2023 HCI Prelim P4 QP

HWACHONGINSTITUTIONC2PreliminaryExaminationHigher2 NAME CTGROUP 22S CHEMISTRYPaper4 Practical CandidatesanswerontheQuestionPaper. 9729/0424August 20232hours30minutes READTHESEINSTRUCTIONSFIRST Writeyournameandclassonalltheworkyouhandin.Givedetailsofthepracticalshiftandlaboratorywhereappropriate,intheboxesprovided.Writeindarkblueorblackpen.YoumayuseanHBpencilforanydiagramsorgraphs.Donotusestaples,paperclips,glueorcorrectionfluid. Answer allquestionsinthespacesprovidedontheQuestionPaper. Theuseofanapprovedscientificcalculatorisexpected,whereappropriate.Youmaylosemarksifyoudonotshowyourworkingorifyoudonotuseappropriateunits.QualitativeAnalysisNotesareprintedonpages19and20. Attheendoftheexamination,fastenallyourworksecurelytogether.Thenumberofmarksisgiveninbrackets[ ]attheendofeachquestionorpartquestion. Shift Laboratory For Examiner’sUse 1 /16 2 /23 3 /16 Total ______________________________________________________________________________Thisdocumentconsistsof 19printedpagesand1blankpage.

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3 Answer allthequestionsinthespacesprovided. DONOTWRITEINTHISMARGIN 1 Determinationof amolar enthalpychangebyanindirect method Citricacid(Mr =192)isanorganictribasicacidanditsstructureisshownbelow. Inthisquestion,theformulaforcitricacidwillbewrittenasC3 H5 O(CO2 H)3 . Citricacidreactswithsodiumhydrogencarbonatetoproducesodiumcitrate,carbondioxideandwaterasshowninequation1andequation2.Themolarenthalpychangeofreactionisdifferentdependingonwhethersolidoraqueouscitricacidisused. equation1C3 H5 O(CO2 H)3 (s)+3NaHCO3 (aq)dC3 H5 O(CO2 − Na + )3 (aq)+3CO2 (g)+3H2 O(l) ΔH1 equation2C3 H5 O(CO2 H)3 (aq)+3NaHCO3 (aq)dC3 H5 O(CO2 − Na + )3 (aq)+3CO2 (g)+3H2 O(l) ΔH2 Youwillperformanexperimenttodeterminethemolarenthalpychangeofreaction,ΔH1 ,in1(a) andplanasetofexperimentstodeterminethemolarenthalpychangeofreaction,ΔH2 ,in1(b). FA1issolidcitricacid,C3 H5 O(CO2 H)3 .FA2is1.00moldm –3 sodiumhydrogencarbonate,NaHCO3 .(Note: FA2isalsousedinquestion3.) Inthespaceprovidedonpage3,preparetablesinwhichtorecordforyourexperiment:● allweighingstoanappropriatelevelofprecision● allvaluesoftemperaturetoanappropriatelevelofprecision. (a) Determinationof themolar enthalpychangeof reaction, ΔH1 (i) Procedure 1. Weighthecappedcontainercontaining FA1.Recordthemassinyourtableonpage3.2. Placeonepolystyrenecupinsideanotherpolystyrenecupandplacebothina250cm 3 glassbeaker.Transfer50cm 3 of FA2intothepolystyrenecupusinga50cm 3 measuringcylinder.3. Placealidonthecupandinsertthethermometerthroughthelid.Stirgentlythesolutionof FA2inthecupwiththethermometer.Measureandrecordtheinitialtemperature,Ti ,of FA2.4. Open the lid and empty the container of FA 1 into the polystyrene cupcontaining FA2.(YoumayneedtotapthecappedcontainertoloosentheFA1 © HwaChongInstitution2023 9729/04/C2Prelim2023 [Turnover

4 solidbeforethetransfer).Closethelidandusethethermometertogentlystirthemixture.Measureandrecordthelowesttemperatureofthemixture,Tf .5. Reweightheemptycappedcontainer.Recordthismassinyourtableonpage3. Results [2] DONOTWRITEINTHISMARGIN (ii) Calculatethetemperaturechange,∆T,producedbytheexperimentin 1(a)(i),andhencecalculatetheheatchange,q. Youshouldassumethatthespecificheatcapacityofthesolutionis4.18Jg –1 K –1 andthedensityofthesolutionis1.00gcm –3 . ∆T=….…………………………… q=………..……………………..[2] (iii) Showthatcitricacidisthelimitingreactantintheexperimentthatyoucarriedoutin1(a)(i).[Ar :H,1.0;C,12.0;O,16.0;Na,23.0] © HwaChongInstitution2023 9729/04/C2Prelim2023 [Turnover

5 [1](iv) Usingyouranswerin 1(a)(ii)and 1(a)(iii),calculatethemolarenthalpychangeofreaction,ΔH1 ,fortheexperimentin1(a)(i). DONOTWRITEINTHISMARGIN ΔH1 =…………………………………[1] (v) PredictandexplainhowthechangeintemperaturewillbeaffectedifthevolumeofFA2addedisdoubled. ………………………………………………………………………………………………… ………………………………………………………………………………………………… ……………………………………………………………………………………………..[1] © HwaChongInstitution2023 9729/04/C2Prelim2023 [Turnover

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7 (b) Planning When aqueous citric acid reacts with aqueous sodium hydrogencarbonate, anendothermicreactionoccurs,asshowninequation2. equation2C3 H5 O(CO2 H)3 (aq)+3NaHCO3 (aq)dC3 H5 O(CO2 − Na + )3 (aq)+3CO2 (g)+3H2 O(l)ΔH2 >0 Theminimumtemperaturechangeoccurringduringthisreactionmaybedeterminedgraphically. Inthisquestion,youaretoplanaseriesofexperimentstodeterminetheconcentrationofasolutionofaqueouscitricacid,named solution A,andthemolarenthalpychangeofreaction,ΔH2 . Youmayassumethatyouareprovidedwith:● solutionA,asolutionofcitricacidofunknownconcentration● FA2,1.00moldm –3 sodiumhydrogencarbonate(NaHCO3 )● apparatuscommonlyfoundinaschoollaboratory Yourplanshouldincludereactingdifferentvolumesof FA2andsolutionAtogetherwhilekeepingthetotalvolumeat50cm 3 ,andmeasuringthetemperaturechange,ΔT,foreachexperiment.Inyourplan,ΔTforeachexperimentshouldbetakenasTfinal –Tweightedinitial, where: Tweightedinitial = 𝑉𝐹𝐴 2 × 𝑇𝐹𝐴 2 ( ) + 𝑉𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝐴 × 𝑇𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝐴 ( )𝑉𝐹𝐴 2 + 𝑉𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝐴 Asuitablegraphisthendrawntoobtainthevolumeof FA2,Vmin ,requiredtocompletelyreactwith solution A.Vmin canthenbeusedtodeterminetheconcentrationof solutionA. The minimumtemperature change, ΔTmin , obtainedfromthegraphcanbeusedtocalculatetheheatchange,q,forthisexperiment,andhencethemolarenthalpychangeofreaction,ΔH2 . DONOTWRITEINTHISMARGIN (i) Yourplanshouldincludebriefdetailsof:● theapparatusyouwoulduse,● thevolumesof FA2andsolutionAyouwoulduseintheformofatable,● theprocedureyouwouldfollow,● themeasurementsyouwouldmaketoallowasuitablegraphtobedrawn. ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… © HwaChongInstitution2023 9729/04/C2Prelim2023 [Turnover

8 ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… DONOTWRITEINTHISMARGIN © HwaChongInstitution2023 9729/04/C2Prelim2023 [Turnover

9 ………………………………………………………………………………………………… ………………………………………………………………………………………………… ……………………………………………………………………………………………..[4] (ii) Ontheaxesgivenbelow,sketchthegraphyouwouldexpecttoobtain,andshowonyoursketchhowyouwouldusethegraphtoobtaintheVmin andΔTmin ofthereaction.Labeltheaxesclearly. [1] DONOTWRITEINTHISMARGIN (iii) Showhowyoumayusetheresultsofyourgraphin1(b)(ii)toderiveanexpressionfortheconcentrationof solutionA. © HwaChongInstitution2023 9729/04/C2Prelim2023 [Turnover

10 [1] (iv) Showhowyoumayusetheresultsofyourgraphin1(b)(ii)toderiveanexpressionforthemolarenthalpychangeofreaction,ΔH2 . [1](c) Thetwomolarenthalpychangesofreaction,ΔH1 andΔH2 ,maybeusedtofindthevalue ofthemolarenthalpychangeΔH3 ,whereΔH3 =ΔH1 –ΔH2 . C3 H5 O(CO2 H)3 (s)+3NaHCO3 (aq)dC3 H5 O(CO2 − Na + )3 (aq)+3CO2 (g)+3H2 O(l) ΔH1 C3 H5 O(CO2 H)3 (aq)+3NaHCO3 (aq)dC3 H5 O(CO2 − Na + )3 (aq)+3CO2 (g)+3H2 O(l) ΔH2 WhatisthethermochemicalprocessthatΔH3 represent? DONOTWRITEINTHISMARGIN …………………………………………………………………………………………………….[1