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) Mo