chemistry in your cupboard: harpic - rsc education

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


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


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.


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.


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


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.


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.


Figure4:Thesurfacetensionofwatercausesittoformdropletsoncleansurfaces

Youcanseethisifyoupouralittletapwaterontoaverycleanporcelainsurfacesuchasaplate‐itwilltendtoformdroplets.Addalittlewashingupliquid(whichcontainssurfactants)andthewaterwillspreadtocoverthewholeplate.

ThefunctionofsurfactantsistoreducethesurfacetensionofwaterandallowtheaqueoussolutiononwhichHARPICisbasedtowettheporcelainsurfaceofthetoiletbowleffectively.Thisallowsproducttospreadoverthesurfaceratherthanformdroplets.

Otheringredients(2of2)PEG‐2hydrogenatedtallowammoniumchlorideandtallowtrimoniumchloridearecationicdetergents.TheyarebasedonquaternaryammoniumcompoundsandhavestructuresliketheoneinFigure5.

+H3N

Cl-

Figure5:Aquaternaryammoniumcompoundsurfactant

Theionicheadmixeswithwaterwhilethehydrocarbontailisnon‐polar.Cationicsurfactantscanbeusedinacidsolution.

AnionicsurfactantssuchassoapsliketheoneinFigure6cannotbeusedinacidsolutionsbecausethehydrogenionsinthesolutionwouldreactwiththenegativeionsandprotonatethem.Thisproducesanon‐ionicandthereforeinsolublecompound


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.


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


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


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

chemistry in your cupboard: harpic - rsc education

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