Class10 ChemistryG12 Notes and Homework

8/4/2019 Class10 ChemistryG12 Notes and Homework

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GRADE12CHEMISTRY

OLYMPIADSCHOOL

WEDNESDAY2:304:30

FRIDAY4:456.45


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,

and

pH


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DifferentcombinationsofstrongArrheniusacidsandbasesreactwiththesameexothermicresult.Measurementsalwaysshowthereleaseof56kJofenergypermoleofwaterformed.Thismakessense,becausethenetionic

equationisthesameregardlessofthespecificneutralizationreactionthatoccurs.

TheArrheniustheoryhaslimitations,however.Forexample,H+(aq),abareproton,doesnotexistinwater.Thepositivechargeonaprotonisattractedtotheregionofnegativechargeonthelonepairofelectronsonawater

moleculesoxygenatom.Thecombinationisahydratedprotoncalleday ron um on, + aq + aq + + aq

Thehydronium ionformshydrogenbondswithotherwatermolecules.Thus,abetterformulafortheionthatispresentinacidicsolutionsis

H H2O n +,

w ere

n

is

usua y

4

or

5.

For convenience,

owever,c emists

usuallyuseasinglehydronium ionwhenwritingequations.

TheArrheniustheoryalsohaslimitationsforexplainingcertainreactions.Forexample,aqueoussolutionso ammoniaarebasic.Theyreactwithacidsinneutralizationreactions,eventhoughammoniadoesnotcontainthe

hydroxideion.Manyaqueoussolutionsofsaltswithnohydroxideionsareas c, oo. omereac ons a ep acew ou any qu so ven . orexample,ammoniumchloridecanbeformedbythereactionbetweenammoniaandhydrogenchloride,whicharebothgases:

g + g s


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TheBrnstedLowryTheory

ThelimitationsoftheArrheniustheoryofacidsandbasesareovercomebyamore eneraltheor calledtheBr nstedLowr theor .Thistheor was

proposedindependently,in1923,byJohannesBrnsted,aDanishchemist,andThomasLowry,anEnglishchemist.Itrecognizesanacidbasereactionasachemicalequilibrium,havingbothaforwardreactionandareversereactionthatinvolvethetransferofaproton.TheBrnstedLowrytheorydefinesacidsandbasesasfollows:

*Anacidisasubstancefromwhichaprotoncanberemoved.(SomechemistsdescribeBrnstedLowryacidsasprotondonors.)

*Abaseisasubstancethatcanacceptaproton.(SomechemistsdescribeBrnstedLowr basesas rotonacce tors.

Notethatthewordprotonreferstothenucleusofahydrogenatom anH+ionthathasbeenremovedfromtheacidmolecule.Itdoesnotrefertoa

rotonremovedfromthenucleusofanotheratom suchasox enorsulfur,thatmaybepresentintheacidmolecule.Asmentionedpreviously,H+ionsshareelectronswithanyspecies(ionormolecule)thathasalone

pairofelectrons.Inaqueoussolution,theprotonbondswithawatermoleculetoformthehydronium ion.UnliketheArrheniustheory,however,theBrnstedLowrytheoryisnotrestrictedtoaqueoussolutions.Forexample,thelonepairofelectronsonanammoniamoleculecanbondwith

+,an qu ammon acanac asa ase.


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ConjugateAcidBasePairs

proceedsinbothdirections.Aceticacidisweak,soonlyafewionsdissociate.Thepositionofequilibriumliestotheleft,andthereversereaction is favoured. In the reverse reaction the h dronium ion ives u aprotontotheacetateion.Thus,theseionsareanacidandabase,respectively.Theacidontheleft(CH3COOH)andthebaseontheright(CH3COO)differbyoneproton.Theyarecalledaconjugateacidbasepair.Similarly,H2OandH3O+areaconjugateacidbasepair.


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Aqueousammoniaisaweakbase,sorelativelyfew

y rox e

ons

orm.

epos on

o

equ r um

es

o

theleft.Intheforwardreaction,thewatermolecule.

canactasaprotondonor(anacid)inonereactionand

beamphoteric.(Wateractsasanacidinthepresence,

strongeracid.


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StrongAcidsandBases

Whenastrongacidorbasedissolvesinwater,almosteveryacidorbasemoleculedissociates.Whiletherearemany

acidsandbases,mostareweak.Thus,thenumberofstrongacidsandstrongbasesisfairlysmall.

StrongAcids

Binaryaci st at avet egenera ormu aHX aq ,w ereX=Cl,Br,andI(butnotF):forexample,hydrochloricacid,HCl,

andhydrobromic acid,HBr (HCl andHBr arehydrohalic

halogenelements).Oxoacids (acidscontainingoxygenatoms)inwhichthenumberofoxygenatomsexceeds,bytwoormore,thenumberofprotonsthatcanbedissociated:orexamp e,n tr cac , ,su ur c ac , ,

perchloric acid,HClO4,andchloric acid,HClO3

Thebinaryacidsofnonmetalsexhibitperiodictrendsin.

trend:theelectronegativity oftheatomthatisbondedtohydrogen,andthestrengthofthebond.

, , .Theyhaveonlyasinglehydrogenatomthatdissociatesinwater.Someacidshavemorethanonehydrogenatomthatdissociates.Theseacidsarecalledpolyprotic acids.


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StrongbasesareconfinedtotheoxidesandhydroxidesfromGroups1(IA)and2(IIA).

rong asesAlloxidesandhydroxidesofthealkalimetals:forexample,

, , ,

Alkalineearth(Group2(IIA))metaloxidesandhydroxides

belowberyllium:forexample,calciumhydroxide,Ca(OH)2,andbariumhydroxide,Ba(OH)2

Thestrongbasicoxideshavemetalatomswithlowe ec ronega v y. us, e on ooxygen s on can srelativelyeasilybrokenbytheattractionofpolarwatermolecules.Theoxideionalwaysreactswithwatermoleculestoproducehydroxideions.O2(aq)+H2O() 2OH(aq)

Magnesiumoxideandmagnesiumhydroxidearenotveryso u e. eyares rong ases, owever, ecause esma amountthatdoesdissolvedissociatesalmostcompletelyintoions.Ber lliumoxideisaweakbase. Itistheexce tionin

Group2(IIA).)Itisarelativelysmallatom,sothebondtooxygenisstrongandnoteasilybrokenbywatermolecules.


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CalculationsThatInvolveStrongAcidsand

Bases enas rongac ssoc a escomp e e y n o ons

inwater,theconcentrationofH3O+(aq)isequaltotheconcentrationofthestrongacid.Similarly,when

astron basedissociatescom letel inwater,the

concentrationofOH(aq)isequaltothe.


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TheEquilibriumofWeakAcidsandBases

solutionproducesionsthatinteractwithwater.ThepHoftheaqueoussolutionisdeterminedbythepositionofequilibriuminreac ons e ween e ons a arepresen nso u onan ewatermolecules.Purewatercontainsafewions,producedbythe

dissociationofwatermolecules:2H2O() H3O+(aq)+OH(aq)

At25C,onlyabouttwowatermoleculesinonebilliondissociate,.

neutralwater,at25C,theconcentrationofhydronium ionsisthesameastheconcentrationofhydroxideions:1.0 107mol/L.

eseconcentrat onsmust et esame ecauset e ssoc at onofwaterproducesequalnumbersofhydronium andhydroxide

ions.Becausethisisanequilibriumreaction,andbecausethepositionofequilibriumofallreactionschangeswithtemperature,[H3O+]isnot1.0 107mol/Latothertemperatures.Thesameistrueof OH .


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TheIonProductConstantforWater

Theequilibriumconstant,Kc,forthedissociationofwaterisgivenbythefollowingexpression.

Sofewionsformthattheconcentrationofwaterisessentiallyconstant.TheproductKc[H2O]2 isequaltotheproductoftheconcentrationsofhydronium ions

andhydroxideions.TheequilibriumvalueoftheconcentrationionproductH3O+ OH at25 Cisca e t e onpro uctconstant orwater.Itisgivent e

symbolKw.

Kc[H2O]2=[H3O+][OH]=1.0 107mol/L 1.0 107mol/L=1.0 1014=Kw

eun tsarecommon y roppe ,as not erequ r umexpress onsyou ave

encountered.TheconcentrationofH3O+inthesolutionofastrongacidisequaltotheconcentrationofthedissolvedacid,unlessthesolutionisverydilute.Consider

+ . .dissociateinwater,formingahydronium ionconcentrationthatequals0.1mol/L.Theincreased[H3O+]pushesthedissociationreactionbetweenwatermoleculesto

theleft,inaccordancewithLeChteliers principle.Consequently,theconcen ra ono y ron um ons a resu s rom e ssoc a ono wa er sevenlessthan1 107mol/L.This[H3O+]isnegligiblecomparedwiththe0.1mol/Lconcentrationofthehydrochloricacid.Unlessthesolutionisverydilute(about1

[H3O+]ofastrongacid.


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Similarly,theconcentrationofhydroxideionscanbe

thesolutionisastrongbase,youcanignorethedissociationofwatermoleculeswhendetermining[OH],unlessthe

. known,youcanusetheionproductconstantforwater,Kw,to

determinetheconcentrationoftheotherion.Althou hthevalueofKw forwateris1.0 1014at25Conly,youcanusethisvalueunlessanothervalueisgivenforadifferent

.

Inanacidicsolution,[H3O+]isgreaterthan1.0 107mol/L

and[OH]islessthan1.0 107mol/L.Inaneutralsolution,both[H3O+]and[OH]areequalto1.0 107mol/L.Inabasicsolution,[H3O+]islessthan1.0 107mol/Land[OH]

. .


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pHandpOH Youcandescribetheacidit ofana ueoussolution uantitativel b statin

theconcentrationofthehydronium ionsthatarepresent.[H3O+]isoftenaverysmallnumber.ThepHscalewasdevisedasaconvenientwayto

represen ac y an , yex ens on, as c y . esca e s ogar m c,

basedon10.Thinkoftheletterpasamathematicaloperationrepresenting

log.ThepHofasolutionistheexponentialpowerofhydrogen(orhydronium)ions,inmolesperlitre.Itcanthereforebeexpressedasfollows:pH= log[H3O+]

T epractica rangeo t epHsca eis rom0to14.Aso utiono astrong

acidthatismoreconcentratedthan1.0mol/LwouldgiveanegativepH.

Since oucandetermine H3O+ ofsuchsolutionsdirectl fromthe

concentrationoftheacid,thepHscaleoffersnoadvantage.Similarly,the

pHofastrongbasethatismoreconcentratedthan1.0mol/Lisgreaterthan

. o e a p sa mens on essquan y. no erwor s, asnoun s.

YoucancalculatethepOH (thepowerofhydroxideions)ofasolutionfrom

=

Kw=[H3O+][OH]=1.0 1014at25C pH+pOH =14

Th A id Di i i C


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TheAcidDissociationConstant

Manycommonfoods(suchascitrusfruits),pharmaceuticals,andsomevitaminsareweakacids.Whenaweakaciddissolvesinwater itdoesnot

completelydissociate.Theconcentrationofthehydronium ions,andtheconcentrationoftheconjugatebaseoftheacidthatisformedinsolution,dependontheinitialconcentrationoftheacidandtheamountofacidthatdissociates.Youcanrepresentanyweakmonoprotic acidwiththegeneralformulaHA.Theequilibriumofaweakmonoprotic acidinaqueoussolution

canbeexpressedasfollows:HA(aq)+H2O(aq) H3O+(aq)+A(aq)

TheequilibriumexpressionforthisreactionisKc =[H3O+][A]/[HA][H2O]

, .

Multiplyingbothsidesoftheequilibriumexpressionby[H2O]givestheproductoftwoconstantsontheleftside.Thisnewconstantiscalledtheaciddissociationconstant Ka.Kc[H2O]=Ka=[H3O+][A]/[HA]

YoucandeterminethevalueofKaforaparticularacidbymeasuringthepH.

WeakacidshaveKavaluesthatarebetween1andabout1 1016.VeryweakacidshaveKavaluesthatarelessthan1 1016.Thesmallerthe

, .


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The Base Dissociation Constant


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TheBaseDissociationConstant

Manycompoundsthatarepresentinplantsareweakbases.

a e ne

nco ee

an

p per ne n

ac

pepper

are

wo

examples.Aweakbase,representedbyB,reactswithwatertoformane uilibriumsolutionofions.

B(aq)+H2O() HB+(aq)+OH(aq)

Theequilibriumexpressionforthisgeneralreactionisgivenasfollows:

Kc =[HB+][OH]/[B][H2O]

Theconcentrationofwaterisalmostconstantindilutesolutions.Multiplyingbothsidesoftheequilibrium

theleftside.Thenewconstantiscalledthebasedissociationconstant,Kb.

Kc[H2O]=[HB+][OH]/[B]=Kb

NitrogencontainingcompoundsareBrnstedLowrybases,

bondwithH+fromwater.


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AcidsandTheirConjugateBases


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j g

Thereisanimportantrelationshipbetweenthedissociationconstantforanacid Ka andthedissociationconstant oritscon u atebase Kb.Consider

aceticacidanditsdissociationinwater.CH3COOH(aq)+H2O(aq) H3O+(aq)+CH3COO(aq)

Theacetateionistheconjugatebaseofaceticacid.Asolublesaltofthe

conjugatebase,

such

as

sodium

acetate,

forms

acetate

ions

in

solution.

The

.

CH3COO(aq)+H2O(aq) CH3COOH(aq)+OH(aq)

Kbisgivenbytheexpression:Kb=[CH3COOH][OH]/[CH3COO]

TheproductKaKb givesaninterestingresult.

KaKb =[H3O+][CH3COO]/[CH3COOH] [CH3COOH][OH]/[CH3COO]

= =

Thus,foranacidanditsconjugatebase(orabaseanditsconjugateacid),KaKb =Kw.Oneinterpretationoftheresultisthestrongeranacid,the

. ,strongacidgivesupaprotonfromeachmolecule.Therefore,itsconjugatebasedoesnotbondwiththeproton.Insummary,then,thestrengthofan

acidisalwaysaweakbase,and,conversely,theconjugateofastrongbase

isalwaysaweakacid.


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Class10 ChemistryG12 Notes and Homework

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