PRSS 4E Chemistry Prelim 2023 ANSWERS (Paper 1 and 2)

SEC4EXPRESS–CHEMISTRY6092PRELIMEXAM2023–ANSWERSPaper 1–MCQ 1 B 11 A 21 B 31 C2 C 12 C 22 C 32 D3 B 13 B 23 B 33 A4 B 14 A 24 C 34 B 5 B 15 C 25 D 35 B6 C 16 C 26 D 36 C7 B 17 A 27 C 37 C8 C 18 D 28 A 38 B9 A 19 B 29 B 39 A10 D 20 C 30 D 40 B

Paper 2–SectionA–StructuredQuestions No SuggestedAnswers MarksA1(a) (i) D(ii) B(iii) A Ingeneral,part(i)and(iii)arenotdonewell.MoststudentsdidnotrelatemetalwithlowmeltingpointasgroupIelement. [3] A1(b) Ehas achieved stable noble gas configuration / full valence shell ofelectrons.[1/2]Hence, Edoesnot havetogain, loseor shareelectrons.[1/2] Moststudentsfinditchallengingtorelatewhyatomreactsas“gain/loseorshare”electrons. [1] A2(a) Carbondioxidehasa simple molecular structure[1/2]where discretemolecules are held together by weak intermolecular forces ofattraction[1/2]. Little amount of energyisrequiredto overcometheseforces[1/2]. Silicondioxidehasa giant molecular structure[1/2]where atoms areheld together bystrongcovalent bonds[1/2]. Largeamount of energyisrequiredtobreakthesebonds[1/2]. Hence, carbon dioxide has a lower meltingpoint thansilicondioxide.(Minus½ifthispointismissing) [3] A2(b) (i) Correctnumberofchargesforeachion–[1]Correctvalenceelectronsdrawnforeachion–[1] (ii) Na2 O contains ionic bonds / involves transfer of electronsfrom Na to O[1/2] where oppositely charged ions are heldtogether bystrongelectrostaticforcesof attraction[1/2]. Commonerror:atomsinsteadofionsarementionedforNa2 O. Cl2 O involves sharing of valence electrons[1/2]where atomsareheldtogether bystrongcovalent bonds[1/2]. Accept: contains covalent bonds(alternativeformarkingpointon“sharingof valence”, ifstudentsdidnotmention“atomsareheldtogetherbystrongcovalentbonds” [4]

Most students compare differences in structure which is notrequiredinthequestion. A3(a) (i) T: iron(II) chloride(ii) U: carbondioxide(iii) V: iron(II) hydroxide(iv) W: silver chloride Alsoacceptchemicalformulae [4] A3(b) MixtureSlikelycontainsiron(II), carbonateandnitrateions.Allthreeionsmustbecorrect.Alsoacceptformulaeofions:Fe 2+ ,CO3 2– ,NO3 – Reject:chloride(couldhavecomefromhydrochloricacid) [1] A3(c) CO2 +Ca(OH)2 →CaCO3 +H2 OFe 2+ (aq)+2OH – (aq) →Fe(OH)2 (s)Ag + (aq)+Cl – (aq) →AgCl(s)CO3 2– (aq)+2H + (aq) →CO2 (g)+H2 O(l) Acceptanyoftheaboveoranyotherpossiblereactions.Commonerror:incorrectformulaeofsilverchloride,iron(II)nitrateetc [1] A4(a) Repeat the experiment with known titre value without addingindicator[1/2]Alsoacceptanswerifstudentswrotetheprocedureswithvaluesindicated(e.g.25.0cm 3 ofLiOHaddedto…) Heat themixtureuntil asaturatedsolutionisformed.[1/2]Coolthesolutionfor crystalstoform.[1/2]Filterthemixturetoobtainthecrystals. Drythecrystalsbetweensheetsoffilterpaper.[1/2] [2] A4(b) Magnesiumhydroxideissparinglysoluble/ insolubleinwater./Itisdifficult todeterminetheend-point of theneutralisation(whenmagnesiumhydroxideexistsasasolid).OR Mixturethat isformedmaycontainexcessacid.[1]Reject:reactionbetweenmagnesiumhydroxideandacidisreactive/unsafe. [1] A4(c) Molesofhydrochloricacid=2.70x(22.50/1000)=0.06075mol--------------------------------[1] Sincehydrochloricacidandlithiumhydroxidereactsin1:1ratio,molesoflithiumhydroxide=0.06075mol Concentrationoflithiumhydroxide=0.06075÷(25/1000)=2.43mol/dm 3 -----------------------------[1] [2]

A5(a) Bothmeltingpointsandboilingpointsof elements increase[1/2]downGroupVII. DownGroupVII,the intermolecular forces of attractionincreases[1/2]as the molecular size of halogen increases[1/2]. Hence, a largeramount of energyisrequiredto overcometheseforcesof attraction.[1/2] misconception: studentsrelatetheincreaseinmp/bptowithintheatomitself, where a larger amount of energy is required to overcomeelectrostaticforcesof attractionbetweennucleusandvalenceelectron(downthegroup) [2] A5(b) (i) Fluorine/ chlorine[1],astheyaremorereactivethanbromine[1/2],andcandisplacebrominefromitsaqueoussolution[1/2]. (ii) F2 (aq) +2Br – (aq) →Br2 (aq) +2F – (aq)ORCl2 (aq) +2Br – (aq) →Br2 (aq) +2Cl – (aq) [1] notpenalisedformissing/incorrectstatesymbolsinthisexam [3] A6(a) 1. transitionelementsexhibit variableoxidationstatesintheircompounds/ formcompoundsthat havevariableoxidationstates[1]2. transitionelementscanformcolouredcompounds[1] [2] A6(b) (i) CuOgainedelectronsandreducedtoformCu2 OORCu 2+ inCuOgainedelectronsandreducedtoformCu + inCu2 O.[1] (ii) Theoxidationstateof oxygeninCuOincreasesfrom-2inCuOto0inO2 .[1] [2] A6(c) (i) Bluesolutionturnedgreen/ palegreen[1/2](notcolourless)Reddish-brown/ pinkdepositscanbeobserved.[1/2] Ironismorereactivethancopper,[1/2]henceironwilldisplacecopper fromitsaqueoussolutions.[1/2] Fe+CuSO4 →FeSO4 +Cu[1] (ii) Electrolysis/ usingelectricity.[1] [4]

A7(a) Anelectrolyticcellconvertselectrical energytochemical energy,[1/2]whileasimplechemical cellconvertschemical energytoelectrical energy.[1/2] Anelectrolyticcell requiresbatteries/ electriccells,[1/2]whileasimplechemical cell requiresacontinuoussupplyof reactants.[1/2]Reject:simplecelldoesnotrequirebatteries [2] A7(b) (i) cathode: Cu 2+ (aq) +2e – →Cu(s) [1]nostatesymbolminus½(ii) anode: 4OH – (aq) →2H2 O(l) +O2 (g) +4e – [1]nostatesymbolminus½ [2] A7(c) Acceptanytwoobservationsbelow[1/2markeach]:- Effervescencecouldbeobservedattheanode- Reddish-brown/pinkdepositscouldbeobservedatthecathode/ cathodeincreasesinsize- Blueelectrolyteturnscolourless Acceptanytwomatchingexplanation[1/2markeach]:- hydroxideionsaredischargedat theanodetoformoxygengas- copper(II) ionsaredischargedat thecathodetoformcoppermetal- concentrationof copper(II) ionsdecreases Commonerror: studentsdidnot EXPLAINtheobservationsasstatedinthequestion [2] A7(d) Zinccouldhavereactedwithwaterintheaqueouscopper(II)nitratetoformhydrogengas. Thisquestionispoorlyattempted.Candidatesarenotsensitisetotheoccurrenceofsidereaction(e.g.betweenmetalandwatertoproducehydrogengasasoneoftheproducts) Commonerror:- zincisabovehydrogenandhencehydrogenispreferentiallydischarged.notethatthisisnotpossiblewhenzincistheANODE,whichwillbeoxidised.- hydroxideionsaredischargedatanode(notpossibleasanodeisZINC/reactiveelectrode) [1]

A8(a) (i) Marker to note: to mark strictly for the ARROW. Students areadvised to use a ruler to draw their lines. Some students did notdraw vertical straight line and/or horizontal line that should beparallel tobothy-axisandx-axisrespectively. correctprofileofdiagram–[1]labelreactantsandproducts–[1]labelenthalpychangeandactivationenergycorrectly–[½x2] (ii) The amount of energy required to break bonds in hydrogensulfide and oxygen is less than the amount of energyreleasedtoformbondsinwater andsulfur dioxide. correctcomparisonintermsofamountofenergy–[1]correctidentificationofthebondstobreak/bondstoform–[1] [5]

A8(b) Totalenergyrequiredforbondbreaking=(2x2x364)+(3x498)=2950kJ-------------[1] Totalenergyreleasedduringbondforming=(2x2x464)+(2x2xX)=(1856+4X) kJ-------------[1] Enthalpychangeofthereaction=–1034kJ+2950–1856–4X=–1034kJ+1094–4X=-1034kJX=532kJ-------------[1]AlsoacceptX=532kJ/mol Allow e.c.f. for total energy calculated for bond breaking and bondforming. Somecandidatesfindit challengingtocount thenumberof bondstobreak/form, andshouldhavemadereferencetothebalancedchemicalequationgiven.Somestudentsmixedupthesigns(+/-)requiredduringcalculation for enthalpy change for bond breaking (+) and enthalpychangeforbondforming(-). [3]

Paper 2–SectionB No SuggestedAnswers MarksB9(a) (i) Biofuel iscarbonneutral asthereis no net gain/loss in theamount of carbonreleasedtotheenvironment[1] Biofuel contains carbon that comes from biomass ofanimals/ the uptake of carbon dioxide by plants[½]duringphotosynthesis, and releases carbon in the formof carbondioxideduringthecombustionof biofuel.[½] Mostcandidateshavenoconceptofthecarboncycle.Alargenumberrelates ‘neutral’ to ‘neither acidic nor alkaline’ instead of ‘no netgain/lossincarbon’ (ii) Process 1 should include processes to remove any acidimpuritiesbeforetransesterificationprocess.[1] [3] B9(b) (i) Transesterificationisthe reaction between fats/ oils/ esters(NOT carbox