2023 HCI Prelim P4 Mark Scheme.docx

HWACHONGINSTITUTION2023C2H2CHEMISTRYPRELIMINARYEXAMINATIONMARKINGSCHEME Paper 4 1 (a) (i) Sampletable: massofcappedcontainer+FA1/g 8.50massofcappedcontainer+remainingFA1/g 5.49massof FA1used/g 3.01 initialtemperatureof FA2,Ti / o C 29.1lowesttemperatureofmixture,Tf / o C 25.4changeintemperature/ o C ‒3.7 ● correct headers+units+massrange(3.00g±0.10g) [1]neednotshow‘changeintemperature’ ● correct precision[1]o massesto2d.p.o temperaturesto1d.p. (ii) ∆T=Tf ‒Ti =25.4–29.1=‒3.7 o C(1d.p.) [1]q=mc∆T=50.0×1.00×4.18×3.7=773.3J=773J(3s.f.) [1] (iii) ηcitricacid used= =0.01568mol=0.0157mol(3s.f.) 3.01192 ηNaHCO3 used= ×1.00=0.0500mol(3s.f.)(> ) [1] 501000 3×η𝑐𝑖𝑡𝑟𝑖𝑐 𝑎𝑐𝑖𝑑 = 0.0471Citricacidisthelimitingreagent(shown) (iv) ∆H1 =+ =+49307Jmol –1 =+49.3kJmol –1 [1] 7730.01568 (v) q=mc∆TΔ𝐻 = 𝑞𝑛𝑙𝑖𝑚𝑖𝑡𝑖𝑛𝑔 𝑟𝑒𝑎𝑔𝑒𝑛𝑡 As FA 2isinexcess, doublingitsvolumedoesnot changetheamountofheatabsorbed,q,since∆H×narebothunchanged,butitdoublesthemassofsolutionthat thesameamount ofheatabsorbedmustcooldown.Hencethetemperaturechangewillbehalfthatmeasuredintheexperiment.[0.5]sameamountofheatabsorbed+mass/volumeofsolutiondoubled[0.5]temperaturechangehalved (b) (i) Sampletable: VFA2 /cm 3 VsolutionA /cm 3 10.00 40.015.00 35.0 2023HCIC2H2ChemistryPreliminaryExaminationMarkingScheme/Paper4 1

20.00 30.025.00 25.030.00 20.040.00 10.0[1]● 6setsofVFA2 +VsolutionA ● Vtotal =50cm 3 ● chosenvaluesofVFA2 shouldbewell-spaced Procedure1.FilltheburettewithFA2.2. Placetwopolystyrenecupstogetherandputthemina250cm 3 glassbeaker.Transfer40cm 3 of FA2intothepolystyrenecupusingtheburette.3.Measuretheinitialtemperatureof FA2usingthethermometer.RecordtheinitialtemperatureasTFA2 .4.Washanddrythethermometer.5.Useameasuringcylindertomeasure10cm 3 of solution A.Measuretheinitialtemperatureof solution Ausingthethermometer.RecordtheinitialtemperatureasTsolutionA .6.Add solution Ato FA2inthepolystyrenecup.Stirthemixturegentlyusingthethermometerandrecordtheminimumtemperature,Tmin ,reached.7.Washanddrythepolystyrenecup.8.Repeatsteps2to6usingthevolumesof FA2and solution Aasgiveninthetableabove.9. The weighted initial temperature should be calculated for each experimentaccordingtotheformulagivenandrecordedasTweighted,initial :10. Calculate the temperature change of each experiment by taking (ΔT =Tmin −Tweighted,initial ),andrecorditasΔTasshowninthetableabove. [0.5]correct apparatus(burettefor FA 2&measuringcylinderfor solution A+polystyrenecupsascalorimeter.AlsoacceptuseofmeasuringcylindersforbothFA2andsolutionA)[0.5]wash+drythermometer&polystyrenecup[0.5]measures&recordsTFA2 &TsolutionA [0.5]measures&recordsTmin &calculatesTweightedinitial [0.5]repeatsexperiments[0.5]calculates&recordsΔTforeachexperiment (ii) 2023HCIC2H2ChemistryPreliminaryExaminationMarkingScheme/Paper4 2

● correctaxeslabels+shapeofgraph● markingofΔTmin &Vmin ● graphmustpassthroughorigin+x-axisat50cm 3 mark (iii) ηNaHCO3 (FA2)used= ×1.00mol 𝑉𝑚𝑖𝑛 1000 ηcitricacid (solutionA)presentinsolution= mol [0.5] 𝑉𝑚𝑖𝑛 1000 × 1.00 × 13 [citricacid]= moldm