2025 GDLSS MS

1 2025 Preliminary Examination Chemistry Secondary 4 Express Answers Section A 1 2 3 4 5 6 7 8 9 10 D D A C B D B C C C 11 12 13 14 15 16 17 18 19 20 D A B A B A C C D C 21 22 23 24 25 26 27 28 29 30 A A D D B C D A A B 31 32 33 34 35 36 37 38 39 40 B B B A B D C B D B

2 Section B Qn Answer Mark 1(a) K AND Mg [1] 1(b) S [1] 1(c) N AND H OR N AND Br [1] 1(d) Br [1] 1(e) K [1] TOTAL [5] 2(a) 1; 3 [2] 2(b)(i) T– [1] 2(b)(ii) CaT2 [1] 2(c) liquid AND simple covalent compound / simple molecular structure / weak intermolecular forces / as it is an isotope of it which formed water. [1] TOTAL [5] 3(a) N2 + 3H2 ⇌ 2NH3; ratio of reacting volume is the same as ratio of moles for gases at the same conditions /r.t.p [2] 3(b)(i) Iron [1] 3(b)(ii) increase surface area (of contact with reactant molecules) [1] 3(c) higher than other gases [1] TOTAL [5] 4(a) +1 AND +2 [1] 4(b) correct amount of Cu AND correct amount of O e.g. amount of Cu = 4.48 / 64 = 0.07 mol amount of O = 1.12 / 16 = 0.07 mol [1] simplest mole ratio AND correct name of ore e.g. ratio = 1:1 AND tenorite [1] 4(c) correct approach with a control; - same volume, same mass of zinc, same concentration of acid, repeat experiment with same conditions but with copper added - measure time taken to collect fixed volume of gas results - shorter time taken for experiment using copper. [2] TOTAL [5] 5(a) concentration = 0.75 mol/dm3; volume = 25 cm3 [2] 5(b)(i) sulfuric acid is a dibasic acid AND higher concentration of hydrogen ions/ more hydrogen ions per unit volume; [2]

3 Qn Answer Mark higher frequency of collision/ collide more often 5(b)(ii) Insoluble layer of calcium sulfate prevent acid from reacting with carbonate [1] TOTAL [5] 6(a)(i) propanoyl chloride AND C2H5COCl; [2] 6(a)(ii) same general formula; CnH2n+1COCl [2] 6(a)(iii) C8H17COCl [1] 6(b)(i) methyl butanoate [1] 6(b)(ii) C3H7COOH + CH3OH → C3H7COOCH3 + H2O [1] 6(b)(iii) similarity same ester formed / esterification difference different byproduct / need catalyst when using ethanoic acid [2] TOTAL [9] 7(a) C2H4 + H2O → C2H5OH; Any two from (one mark for two points): • acid catalyst/ phosphoric acid • high temperature • high pressure [2] 7(b) amount of glucose = 36 x 106 / 180 = 200 000 mol amount of ethanol = 2 (200 000) = 400 000 mol [1] mass of ethanol = 400 000 x 46 = 18.4 tonnes / 18.4 x 106 g [1] 7(c) ethene obtain from crude oil / petroleum / fossil fuels which is a finite resource / will run out. glucose obtained from plants so continuous supply / will not run out / regrown quickly [2] 7(d)(i) 1371/ Mr of ethanol = 1371 / 46 = 29.8 kJ / g; = 29 800 kJ / kg; [3]

4 Qn Answer Mark OR 1000 / Mr of ethanol = 1000 / 46 = 21.74 mol; 21.74 x 1371 = 29 804 kJ/ kg statement linking 29 800 to 50 450; 7(d)(ii) ethanol has fewer carbon (and hydrogen) atoms / statement that compares the difference in energy absorbed to break bonds and the difference in energy released when bonds form in each reaction [1] TOTAL [10] 8(a) positive electrode 4OH– → 2H2O + O2 + 4e– negative electrode Cu2+ + 2e– → Cu [2] 8(b) concentration of Cu2+ decreases/ Cu2+ ions removed from solution / Cu2+ discharged/ reduced. concentration of hydrogen ions is higher than concentration of hydroxide ions / sulfuric acid is formed [2] 8(c) 750 x 4 = 3000 coulombs; 3000 x 64 = 192 000 coulombs [2] TOTAL [6] 9(a)(i) repeating unit : -NH-(CH2)6-NHCO-(CH2)4-CO- relative molecular mass (C12H22O2N2) = (12 x 12) + 22 + (16 x 2) + (14 x 2) = 226 OR total molecular mass of molecules A and B – molecular mass of two water molecules [2] 9(a)(ii) eye irritation / lungs inflammation form acid AND /corrode limestone/metal building / kills aquatic lives / leaches nutrients from soil causing poor plant growth [2] 9(b)(i) it contain ester linkage ( - COO - ) [1] 9(b)(ii) similarity (any one for 1 m) MAX 1; - small molecules joined together to form long-chain polymer/molecules [3]

5 Qn Answer Mark - polymeric product has higher molecular mass than the monomer Differences (any one for 1 m) MAX 2 - mass of poly(ethene) is the same as total mass of monomers used but mass of polymeric product in condensation polymerisation is lesser than total mass of monomers used - condensation polymer has lesser atoms than the total number of atoms of the monomers used, addition polymer has the same number of atoms as the monomers used - addition polymerisation has only one product and condensation polymerisation has two products - no other product are formed in addition polymerisation but water is formed in condensation polymerisation - addition polymerisation one functional group (-C=C-) but condensation polymerisation involve two different functional groups TOTAL [8] 10(a) [2] 10(b) Ozone protects us from UV–B and UV–C but not UV–A; Step 1 absorb UV-C, Step 3 absorb UV-B and UV-C to break the bonds; By absorbing these medium and high energy radiation, it prevents the radiations to reach the earth surface [3] 10(c)(i) O + O2 → O3 [1] 10(c)(ii) produce oxygen AND oxygen is one of the reactants in both Step 1 and 2 of Chapman Cycle to produce ozone. [1] 10(d)(i) CFC / Chlorofluorocarbon [1] 10(d)(ii) Y is used up in Step 1 but regenerate in Step 2 (thus not used up and can continuously participate in the breakdown of ozone). [1] 10d(iii) 2O3 → 3O2 [1] 10(e) product has lower energy AND ∆H (correct direction of arrow); Ea (correct label) [2] TOTAL [12]

6 Section C Qn Answer Mark 11(a) X, Y, Z, W [2] 11(b) effervescence / bubbles of gases produced/seen / fizzing; hydrogen gas is produced [2] 11(c) approach (M1) - same volume, same concentration of hydrochloric acid, fixed mass of metal measurement (M2) - time taken to collect fixed volume of gas / collection of gas over a period of time results (M3) - time taken should be longer than X, Y and Z but shorter than W / volume of gas collected should be less than X, Y and Z but more than W [3] 11(d)(i) Z; Z is less reactive than X and Y but more reactive than W. W will occurs uncombine. X and Y will require electrolysis to extract from its ores. [2] 11(d)(ii) iron / zinc / lead/ copper / silver [1] TOTAL [10] 12(a) iron is more reactive than copper / higher in the reactivity series than copper; iron rod becomes smaller AND iron displace copper from its solution/ iron becomes ions OR iron rod plated with red-brown solid AND Cu2+ ions become copper; blue solution becomes lighter blue AND lesser Cu2+ in solution / concentration Cu2+ ions in solution decrease OR solution becomes colourless AND Cu2+ ions are used up/ Cu2+ formed Cu; [3] 12(b)(i) copper, iron , zinc, X a more reactive metal will displace the less reactive metal from its salt solution [1] [1]

7 Qn Answer Mark 12(b)(ii) magnesium / aluminium / calcium [1] 12(c) approach (M1) - same mass of carbonate, heating of carbonate measurement (M2) - time taken to collect fixed volume of gas / collection of gas over a period of time / time taken to form white precipitate in limewater results (M3) - shortest time taken to collect fixed volume of gas/ shortest time taken for white ppt to form in limewater/ rate of gas collection highest for copper carbonate order (M4) highest thermal stability to lowest X carbonate, zinc carbonate, iron carbonate, copper carbonate [4] TOTAL [10]