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HarpicisaregisteredtrademarkoftheReckittBenckisergroupofcompanies.LastreviewedbyReckittBenckiserin2013
Chemistryinyourcupboard:Harpic
Introduction
Limescale is a hard,white, chalkydeposit found insidekettles and around taps, sinksandtoiletbowls.Whenrainwaterseepsthroughlimestonetheacidityoftherainallowsittoreacttoformsolublecalciumhydrogencarbonate.Whenthewaterevaporates(inakettleforinstance)itleavesbehindinsolublecalciumcarbonatewhichcanberemovedby reacting itwith acid.This forms thebasis of the coreproductmarketedunder thebrandnameHarpic®,Harpiclimescaleremover.
Linkstothecurriculum
This section contains information relevant to the following areas of your chemistrycurriculum:
Bonding Acidrain EquilibriumandLeChatelier’sprinciple Strongandweakacids
HARPIC
Likemanyproducts,Harpic®isabrand,ratherthanasingleproduct.Thebrandcoversarangeofproductsassociatedwithcleaningandkillinggermsinandaroundthelavatorybowl.Thisarticlewillconcentrateonthe‘core’product,Harpiclimescaleremover.
Figure1:Harpiclimescaleremover
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Whatislimescale?Limescaleisahard,whitish,chalkydepositfoundinsidekettlesandalsoaroundtaps,sinksandtoiletbowls.Itisformedwhendissolvedsaltspresentinhardwatercomeoutofsolutionasthewaterevaporates.Amajorcomponentiscalciumcarbonatewhichcomesfromlimestone.
Whenrainwaterseepsthroughlimestonerockonitswaytoareservoir,itdissolvessomeofit.Thisistheoriginoflimestonecaves.Limestoneismainlycalciumcarbonate,whichisalmostinsolubleinpurewater,butrainwaternaturallycontainsdissolvedcarbondioxidewhichmakesitacidic.Thefollowingreactionoccurs,dissolvinglimestoneascalciumhydrogencarbonate(calciumbicarbonate)andsocalciumandhydrogencarbonateionsarepresentdissolvedintapwater‐seeCalgon.
CaCO3(s)+H2O(l)+CO2(aq) Ca2+(aq)+2HCO3‐(aq)ΔH‐ve
Asimilarreactioncanoccurwithmagnesiumcarbonate.Noticethatrainwaterisnaturallyacidicbecauseofdissolvedcarbondioxide,whichisnormallypresentintheairandtakespartintheequilibria:
H2O(l)+CO2(aq) HCO3‐(aq)+H+(aq) H2CO3(aq)
AcidrainAtthepresenttime,rainwaterisbecomingmoreacidicduetotheactivitiesofmankind:
theburningfossilfuels,whicharecarbon‐basedcompounds,isincreasingthelevelofcarbondioxideintheair;
burningthesulfur‐containingimpuritiespresentinmanyfossilfuelsleadstosulfurdioxidewhichisoxidisedtosulfurtrioxideandthenconvertedtosulfuricacidintheatmosphere;
hightemperaturecombustionprocessescombineoxygenandnitrogenfromtheairtoformnitrogenoxideswhichthenreactwithwatertoformnitricacid.
Figure2:Effectofacidrainontrees
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Question1Writebalancedequationstoshowhowa)sulfurburnsinoxygentoformsulfurdioxideb)reactswithfurtheroxygentoformsulfurtrioxidec)howsulfurtrioxidereactswithwaterintheatmospheretoformsulfuricacid
Asthewaterevaporates,theequilibrium
CaCO3(s)+H2O(l)+CO2(aq) Ca2+(aq)+2HCO3‐(aq)∆H–ve
movestothelefttodepositcalciumcarbonate‐limescale.
Question2Suggesttworeasonswhylimescaletendstoformmorearoundthehottapthanthecoldtapinabathroomsink.Dissolvinglimescale(1of2)Calciumcarbonateisvirtuallyinsolubleinwaterbut‘dissolves’readilyinacids.Strictlyitisreactingwiththemratherthansimplydissolving.
Harpiccontainshydrochloricacidwhichdissolveslimescale,calciumcarbonate(CaCO3),asshown.
CaCO3(s)+2HCl(aq) CaCl2(aq)+CO2(g)+H2O(l)
Hydrochloricacidisastrongacidwhichmeansthatitdissociatescompletelyintoions:
HCl(aq) H+(aq)+Cl‐(aq)
Question3Hydrochloricacidisasolutionofhydrogenchloridegas.a)Workoutthevolume(atroomconditions)ofhydrogenchloridedissolvedin1dm3Harpic.b)Workoutthemassofhydrogenchloridedissolvedin1dm3Harpic.
TheconcentrationofHClinHarpicisaround10%,whichequatestoapproximately3moldm‐3.
Question4Calculate[HCl]toproduceapHof0
Otheracidsthataresometimesusedinproductsforremovinglimescalearetheweakacidssulfamicacid(SO3HNH2),formicacid(HCOOH)andphosphoricacid(H3PO4),Figure3.Weakacidsdissociateonlypartlyinsolution.
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sulfamicacid
formicacid
phosphoricacid
Figure3:Structuresofsulfamic,formicandphosphoricacids
Question5Whatisthesystematicnameofformicacid?ThestrengthofaweakacidismeasuredbyitspKavalue:
Sulfamicacid(HOSO2NH2)pKa=0.99
Formicacid(HCOOH)pKa=3.8
Phosphoricacid(H3PO4)pKa1=2.1;pKa2=7.20;pKa3=12.38pKavalueisameasureofhoweasilyaH+ionislost.ThelargerthepKavalue,theweakertheacid.
Dissolvinglimescale(2of2)ImagineaweakacidHAwhichdissociates
HA(aq) H+(aq)+A‐(aq)
Theequilibriumconstantisgivenby:
Kc=eqm(aq)
eqm(aq)eqm(aq)
][HA
][A][H
Foraweakacid,thisisusuallygiventhesymbolKaandcalledtheaciddissociationconstant.
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Ka=eqm(aq)
eqm(aq)eqm(aq)
][HA
][A][H
ThepKaisdefinedasfollowsandisanalogouswithpH=‐log10[H+]:
pKa=–log10Ka
Question6CalculatethepHofa1moldm‐3solutionofsulfamicacid.ThelargerthevalueofKa,thestrongertheacidi.e.thegreater[H+]itproducesondissociation.However,becauseofthenegativesignintheexpressionabove,thelargerthevalueofpKa,theweakertheacid.
Question7a)Explainwhy,whenformicaciddissociates,thehydrogenatombondedtoanoxygenistheonelostasaH+ionratherthantheonebondedtothecarbonatom.b)ThepKaofformicacidis3.8,whatisthevalueofKa?Givethecorrectunitsc)WriteanexpressionforKaofformicacid(representtheacidasHCOOH)d)CalculatethepHofa1moldm‐3solutionofformicacid.e)CompareyouranswerforthepHofformicacidwiththatof1moldm‐3hydrochloricacid(HCl),astrongacid.HowmuchlimescalewillHarpicdissolve?
CaCO3(s)+2HCl(aq) CaCl2(aq)+H2O(l)+CO2(g)
1mol 2mol
Consideratypical‘dose’ofHarpicisapproximately50cm3.
MolesHCl=1000
x VC =1000
50 x 3=0.15mol
Thiswilldissolvehalfthisi.e0.075molCaCO3
MrCaCO3=100gmol‐1
SotheHarpicwilldissolve7.5gcalciumcarbonate.
Mixingacid‐basedandbleach‐basedcleanersAnumberofotherhouseholdcleanerssuchasCillitBangToiletCleanerareformulatedwithachlorine‐basedbleach.Theseareessentiallysolutionsofchlorineinalkaliinwhichthefollowingequilibriumissetup.
Cl2(g)+OH‐(aq) HClO(aq)+Cl‐(aq)
TheactivebleachingagentisHClO(aq),chloric(I)acid.
Itisimportantnottomixchlorine‐basedandacid‐basedcleaners(ofanybrand)andtherearewarningstothiseffectonthebottles.Onaddingacidtobleach,theaddedacidwillreactwiththeOH‐ionstoformwaterand,byLeChatelier’sprinciple,the
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equilibriumabovewillmovetotheleft,withtheconsequentformationofpoisonouschlorinegas.Notagoodidea!
Inotherwords,thereaction
HClO(aq)+HCl(aq) Cl2(g)+H2O(l)
goesessentiallytocompletion.
Question8AsimilarproducttoHarpicstatesonthepack‘thisuniqueformulashowsyouwhereitisworkingbyturningfromgreentoblueandfizzingoncontactwithlimescale’.Explainthefizzingandsggesthowthecolourchangemaybebroughtabout.Question9CalculatehowmuchchlorinecouldbeproducedfromatypicaldoseofHarpicof50cm3assumingthattherewasexcessbleach.Areferencedatabaseonchemicalhazards,CLEAPSSHazcards,givesthefollowinginformationaboutchlorine:‘Toxicbyinhalation.Irritatingtoeyes,respiratorysystemandskin.WEL1.5mgm‐3(STEL)’
Letusputthisincontext.WELstandsforWorkplaceExposureLimitandisthemaximumconcentrationtowhichworkersmaybeexposedbybreathing,andSTELstandsforShortTermExposureLimit,i.e.anexposurefor15minutes.
Imagineasmallishbathroom2mx2mx3m.Thisgivesavolumeof12m3.Fromtheanswertoquestion9,weknowthat3550mgCl2couldbeproducedfromatypicaldoseofHarpicwithexcessbleach.Thisgivesanaveragechlorineconcentrationof3550/12=296mgm‐3.Thisiswellabovethequotedlevelof1.5mgm‐3fora15minuteexposure.Sothisposesasignificantproblem.
Letuscheckourassumptionsandconditions
Manybathroomswillbelargerthantheexamplegiven,thusreducingtheconcentrationofchlorine
Bathroomsarenormallywell‐ventilated(oftenwithafan),sothechlorinewoulddissipate.
Mostpeoplewillnotspendaslongas15minutesintheroomaftercleaningthetoilet.
Thesmellofchlorinewillalertpeopletothedanger. Chlorineisdenserthanairandsowilltendtoremaininthetoiletbowlrather
thanspreadingthroughouttheroom. Chlorineisfairlysolublesosomeofitwouldremaindissolvedinthewaterinthe
toilet.
However,thisisclearlynotagoodidea,andcareshouldbetakennottomixbleachandacid‐basedcleaners.Inparticular,peoplewithbreathingproblemssuchasasthmamightbemoreseverelyaffected.
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Otheringredients(1of2)Harpicisnotjustasolutionofhydrochloricacid.TheingredientsofHarpic100%limescaleremoveroriginalarelistedinFigure3indecreasingorderofquantity.
Figure3:IngredientsofHarpic100%limescaleremoveroriginal
‘Aqua’meanswater.Parfumandcolourdoastheirnamesuggestsandmaketheproductpleasanttouse.
Oftheotheringredients,PEG‐2hydrogenatedtallowammoniumchloride,tallowtrimoniumchlorideandlaureth‐10aredetergentsorsurfactants.Theseare‘tadpole‐shaped’molecules,thatistheyhaveapolarorionic‘head’whichwillmixreadilywithwaterandanon‐polar‘tail’
Thereisstronghydrogenbondingbetweenwatermoleculeswhichformsa‘skin’(knownassurfacetension)atthesurfaceofwaterandtendstopullitintodroplets,Figure4.Surfactantmolecules,asthenameimplies,clusteratthewatersurfacewithheadsinthewaterandtailsstickingoutanddisruptthehydrogenbonding.Thisallowswatertospreadandwetthesurface.
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Figure4:Thesurfacetensionofwatercausesittoformdropletsoncleansurfaces
Youcanseethisifyoupouralittletapwaterontoaverycleanporcelainsurfacesuchasaplate‐itwilltendtoformdroplets.Addalittlewashingupliquid(whichcontainssurfactants)andthewaterwillspreadtocoverthewholeplate.
ThefunctionofsurfactantsistoreducethesurfacetensionofwaterandallowtheaqueoussolutiononwhichHARPICisbasedtowettheporcelainsurfaceofthetoiletbowleffectively.Thisallowsproducttospreadoverthesurfaceratherthanformdroplets.
Otheringredients(2of2)PEG‐2hydrogenatedtallowammoniumchlorideandtallowtrimoniumchloridearecationicdetergents.TheyarebasedonquaternaryammoniumcompoundsandhavestructuresliketheoneinFigure5.
+H3N
Cl-
Figure5:Aquaternaryammoniumcompoundsurfactant
Theionicheadmixeswithwaterwhilethehydrocarbontailisnon‐polar.Cationicsurfactantscanbeusedinacidsolution.
AnionicsurfactantssuchassoapsliketheoneinFigure6cannotbeusedinacidsolutionsbecausethehydrogenionsinthesolutionwouldreactwiththenegativeionsandprotonatethem.Thisproducesanon‐ionicandthereforeinsolublecompound
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Na+O O-
Figure6:Ananionicsurfactant
Question10a)IdentifythefunctionalgroupoftheanionicsurfactantinFigure6.b)Writeanequationtoshowtheprotonationofthisgroup.
Laureth‐10,Figure7,isanon‐ionicsurfactantbasedonthegroup–(CH2CH2O)n‐.Itisrelativelypolarwithanon‐polarhydrocarbontail.
O
OH
n
Figure7:Apolyethyleneethoxylate–anon‐ionicdetergent
Non‐ionicsurfactantscanalsobeusedinacidsolutions.
Afurtherfunctionofthetallowammoniumchlorideisthatitactsasathickeningagentsothattheproductwillclingtothesidesofthetoiletpanandremaineffectivethroughmanyflushes.Harpicalsocontainsabluecolouringagentsothattheusercanseeclearlywheretheproductisandcanmakesurethatthewholeofthetoiletrimiscovered,ataskthatwouldbemoredifficultwithacolourlessliquid.
TheacidinHarpicmeansthatitisaneffectivegermicide,killing‘99.9%ofgerms’.
FurtherinformationHarpicissoldintheUKbyReckittBenckiser(www.reckittbenckiser.com/site/RKBR/Templates/OurBrandsSurfaceCare.aspx?pageid=253).Thereareotherbrandedproductswhichworkinasimilarway.
AcknowledgementsTheRoyalSocietyofChemistrywishestothankChrisJonesandDavidKennedyofReckittBenckiserforhelpinpreparingthismaterial.
TheRoyalSocietyofChemistrygratefullyacknowledgesthatthisprojectwasinitiallysupportedbyReckittBenckiserin2007.ReckittBenckiserconductedafinalreviewin2013sopleasenotethatcertaininformationmaybeoutofdate.
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QUESTIONSANDANSWERS
Question1Writebalancedequationstoshowhowa)sulfurburnsinoxygentoformsulfurdioxideS(s)+O2(g) SO2(g)
b)reactswithfurtheroxygentoformsulfurtrioxideSO2(g)+½O2(g) SO3(g)
c)howsulfurtrioxidereactswithwaterintheatmospheretoformsulfuricacidSO3(g)+H2O(l) H2SO4(aq)
Question2Suggesttworeasonswhylimescaletendstoformmorearoundthehottapthanthecoldtapinabathroomsink.Thewaterevaporatesduetotheheatofthehottap.Theequilibriummovestotheleft(byLeChatelier’sprinciple)iethedirectioninwhichitabsorbsheat.
Question3Hydrochloricacidisasolutionofhydrogenchloridegas.a)Workoutthevolume(atstandardtemperatureandpressure)ofhydrogenchloridedissolvedin1dm3ofHarpic.Thevolumeof1moleofanygasatroomtemperatureisapproximately24000cm3.Sothevolumeofdissolvedgasinasolutionof3moldm‐3HClisapproximately72000cm3.
b)Workoutthemassofhydrogenchloridedissolvedin1dm3Harpic.MrofHClis1+35.5=36.5gmol‐1sothemassof3molHCl=109.5g.
Question4Calculate[HCl]toproduceapHof0 pH=‐log10[H+]
0=‐log10[H+]
[H+]=invlog100
[H+]=1moldm‐3
ComparethistotheconcentrationofHClinHarpicwhichis3timeshigher.InterestinglyHarpichasapHof‐0.48.
Question5Whatisthesystematicnameofformicacid?Methanoicacid.
Question6CalculatethepHofa1moldm‐3solutionofsulfamicacid.ThepKaofsulfamicacidis0.99
pKa=–log10Ka
soKa=10‐0.99
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Ka=0.10moldm‐3
HOSO2NH2⇌OSO2NH2‐+H+
Ka=eqm(aq)22
eqm(aq)eqm(aq)-
22
]NH[HOSO
][H]NH[OSO
[OSO2NH2‐(aq)]eqm=[H+(aq)]eqm
[HOSO2NH2(aq)]eqm=1‐[H+(aq)]eqm≈1
(Wecansafelymakethisassumptionforaweakacid)
So0.10=1
][H eqm2
(aq)
[H+(aq)]eqm=√0.10=0.316moldm‐3
pH=–log10[H+(aq)]eqm=0.5
Question7a)Explainwhy,whenformicaciddissociates,thehydrogenatombondedtoanoxygenistheonelostasaH+ionratherthantheonebondedtothecarbonatom.Oxygenissignificantlymoreelectronegativethanhydrogen(3.5asopposedto2.1).SotheO‐HbondispolarisedOδ‐‐Hδ+makinglossofthehydrogenasH+easy.Carbon’selectronegativityissimilartothatofhydrogen(2.5to2.1)sotheC‐Hbondisrelativelynon‐polarandthereislittletendencyfortheHatomtoacquireapositivecharge.
b)ThepKaofformicacidis3.8,whatisthevalueofKa?GivethecorrectunitspKa=–log10KasoKa=1.6x10‐3moldm‐3
c)WriteanexpressionforKaofformicacid(representtheacidasHCOOH)
Ka=eqm(aq)
eqm(aq)eqm(aq)-
][HCOOH
][H][HCOO
d)CalculatethepHofa1moldm‐3solutionofformicacid.[HCOO‐(aq)]eqm=[H+(aq)]eqm
[HCOOH(aq)]eqm=1‐[H+(aq)]eqm≈1
(Wecansafelymakethisassumptionforaweakacid)
So1.6x10‐3=1
][H eqm2
(aq)
[H+(aq)]eqm=√1.6x10‐3=0.04moldm‐3
pH=–log10[H+(aq)]eqm=1.4
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e)CompareyouranswerforthepHofformicacidwiththatof1moldm‐3hydrochloricacid(HCl),astrongacid.Hydrochloricaciddissociatescompletelyinsolution
HCl(aq) H+(aq)+Cl‐(aq)
So[H+]eqm=1moldm‐3
pH=–log10[H+(aq)]eqm=0
Question8AsimilarproducttoHarpicstatesonthepack‘thisuniqueformulashowsyouwhereitisworkingbyturningfromgreentoblueandfizzingoncontactwithlimescale’.Explainthefizzingandsggesthowthecolourchangemaybebroughtabout.Thefizzingiscarbondioxideproducedbythereactionofhydrochloricacidandcalciumcarbonateinthelimescale.Thecolourchangecouldbebroughtaboutbyaddingasuitableacidbaseindicatorthatchangescolourastheacidisusedup.
Question9CalculatehowmuchchlorinecouldbeproducedfromatypicaldoseofHarpicof50cm3assumingthattherewasexcessbleach.HClO(aq)+HCl(aq) Cl2(g)+H2O(l)
Assumetheequilibriumisforcedcompletelytotheright.
1molHClwillproduce1molchlorine
Ina50cm3doseofHarpicofconcentration1moldm‐3
MolesHCl=1000
x VC =1000
50 x 1 =5x10‐2mol
So5x10‐2molCl2willbeproduced
Atroomtemperature1moleofanygashasanapproximatevolumeof24dm3(24000cm3)
SovolumeCl2=24000x5x10‐2cm3=1200cm3
MrCl2=71,sothisisequivalenttoamassofchlorineof71x5x10‐2g=3.55gor3550mg.
Question10a)IdentifythefunctionalgroupoftheanionicsurfactantinFigure6.Carboxylicacid(thesodiumsaltofacarboxylicacid)COO‐.
b)Writeanequationtoshowtheprotonationofthisgroup.
RCOO‐+H+→RCOOH