFoodscontainproteinsandcomplexcarbohydrates(polysaccharides,suchasstarch).

ProteinsProteinsarenaturalpolyamideswhicharemadefromaminoacidmonomers.Thereare20differentaminoacids.Allhavean—NH2(amine)anda—COOH(carboxylicacid)functionalgroup.Thesegroupsreacttogetherbycondensationpolymerisationtoproduceproteinswhichhaveaminoacidresiduesinasequencewhichisspecifictoeachindividualprotein.

Proteinscontainthesameamidelinkageasthatpresentinsyntheticpolyamides,suchasnylon,althoughbiologistsusuallyrefertoitasapeptidelinkagewhenitexistsinproteins.

ComplexcarbohydratesComplexcarbohydrates,suchasstarch,arenaturallyoccurringcondensationpolymersmadewhenglucosemonomers

jointogetherwiththeremovalofwatermoleculestoformstarch.

HydrolysisofnaturalpolymersProteinsTheconstituentaminoacidsthatarepresentindifferentproteinscanbeidentifiedbyheatingfor24hourswith6.0moldm−3(concentrated)hydrochloricacid.Theproteinundergoesacidcatalysedhydrolysis(breakdownbyreactionwithwater)andisconvertedtoitsconstituentaminoacids.(Thisisthereverseofcondensationpolymerisation.)•separatingtheaminoacidsbypaperchromatography(seeChapter2)•sprayingthechromatogram(chromatographypaper)withalocatingagent(ninhydrinisused),sothatthecolourlessaminoacidsproducevisiblebluespots•determiningRfvaluesandcomparingwithRfvaluesofknownaminoacidsinadatabook(seeChapter2).

ComplexcarbohydratesComplexcarbohydratescanalsobehydrolysedbyhydrochloricacid.Thisbreaksthecomplexcarbohydratedowntosimplesugars.Thecomplexcarbohydratestarchwouldbebrokendownintoglucosebyhydrolysis.Hydrolysiscanalsobecatalysedbyenzymes.Starchisbrokendownbythe

enzymeamylaseinsalivatoproducemaltose(adisaccharidemadefromtwoglucoseunits).

Exam-stylequestions1Threecompounds,A,BandC,allhavethemolecularformulaC3H8O.Areactswithethanoicacidtoproduceacompoundwiththestructureshownbelow.

BreactswithethanoicacidtoproduceacompoundwiththemolecularformulaC5H10O2.

Cdoesnotreactwithethanoicacid,butitundergoescompletecombustionwhenburnedinexcessoxygen.

aWhatismeantbytheterm‘molecularformula’?[1mark]

bWhatisthetermusedtodescribecompoundswiththesamemolecularformulabutdifferentstructuralformulae?

[1mark]cWhatistheempiricalformulaofthecompoundwiththemolecular

formulaC5H10O2?

[1mark]dWhattypeofreactionoccursbetweenAandethanoicacid?

[1mark]eWhataretheconditionsthatarerequiredforAtoreactwithethanoicacid?

[2marks]fCompletethechemicalequationforthereactionoccurringwhenC

undergoescompletecombustioninexcessoxygen.Statesymbolsarenotrequired.

[2marks]

gGivethestructuresofmoleculesA,BandC.Showalltheatomsandallthebonds.

[3marks][Total:11marks]

2Thediagramshowspartofapolymer.

aWhattypeofapolymerisshown?[1mark]

bDrawacirclearoundonerepeatunitofthepolymer.[1mark]

ciDrawthestructureofonemoleculeofthemonomer.Showalltheatomsandallthebonds.

iiNamethemonomer.[2marks]

[Total:4marks]3Thediagramshowspartofapolymerwhichisformedbycondensationpolymerisation.

aWhatismeantbycondensationpolymerisation?[2marks]

bWhattypeofcondensationpolymerisshown?[1mark]

cDrawacirclearoundonerepeatunitofthepolymer.Labeltherepeatunit.[1mark]

dDrawacirclearoundthelinkageinthepolymer.Labelthelinkage.[1mark]

eWhattypeofbiologicalmoleculecontainsthesamelinkageasthepolymershown?

[1mark]fDrawthestructuresofthetwomonomers,showingalltheatomsand

bondsinthefunctionalgroups.(YoumayleaveC6H4asitiswritten.)[2marks]

[Total:8marks]4Thediagramshowspartofacondensationpolymermadefromonemonomer.

aWhattypeofacondensationpolymerisdrawn?[1mark]

bDrawacirclearoundtherepeatunitofthepolymer.[1mark]

cDrawthestructureofthemonomershowingallatomsandbonds.[1mark]

dWhatarethenamesofthetwofunctionalgroupsinthemonomer?[2marks]

[Total:5marks]

16Experimentalchemistry

KeyobjectivesBytheendofthissection,youshouldbeabletodescribeteststoidentify

•aqueouscations:aluminium,ammonium,calcium,chromium(III),copper(II),iron(II),iron(III),andzinc•cations:useofflameteststoidentifylithium,sodium,potassium,copper(II)•anions:chloride,bromide,iodide,carbonate,sulfite,sulfateandnitrate•gases:ammonia,carbondioxide,oxygen,hydrogen,chlorineandsulfurdioxide.

Testingforcations(positiveions)Cations(positiveions)canbeidentifiedusingaqueoussodiumhydroxide•aqueousammonia•flametests.

Usingaqueoussodiumhydroxide

Usingaqueousammonia

FlametestsFlametestscanbecarriedoutonsolidsoronaqueoussolutions.

•Startingwithasolid,afewdropsofconcentratedhydrochloricacidshouldbeaddedtoasampleofthesolidonawatchglass.Dilutehydrochloricacidcanbeusedtoavoidsafetyissues.

•Asmallamountofthemixtureshouldthenbeplacedonanichromewire.•ThenichromewirecontainingsomeofthemixtureisthenplacedinthehotpartofaBunsenflame.

•Thecolouroftheflameidentifiesthecation(positiveion)(Table16.1).

Table16.1

Cation(positiveion) FlamecolourLithium,Li+ Red

Sodium,Na+ Yellow

Potassium,K+ Lilac

Copper(II),Cu2+ Blue-green

Testingforanions(negativeions)TestsforanionsandtheirresultsareshowninTable16.2.

Table16.2

Test Result Anion

Adddilutenitricacidfollowedbyaqueoussilvernitrate

Whiteprecipitate Chloride,CI−

Creamprecipitate Bromide,Br−

Yellowprecipitate Iodide,I−

Addanydiluteacid

BubblesGasgivenoffturnslimewatermilky(gasisCO2)

Carbonate,

GasgivenoffwhenwarmedGasturnsacidifiedaqueouspotassiummanganate(VII)colourless(gasisSO2)

Sulfite,

Adddilutenitricacidfollowedbyaqueousbariumnitrate

Whiteprecipitate Sulfate,

Addaqueoussodiumhydroxidefollowedbyaluminium.Warmgently

Gasgivenoffturnsdampredlitmuspaperblue(gasisNH3)

Nitrate,

TestingforgasesTestsforgasesandtheirresultsareshowninTable16.3.

Table16.3

Test Result GasDampredlitmus Turnsblue Ammonia,NH3Limewater Turnsmilky Carbondioxide,

CO2Glowingsplint Lights Oxygen,O2Burningsplint Pops Hydrogen,H2Damplitmuspaper Bleached Chlorine,Cl2Acidifiedaqueouspotassiummanganate(VII)

Changesfrompurpletocolourless

Sulfurdioxide,SO2

Answerstoexam-stylequestions

Chapter1

1Solid[1]Bothmeltingpointandboilingpointareabove50°C.[1]Althoughthemeltingpointbeingabove50°Cisreallyenoughofanexplanation,itisadvisabletorefertobothmeltingpointandboilingpoint.

2aC[1]Solidshaveparticlesthatareveryclosetogether,orderedandvibrate

aboutafixedposition.bB[1]Liquidshaveparticlesthatarefairlyclosetogether,irregularlyarranged

andmoveslowly.cD[1]Gaseshaveparticlesveryfarapart,arrangedrandomlyandmovingathigh

speeds.dA[1]Particlesthatareveryfarapartmustbeinagaswhichmeanstheycannot

beorderedorvibrateaboutfixedpositions.Thequestiondoesnotaskforexplanations,soitisunnecessarytogiveany.

Chapter21aDissolving(sugarinwater)[1]Filtration(toremovesand)[1]Crystallisation[1]bDistillation[1](Simple)distillation,ratherthanfractionaldistillationisusedtoseparatea

pureliquidfromasolution.cFractionaldistillation[1]Fractionaldistillationisusedtoseparateamixtureoftwoormoreliquids

withdifferentboilingpoints.(Simple)distillationisincorrect.dFiltration[1]Washingwithdistilledwater[1]Dryingonawarmwindowsillorinalowoven[1]Aprecipitateisaninsoluble/undissolvedsolid.Centrifugationisanother

alternative.Neitheroftheseprocesseswillproduceapuresolid,becausethesolidwillbecontaminatedwithasmallamountofthesolutionitwasseparatedfrom.Therefore,washingwithdistilledwateranddryingarebothrequiredinaddition.

2Heatuntilcrystalsformonaglassrodplacedinthesolutionandwithdrawn.[1]

Leavethehotsaturatedsolutiontocoolslowly.[1]Crystalsshouldthenform.Removecrystals(byfiltrationifthereisanyliquidleft).[1]Washwithasmallamountofcolddistilledwaterandthendryinalowovenoronawarmwindowsill.[1]

Commonmistakesinclude•Notstatingwhentheheatingshouldbestopped.•Evaporatingtodryness.Inthiscasewaterofcrystallisationwouldbe

removed,leavingtheanhydroussalt.•Notspecifyingthatasmallamountofcolddistilledwaterbeusedfor

washing.Thereisadangerofdissolvingthecrystalsaswellasremovingimpuritiesiftoomuchisusedorifthewaterisnotcold.

•Ifanovenisusedfordryingitshouldbelow.Toomuchheatwouldcause

thecrystalstodecompose(particularlythosethatcontainwaterofcrystallisation).

3Carryoutpaperchromatographyusingasuitablesolvent.[1]Allowsolventtoreachthetopofthechromatographypaper.[1]Removechromatographypaperandallowtodry.[1]Spraywithlocatingagent.[1]MeasureRfvaluesandcomparewithdatabookvaluestoidentifyaminoacids.[1]

Asimplediagramofhowpaperchromatographyiscarriedoutisthebestwaytodescribehowtheapparatusissetupevenifthequestiondoesnotaskforadiagram.

Itisacommonerrortodrawthesolventlevelabovethestartinglineonthechromatographypaper.

Chapter31

2a

b

c

d

e

[Total:5marks]Studentsareencouragedtousethetypeofdiagramsshown.Inmanycasesitisverydifficulttodecidewhichelectronsaresharedifdiagramsarebadlydrawn.

‘Outershellelectrons’onlyarerequested.3aMg(OH)2bCaCl2c(NH4)3PO4dLi2S

ePb(NO3)2fCaCO3gAl(NO3)3hK2SO3iZnSO4j(NH4)2SO4

[1markeach][Total:10marks]

Pleasenote:(d)asksforasulfideand(h)asksforasulfite,whereasboth(i)and(j)askforasulfate.

4aB,C,DandESolidshavemeltingpointsandboilingpointsabove25°C.bAAhasameltingpointbelow25°Candaboilingpointabove25°Csoitis

aliquidat25°C.cFFhasbothmeltingpointandboilingpointbelow25°Csoitisagasat25

°C.dDDistheonlyonetoconductelectricitywhensolid;thereforeDcouldhave

agiantmetallicstructure.(Itcouldalsobegraphiteorgraphene.)eCChasahighmeltingpointandahighboilingpointandconductselectricity

whenmoltenbutnotwhensolid.ThereforeChasagiantionicstructure.fEEhasahighmeltingpointandahighboilingpointwhichmeansithasa

giantstructure.Eisanon-conductorbothwhensolid(soitisnotagiantmetallicstructure)

andwhenmolten(soitisnotagiantionicstructure).ThereforeEhasagiantmolecularstructure.

[1markeach][Total:6marks]

Chapter41MrofH2=(1×2)=2MolesofAl=8.1÷27=0.3moles[1]Moleratiofromtheequation2moleAl:3moleH20.30molesAl:0.30×3/2=0.45molesH2[1]MassofH2=moles×massof1mole=0.45×2=0.90g[1]

[Total:3marks]2MrofKO2=39+(16×2)=71MolesofKO2=0.142÷71=0.002moles[1]Moleratiofromtheequation4moleKO2:3moleO20.002molesKO2:0.002×3/4=0.0015molesofO2[1]VolumeofO2(g)=moles×volumeofonemoleofgas=0.0015×24=0.036dm3[1]

[Total:3marks]3MrofCaC2=40+(12×2)=64MolesofC2H2(g)=120÷24000=0.005molesC2H2(g)[1](Thevolumesmustbothbeinthesameunits:seebelow.)Becausethevolumeinthequestionisgivenincm3,thevolumeof1moleofagasmustbeconvertedfrom24dm3to24000cm3.Alternatively,120cm3couldbeconvertedto0.120dm3.

Moleratiofromtheequation1moleC2H2:1moleCaC20.005molesC2H2:0.005molesCaC2[1]MassofCaC2=moles×massof1mole=0.005×64=0.32g[1]

4Molesof [1]

Moleratioinequation

1moleH2SO4:2molesKOH0.007molesH2SO4:2×0.007=0.014molesKOH[1]aconcentrationofKOH

=0.70moldm−3[1]bMrofKOH=39+16+1=56

Toconvertconcentrationinmoldm−3toconcentrationingdm−3Mass(grams)=moles×Mr=0.70×56=39.2gdm−3[1]

[Total:4marks]5aCalculatethenumberofmolesofatomsofeachelement.Carbon,C54.5÷12=4.54Hydrogen,H9.1÷1=9.1Oxygen,O36.4÷16=2.275DividealltheabovebythesmallestC4.54÷2.275=2H9.1÷2.275=4O2.275÷2.275=1Empiricalformula=C2H4ObIftheMrofthecompound=44TheMrofC2H4O=(12×2)+(1×4)+16=44n=Mrofthecompound÷Mrofempiricalformulan=44÷44=1Therefore,molecularformula=C2H4O×1=C2H4O6MrofTiCl4=48+(35.5×4)=190MolesofTiCl4=0.38÷190=0.002moles[1]Moleratio1moleofTiCl4:1moleofTiTherefore,0.002molesofTiCl4:0.002molesofTi[1]

MassofTi=0.002×48=0.096g[1]0.096gis100%.Buttheyieldisonly0.024g.Percentageyield=actualyield÷100%yield×100%Percentageyield=0.024÷0.096×100=25.0%[1]

[Total:4marks]Itisessentialtoshowallworkingoutinallcalculations.

Chapter51

Electrolyte Nameofproductatanode(+)

Nameofproductatcathode(−)

Moltenpotassiumbromide

Bromine[1] Potassium[1]

Aqueouspotassiumbromide

Bromine[1] Hydrogen[1]

Moltenleadiodide Iodine[1] Lead[1]Aqueouscopper(II)chloride

Chlorine[1] Copper[1]

Aqueoussodiumsulfate Oxygen[1] Hydrogen[1]

Moltenelectrolytescanonlyproducethetwoelementspresentinthemoltencompound.

Aqueouselectrolytesproduce•halogensoroxygenattheanode(exceptionsapplywhentheanodeismade

ofanunreactivemetalsuchascopper)•metalsbelowhydrogeninthereactivityseriesorhydrogenatthecathode.

Metalsabovehydrogeninthereactivityseriesareneverproducedbyelectrolysisofaqueoussolutions.

2aNickelistheanode.[1]bAsolutioncontainingnickelionsistheelectrolyte.[1]cTheobjecttobeplated,i.e.theknife,isthecathode.[1]Studentsmaynotbefamiliarwithnickelcompounds,butifaskedtonamea

solublecompounditissafetoassumethatnitratesarealwayssolubleinwater.Thereforeaqueousnickelnitratecouldbechosenastheelectrolyte.

3aIodine[1]Halogensarealwaysproducedattheanodefromhalides(inpreferenceto

oxygen).b [1]Potassiumisneverproducedbyelectrolysisofanaqueoussolution.

Itisunnecessarytogivestatesymbolsinequationsunlessrequested.cOxidation[1]Oxidationalwaysoccursattheanodebecauseelectronsarealwayslostby

ionsbeingdischargedattheanode.d(Aqueous)potassiumhydroxide[1]Hydrogenions(fromwater)aredischargedatthecathodeandiodideions

aredischargedatthecathode.Thisleavespotassiumions,K+,andhydroxideions,OH−(fromwater),inthesolution,whichbecomes(aqueous)potassiumhydroxide.

Thisissimilartotheelectrolysisofconcentratedaqueoussodiumchlorideinwhichhydrogenandchlorineareproducedattheelectrodesandtheelectrolytebecomesaqueoussodiumhydroxide.

eElectrons[1]Conductingwires,beingmetallic,conductelectricitybecausetheycontain

movingelectrons.‘Ions’isacommonincorrectanswer.f(K+andI−)ions[1]Electrolytesconductelectricitybecausetheycontainmovingions.‘Electrons’isacommonincorrectanswer.

Chapter61a

[2]b2C–C,8C–H,5O=O[2]Itisacommonerrortoinclude3C–CbondsbecausetheformulaisC3H8.c(2×347)+(8×435)+(5×497)=6659kJ[1]d6C=O,8O–H[2]e(6×803)+(8×464)=8530kJ[1]f8530−6659=1871kJ/mole.Thereactionisexothermic.[3]Becausetheamountofenergygivenoutwhennewbondsforminthe

products(8530)isbiggerthantheamountofenergytakenintobreakbondsinthereactants(6659),theoverallenergychangeisexothermic.

Chapter71aPhysical[1]Thisproducesaqueoussodiumchloride.Thesodiumchlorideandwater

arenotchangedintoanysubstancesthatarenotthereatthestart.Sinceanewchemicalsubstanceisnotproduceditisaphysicalchange.

bChemical[1]Thisproducesthreenewchemicalsubstances,i.e.chlorine,hydrogenand

sodiumhydroxide.Therefore,achemicalchangeoccurs.cChemical[1]Thesilverchlorideischemicallychangedintosilverandchlorine.dPhysical[1]Thisisamethodofseparationanddoesnotproduceanynewchemical

substances.ePhysical[1]Thisisamethodofseparationanddoesnotproduceanynewchemical

substances.2

Thevolumeofhydrogenproduceddependsonlyonthenumberofmolesofhydrochloricacidused(becausethecalciumcarbonateisinexcess).

Experiment2istheonlyexperimentinwhichmoremolesofhydrochloricacidareproduced.Thenumberofmolesofhydrochloricacidisdoubledandsothevolumeofhydrogenisdoubled.Becausetheconcentrationofhydrochloricacidisgreater,therateofreactionisfasterandthegraphissteeper(GraphD).

Increasingthesurfacearea(Experiment3)andincreasingthetemperature(Experiment5)bothincreasetherateofreactionwithoutchangingthevolumeofhydrogen.Thusthegraphissteeperinbothcasesandlevelsoffatthesamevolume(GraphB).

InExperiment4,decreasingthetemperaturedecreasestherateandthegraphislesssteepbutlevelsoffatthesamevolumebecausethevolumeofhydrogenisunchanged(GraphE).

3aTherearethesamenumbersofgasmolecules(2)onbothsidesofthesign;thereforeincreasingthepressuredoesnotfavoureitherreaction.[1]

bTherearetwomoleculesofgasontherightandthreemoleculesofgasontheleftofthe sign;thereforeincreasingthepressurecausesequilibriumtoshiftinthedirectionoffewergasmolecules,whichmeanstotheright.[1]

cTherearetwomoleculesofgasontherightandonemoleculeofgasontheleftofthe sign;thereforeincreasingthepressurecausesequilibriumtoshiftinthedirectionoffewergasmolecules,whichmeanstotheleft.[1]

4aTheforwardreactionisexothermic,whichmeansthatthereversereactionisendothermic.Whentemperatureisdecreased,equilibriumalwaysshiftsintheexothermicdirection.Inthiscase,equilibriumshiftstotheright.[1]

bTheforwardreactionisendothermic,whichmeansthatthereversereactionisexothermic.Whentemperatureisdecreasedequilibriumalwaysshiftsintheexothermicdirection.Inthiscase,equilibriumshiftstotheleft.[1]

5aMg(s)+Cu2+(aq)→Mg2+(aq)+Cu(s)[1]

Thesulfateions,So42+,arespectatorions.Statesymbolsneednotbegiveninequationsunlessaskedfor.bOxidation:Mg(s)→Mg2+(aq)+2e−[1]

Reduction:Cu2+(aq)+2e−→Cu(s)[1]Thetotalnumberofchargesonbothsidesofanyequationmustbeequal.cCu2+[1]isanelectronacceptor/gainselectrons.[1]Oxidisingagentsarealwaysreduced.

Oxidisingagentsarealwaysonthesamesideastheelectronsinanionichalf-equation.

dMg[1]isanelectrondonor/loseselectrons.[1]Reducingagentsarealwaysoxidised.Reducingagentsarealwaysontheoppositesidetotheelectronsinanionic

half-equation.

Chapter81aiMethod1ii(Dilute)hydrochloricacid[1]Hydrochloricacidproduceschlorides.iiiCoCO3+2HCl→CoCl2+CO2+H2O[1formulae,1balancing]Cobaltcompoundsarenotallthatwellknown,butthe(II)incobalt(ii)

carbonateandcobalt(II)chloridemeanthatacobaltionisCo2+,thusmakingitstraightforwardtoderivetheformulaeofthecompounds.

biMethod3ii(Aqueous)sodiumiodide[1]Todecideonachoiceofanaqueoussolutioncontainingiodideions,you

shouldrememberthatallsodium(orpotassium)saltsaresolubleinwater.

iii2NaI+Pb(NO3)2→PbI2+2NaNO3[1formulae,1balancing]ciMethod2[1]ii(Dilute)nitricacid[1]Nitricacidproducesnitrates.iiiKOH+HNO3→KNO3+H2O[1formulae,1balancing]2•Pourdilutesulfuricacid[1]intoabeaker.•Addmagnesiumcarbonate.[1]•Stirorwarm.[1]•Stopaddingmagnesiumcarbonatewhensomeremainsundissolved/nomore

bubblesofgasevolved.[1]•Filteroffexcessmagnesiumcarbonate.[1]•Heatthefiltrateuntilcrystalsformonaglassrodplacedinthesolutionand

withdrawn.[1]•Leavethehotsaturatedsolutiontocoolslowly.[1]Crystalsshouldthen

form.•Removecrystals(byfiltrationifthereisanyliquidleft).[1]•Washwithasmallamountofcolddistilledwaterandthendryinalowoven

oronawarmwindowsill.[1]•MgCO3+H2SO4→MgSO4+CO2+H2O[1]3•Addaqueoussodiumhydroxideorpotassiumhydroxide.[1]•Stir/warmtodissolvescandiumoxide.[1]•Filteroffcopper(II)oxide.[1]•Washwithdistilledwater.[1]•Drythecopper(II)oxideonawarmwindowsill/inalowoven.[1]

Chapter91aB[1]GroupIelementsbecomemorereactivedownthegroup.bD[1]GroupVIIelementsbecomemorereactiveupthegroup.cEitherForG[1]dA[1]Periodsarethehorizontalrows.eA[1]AtomsofGroupIVelementshavefourelectronsintheiroutershells.All

atomshavethesamenumberofouterelectronsastheirgroupnumbers.2aBubbles,orlithiumfloats,orlithiumdisappears,orlithiummelts,orlithium

movesaroundAnythree[3]Itisacommonerrortosaylithiumburstsintoflame.Onlytheelements

frompotassiumdownwardsburstintoflame.b2Li(s)+2H2O(l)→2LiOH(aq)+H2(g)[1formulae,1balancing,1state

symbols]Thisequationisverycommonlyaskedfor(withanyGroupIelement)in

exams.TheformulaeandbalancingnumbersarethesameforallGroupIelements;onlythesymboloftheelementchanges.

cYellow[1]Thecoloursofmethylorangeandlitmusinacidicandalkalinesolutions

shouldbememorised.3aCl2(g)+2KI(aq)→2KCl(aq)+I2(aq)[1formulae,1balancing,1statesymbols]Cl2(g)+2I−(aq)→2Cl−(aq)+I2[1]bBr2(l)+2KI(aq)→2KBr(aq)+I2(aq)[1formulae,1balancing,1state

symbols]Br2(l)+2I−(aq)→2Br−(aq)+I2(aq)[1]I2(s)isacceptableinbothcasesbecausesomeofitmayformasa

precipitate.

Thehalogenelementsareoftendissolvedinwatertocarryoutthesereactions,inwhichcase(aq)wouldbeusedasthestatesymbol.

Potassiumionsarespectatorionsinbothreactions.Ifaqueoussodiumiodidewasusedineitherorbothcases,thesodiumionswouldbespectatorionsandtheionicequationswouldbeexactlythesame.

4aF2+2KCl→2KF+Cl2[1formulae,1balancing]bNoreaction[1]cBr2+2KAt→2KBr+At2[1formulae,1balancing]dNoreaction[1]HalogensnearerthetopofGroupVIIdisplacethoselowerdownbutnot

viceversa.Reactionsinvolvingfluorineonlyoccurintheory,becauseinpractice

fluorinereactsviolentlywithwatersocouldnotbeused.Astatineisradioactive.5aCu2O[1]bCu(NO3)2[1]cFeCl2[1]dFe2(SO4)3[1]Theromannumeralsusedtorepresentoxidationstatesarethesameasthe

numberofpositivechargesonthecations:copper(I)referstoCu+,iron(II)referstoFe2+andiron(III)referstoFe3+.

Chapter101aB,C,A,D[1]bB+A(NO3)2→A+B(NO3)2[1]

cC+D2+→D+C2+[1]BecausethenitrateionisNO3−,itcanbededucedthatallthechargeson

themetalionsare2+because1metalioniscombinedwith2nitrateionsineachcompoundascanbeseenfromtheformulae.

dB[1]Themostreactivemetalisalwaysthenegativeterminalinacellcontaining

twodissimilarmetalsinanelectrolyte,becausethemostreactivemetalhasthegreatesttendencytoreleaseelectrons(whichmakesitnegative)asitformspositiveions.

eB+DO→BO+D[1]BecauseBandDhaveionswiththeformulaB2+andD2+,andbecausethe

oxideionisO2−,theformulaeofthemetaloxidesareBOandDO.2•Addadiluteacidinexcess(preferablyhydrochloricorsulfuricbutnot

nitric).[1]•Stirorwarmorboth.•Zincreactsanddissolvesbutcopperdoesneither;bubblingisalsoseenwith

zincbutnotwithcopper.[1]•Filteroffcopper.[1]•Washwithdistilledwateranddryinalowoven.[1]

[Total:4marks]Studentsshouldmemorisethepositioninthereactivityseriesofthose

elementsreferredtoonthesyllabus.Zincisabovehydrogenbutcopperisbelowhydrogen.Thuszincreactswithdilutehydrochloricandsulfuricacidstoproduceanaqueoussaltsolution.Hydrogengasisevolved.

3

[Total:4marks]Becausebothelectrodescontainnickel,theanodemustbeclearlylabelledasimpurenickelandthecathodeaspurenickel.Studentsshouldrememberthatallnitratesaresolubleinwater;thereforeaqueousnickelnitrate(ornickelnitratesolution)isasuitableelectrolyte.

4aB,copper(II)nitrate[1]C,copper(II)oxide[1]D,nitrogendioxide[1]E,oxygen[1]Studentsareaskedtonamethesubstances.Itisacommonerrortowrite

formulae.biCuO+2HNO3→Cu(NO3)2+H2O[1formulae,1balancing]ii2Cu(NO3)2→2CuO+4NO2+O2[1formulae,1balancing]cCopper(II)hydroxide[1]Copper(II)carbonate[1]Bases(copper(II)hydroxide)andcarbonates(copper(II)carbonate)are

usedtoreactwithdiluteacidstoformaqueoussolutionsofsolublesalts.Coppermetalisnotsuitablebecausecopperisbelowhydrogeninthe

reactivityseriesandthereforeitdoesnotreactwithdiluteacids.

Chapter111aBrown[1]toblack[1]Acolourchangeisrequested;thereforeitisessentialtogivetheoriginal

andfinalcolours.b2Cu+O2→2CuO[1formulae,1balancing]cOxygengasistakenout/removedfromtheair.[1]d‘Excess’inthiscasemeansthatthereismorethanenoughcoppertoreact

withalltheoxygen.Somecopperwillbeleftoverwhenalltheoxygenhasreacted.[1]

Studentscommonlyanswerthatexcessmeans‘morethanenough’butitisimportanttoaddmorethanenoughtoreact.Thepointisemphasisedbysayingsomewillbeleftoverafterthereaction.

eNitrogen[1]2aAcatalystincreasestherateofachemicalreaction[1]andischemically

unchangedattheendofthereaction.[1]b

Yieldofammonia

Rateofproductionofammonia

Highertemperaturethan450°C Decrease[1] Increase[1]Higherpressurethan200atmospheres

Increase[1] Increase[1]

Notusingacatalyst Nochange[1] Decrease[1]

Thetablemakesdistinctionsbetweenyield(equilibriumposition)andrateofreaction.Studentsshouldmakesurethattheydonotconfuseonewiththeother(seeChapter7).Wheretherearechangestheanswersmustbecomparative,e.g.loweryieldandnotlowyield.

ciSulfuricacid[1]ii2NH3+H2SO4→(NH4)2SO4[1formulae,1balancing]d3NH3+H3PO4→(NH4)3PO4[1formulaof(NH4)3PO4,1balancing]

Chapter121aVanadium(V)oxideorvanadiumpentoxide[1]Theoxidationstateofvanadiumisanessentialpartofthename

vanadium(V)oxide.Iron(thecatalystintheHaberprocess)isacommonerror.bBelow450°C,therateofreactionwouldbeslower.[1]cTherateofreactionwouldincreaseatahigherpressure[1]becausethegas

moleculeswouldbeclosertogetherandthereforetherewouldbemorecollisionsinanygiventime.[1]

Statingthatgasmoleculeswouldbeclosertogetherthereforetherewouldbemorecollisionsisnotenoughforthesecondmark.Referencetotimeisessential.

dTheyieldofsulfurtrioxideandtherateofreactionwouldbehighenoughat1–2atmospheres.

orTherewouldberisksofexplosions/leakagesathigherpressure.orItwouldbetooexpensivetobuildanindustrialplanttowithstandhigher

pressure.Anytwo[2]eSulfurtrioxideisdissolvedin98%concentratedsulfuricacidtoproduce

oleum.SO3+H2SO4→H2S2O7[1]Theoleumisthenaddedtothecorrectamountofwatertoproducesulfuric

acidoftherequiredconcentration.H2S2O7+H2O→2H2SO4[1]2aH2SO4+CuCO3→CuSO4+CO2+H2O[1formulaeofCuCO3andCuSO4,1restofequationcompletelycorrect]bH2SO4+2NaOH→Na2SO4+2H2O[1formulae,1balancing]cH2SO4+KOH→KHSO4+H2O[1]dH2SO4+Zn→ZnSO4+H2[1]

Chapter131PlantsgrowbetterinsoilsatspecificpHvalues.[1]Calciumoxideorcalciumhydroxide[1]Lime,quicklimeorslakedlimearecommonnames(asalsoarelimestoneandlimewater)andnotchemicalnames.

2ai2C8H18+25O2→16CO2+18H2O[1formulae,1balancing]Fractions/multiplesareacceptableinequationsunlessstatedotherwise.iiCaCO3→CaO+CO2[1]iiiFe2O3+3CO→2Fe+3CO2[1formulae,1balancing]ivCaCO3+2HCl→CaCl2+CO2+H2O[1formulae,1balancing]biWater[1]iiGlucose[1],oxygen[1]iiiUVlight/sunlight[1],chlorophyll[1]

Chapter141

[2]Whendrawingmoleculesitisessentialthatallcarbonatomshavefourbondsonly.

Itisalsoessentialtodrawtwodifferentmolecules,andnotdrawthesamemoleculetwiceasinthefollowingexamples.

Theseshowthesamemoleculedrawnseveraldifferentways.Inalltheexamples,thedoublebondisbetweenthefirsttwocarbonatomsandallfivecarbonatomsareina‘straightchain’eveniftheyaredrawnatanangleinsomecases.

2aUltravioletlight[1]bChloroethane[1]cAnyoneofthefollowing:C2H4Cl2/C2H3Cl3/C2H2Cl4/C2HCl5/

C2Cl6[1]

Someorallofthehydrogenatomsinethanecanbesubstitutedbychlorineatoms.Thetotalnumberofatomsbondedtothetwocarbonatomsmustbe

sixineachcase.3aAddition[1]bThebrominechangescolourfromyellow/brown[1]tocolourless.[1]Whenthebrominetestforunsaturationisdescribed,itisessentialtogive

theinitialandfinalcolours.Ifexcesspropeneisnotusedtheremaybesomebromineleftandsothe

colourofthebrominemaystillbevisible.ciC3H6Br2[1]Twobromineatomsareaddedtothepropene.iiC3H8[1]Twohydrogenatomsareaddedtothepropene.iiiC3H8O[1]Awatermoleculeisaddedtothepropene,i.e.twohydrogenatomsand

anoxygenatomareadded.dNickel[1]4aC9H20[1]Thecorrectanswerisobtainedusingn=9inthegeneralformulafor

alkaneswhichisCnH2n+2.bAnyonefromthefollowing:

[formula1,name1]Allowstructuralformulaeordisplayedformulaebecause‘showingallthe

atomsandbonds’isnotrequested.StructuralformulaemustshowtheC=Cdoublebond.

ciC8H18→C4H10+2C2H4[1]iiC8H18→C5H10+C3H6+H2orC8H18→C2H4+C6H12+H2orC8H18→C2H4+2C3H6+H2[1]ThereareacceptableanswerswithanalkyneCnH2n−2andanalkaneas

theproductsaswellashydrogen.

Chapter151aTheformulathatshowsthenumberofatomsofeachelementinone

moleculeofanelementorcompound.[1]bStructuralisomers[1]Theword‘structural’isoftenomitted.cC5H10O2[1]Thisformulacannotbesimplifiedanyfurther.Thereforethemolecular

formulaandtheempiricalformulaarethesame.dEsterification[1]Condensationwouldalsobeacceptable.eHeat[1]andacatalystofconcentratedsulfuricacid[1]f2C3H8O+9O2→6CO2+8H2O[1formulae,1balancing]Fractions/multiplesareacceptedinequations.g

[3]ThediagrambelowshowspartsoftheestersformedfromAandB.

TheoxygenatomintheesterformedfromAisjoinedtocarbonnumber2.ThereforetheO–HgroupinalcoholAmusthavebeenoncarbonnumber2.

TheoxygenatomintheesterformedfromBisjoinedtocarbonnumber1.ThereforetheO–HgroupinalcoholBmusthavebeenoncarbonnumber1.

CdoesnotreactwithethanoicacidwhichmeansthatCisnotanalcoholanddoesnothaveanO–Hgroup.TheonlymoleculethatcanbedrawnwiththemolecularformulaC3H8Othatisnotanalcoholandobeystherulesthat

•carbonatomshaveonlyfourbonds•oxygenatomshaveonlytwobonds,

and•hydrogenatomshaveonlyonebondis2aAdditionpolymer[1]b

[1]Thecircleshouldbedrawnaroundtwoconsecutivecarbonatomsinthe

mainchainandalltheatomsandgroupsofatomsjoinedtothem.Oneexampleisgiven.

ci

[1]iiBut-1-ene[1]3aCondensationpolymerisationistheformationofalong-chainmolecule(the

polymer)fromsmallmolecules(monomers)[1].Asimplemoleculesuchaswateriseliminatedasthemonomersjointogether.[1]

bApolyamide[1]c,d

[1each]eAprotein[1]f

[2]Incarboxylicacidsandalcohols,itisacommonerrorforstudentsnotto

drawthebondbetweenOandHatoms,i.e.theydraw–OHinsteadof–O–H.

4aApolyester[1]b

[1]c

[1]dCarboxylicacid[1]Alcohol[1]

Index

Aacidicoxides60acids57–9activationenergy51additionpolymerisation103additionreactions93air79–80airpollution80alcohols98–100alkalimetals64alkalis58,59alkanes92–3alkenes93–7allotropes91alloys69aluminium(Al)71,73–4,76aluminiumchloride20ammonia(NH3)21,82,111ammoniumsalts59,61amphotericoxides60anions,testingfor111anode37,39aqueousammonia111aqueouspotassiumiodide55aqueouspotassiummanganate55aqueoussodiumhydroxide110argon(Ar)67atomicnumber(protonnumber)14atomicstructure14–15atoms,electronarrangement15–16

Avogadroconstant26

Bbases57,58,59basicoxides60benzene33boiling1,2,3boilingpoint1,2,69bondenergies43,45bromide(Br−),testingfor111bromine(Br)65–6,95Brownianmotion1,4butane33,91,92

Ccalcium(Ca),reactionwithwater70carbohydrates,fermentation98carbonates(CO32−)58,61,72,88,111carboncycle88carbondioxide(CO2)21,87,111carbon,inorganicchemistry87–9carbonmonoxide(CO)80carboxylicacids98,100–1catalysts47catalyticconverters78,80catalyticcracking96cathode37,39cations,testingfor110–11cells(batteries)41ceramics,usesfor41changesofstate2chemicalenergetics43–6chemicalequilibrium52–3chemicalpropertiesofasubstance48chemicalreactions47–56chlorides(Cl−)61,111

chlorination78,79chlorine(Cl2)21,65–6,93,111chromatography8–10cobaltchloride78collisiontheory49–51combustion,alkanes93completecombustion93complexcarbohydrates107compounds7–13condensation1,2condensationpolymerisation103conductors37coolingcurves3copper(Cu)75–6copper(II)sulphate40,78covalentbonding21cracking96crystallisation10–11

Ddensity,metals69diamond23–4diffusion1,4displacementreactions,metals70displayedformula91dissolving10–11distillation11–12drinkingwater79ductility,metals69

Eelectricalconductivity,metals69electricity37–42electrodes37electrolysis37,38–40,41electrolytes37–8electrons14,15–16

electronshell15electroplating40–1elements7–13empiricalformula26,32–3,90,91endothermicreaction43,44energyleveldiagrams43enzymes47equations,Molecalculation30equilibriumsystems52–4esters101–3,102ethane92ethanol98–9ethene(C2H4)21,98evaporation1,2exothermicreaction43experimentalchemistry110–11extractionofmetals73–6

Ffertilisers78,82–3filtration10–11,78,79flametests111formulae18–19,19–20,91fractionaldistillation11–12,78,79freezing1,2freezingpoint1fuelcells44fuels44,96functionalgroups90

Ggalvanising81gases1–2,4–5,111gasvolume,Molecalculation29giantcovalentstructure(macromolecularstructure)22giantionicstructure22,23giantmetallicstructure22,23

giantmolecularstructure23glucose33graphite23–4greenhouseeffect87greenhousegases87Group0elements66–7GroupIelements65GroupVIIelements65–6groups,PeriodicTable64,65–7

HHaberprocess53–4,82halogens64,65–6heatingcurves3helium(He)67homologousseries90hydrocarbons92hydrogen(H2)21,44,95,111hydrogenperoxide33hydrolysis,naturalpolymers107hydroxides,thermaldecomposition72

Iincompletecombustion93indicators60inorganiccarbonchemistry87–9insolublebases58iodide(I−),testingfor111iodine(I)65–6ionicbonding16–20ioniccompounds16–20ionicequations28iron(Fe)70,74,75,76iron(III)sulfate20isomers91isotopes14,91

Kkinetictheory3

Llattices14leadsalts61,80limestone88limitingreactants33liquids1–2,4litmus60

Mmacromolecularstructure(giantcovalentstructure)22magnesiumfluoride,ionicbonding18magnesiumhydroxide20magnesium(Mg),reactionwithwater70malleability,metals69masses,Molecalculation29massnumber(nucleonnumber)14matter,particulatenatureof1–6melting1,2,3meltingpoint1,69metallicbonds22metalliccompounds,thermaldecomposition72metals7,69–77extractionof41,73–6reactionswithacids57–8usesof41,76

methane(CH4)21,33,87,92methanol(CH3OH)21methylorange60mixtures7,8–12Mole26,29molecularformulae26,33–4,90,91

N

naturalpolymers106–7neutraloxides60neutron14nitrates61nitrates(NO3–)72,111nitrogen(N2)21,82–3noblegases64,66–7non-metals7nucleonnumber(massnumber)14nucleus15nylon106

Oorganicchemistry90–109oxidation47oxides60oxidisingagents47,54,55oxygen(O2),testingfor111

Ppercentagepurity35percentageyield34PeriodicTable16,19,64–8periods,PeriodicTable64petroleumindustry96photochemicalreactions51pHscale60physicalpropertiesofasubstance48plastics,usesfor41pollution,air80polyamides105polyesters104polyethene103polymerisation98,103polymers98,103–7potassiumsalts61

propane92proteins106–7proton14protonnumber(atomicnumber)14

Qquicklime88

Rrateofreaction48reactivityseries,metals70–2redoxreactions54–5reducingagents47,55reduction47,54relativeatomicmass(Ar)8,26relativemolecularmass(Mr)8,26,28reversiblereactions52rust78,81

Ssalts57,61–2separationofmixtures8–12silicon(IV)oxide(silicondioxide)24simpledistillation11simplemolecularstructures23slakedlime88sodiumchloride,ionicbonding17sodiumhydroxide(NaOH)110sodiumsalts61solids1–2,22–3solutes7solutions7,29solvents7stainlesssteel,usesof76starch107state,changesof2steam,reactionwithalkenes95

steel75,76stoichiometry26–7strongacids57,58strongbases57structuralformula90,91structuralisomerism90,91–2,94sublimation1substance7substitutionreactions93sulfate(SO42–)61,111sulfite(SO32–),testingfor111sulfurdioxide(SO2)80,84–6,85,111sulfuricacid84–5symbolequations26

Tterylene105thermalconductivity,metals69thermalcracking96thermaldecomposition,metalliccompounds72transitionelements64,67

Uunbranchedesters102universalindicators60unsaturationtesting,alkenes94

Wwater(H2O)21,65,70,78–9weakacids57,58wordequations26

Zzinc(Zn)76,81