2025 CHIJ SEC (TP) Chemistry Prelim Paper 1 and Paper 2 MS

CHIJ SECONDARY Sec 4 Chemistry Prelim Exam 6092_2025 Mark Scheme / P1/10 CHIJ SECONDARY Sec 4 Preliminary Examination 2025 Chemistry 6092 Mark Scheme Updated on 14/09/2025 Total Score = (P1/40×30%) + (P2/80×50%) + (P3/40×20%) Paper 1 (40 marks) No Answer No Answer No Answer No Answer 1 A 11 C 21 C 31 B 2 B 12 A 22 C 32 C 3 B 13 B 23 B 33 C 4 D 14 D 24 A 34 B 5 C 15 B 25 D 35 B 6 D 16 D 26 D 36 B 7 D 17 C 27 D 37 A 8 A 18 A 28 C 38 B 9 C 19 C 29 A 39 B 10 D 20 C 30 C 40 A Abbreviations: ORA: or reverse argument AW: alternate wording ECF: error carried forward (for calculations only)

CHIJ SECONDARY Sec 4 Chemistry Prelim Exam 6092_2025 Mark Scheme / P2/10 Paper 2 Section A (70 marks) Question Answers Marks Total Comment 1 (a) Zn 1 5 (b) Br 1 (c) Li 1 (d) Ar 1 (e) Ag 1 ZnO is white when hot and yellow when cold, so it only forms 1 coloured compound. 2 (a) The black dye is impure. (no mark) There is more than one component/spot on the chromatogram. 1 6 (b) Boil/freeze the black dye / Determine the boiling point/freezing point of the black dye. If the black dye boils/freezes at a fixed temperature, it is pure. or If the black dye boils/freezes over a range of temperatures, it is not pure. 1 1 Reject methods relating to melting. (c) Dye B is insoluble in the solvent/ethanol. 1 (d) Dye A Distance of A from start line = 2.5 cm Distance of solvent front from start line = 5.5 cm (values must be correct) Rf value = 2.5 / 5.5 = 0.45 OR 0.45 = distance moved by solute ÷ (7 – 1.5) distance moved by solute = 0.45 × 5.5 = 2.475 Position on chromatogram = 2.475 + 1.5 = 3.975 ≈ 4cm 1 1 3 (a) nitrogen = 78% oxygen = 21% 1 7 % sign is not required (b) 32 / 24 = 1.33 g/dm3 1 (c) (i) The total percentage composition of nitrogen and oxygen on Mars is much lower at 2.8% as compared to 99% on Earth / The percentage composition of nitrogen and oxygen on Mars is much lower at 2.6% and 0.3% compared to 78% and 21% on Earth. At high temperatures (in engines of vehicles or in factories or during lightning activity ), nitrogen and oxygen in the air can react together to form nitrogen monoxide , forming more nitrogen oxides on Earth. 1 1

CHIJ SECONDARY Sec 4 Chemistry Prelim Exam 6092_2025 Mark Scheme / P3/10 Question Answers Marks Total Comment (ii) In step 1, NO increased the rate of breakdown of ozone by providing alternative reaction pathway of lower activation energy; In step 2, the regeneration of NO showed that it remained chemically unchanged at the end of the reaction. 1 1 (d) Mars has a high composition of carbon dioxide which traps heat/causes global warming. 1 4 (a) (i) Y (Anode): 2Cl − → Cl2 + 2e− Z (Cathode): 2H+ + 2e−→ H2 1 1 8 (ii) Universal Indicator turns from green to purple/violet. Since Cl− and H+ ions are discharged, K+ and OH− ions (or KOH) are left behind / the relative concentration of OH− ions increases, forming a strong alkali. 1 1 (b) Similarity: A red-brown deposit/coating forms at electrode X (cathode) / electrode X (cathode) increases in size. Reasoning (Similarity): In both cases, copper is less reactive than hydrogen, so copper(II) ions from the solution are preferentially discharged/reduced over hydrogen ions, and are reduced to form copper metal. Difference: When copper electrodes are used, electrode W decreases in size. When graphite electrodes are used, effervescence is observed at W / the electrodes do not change in size. Reasoning (Difference): • When copper electrodes are used, copper anode W oxidises to form Cu2+. • When graphite electrodes are used, the OH− ions are preferentially discharged over SO42− ions to form oxygen gas, Or Difference: When copper electrodes are used, the blue solution does not decolourise. When graphite electrodes are used, the blue solution decolourises. Reasoning (Difference) : 1 1 1 1 1 Similarity 1M Difference 1M Reasoning 1M (for similarity) 1M (for difference)

CHIJ SECONDARY Sec 4 Chemistry Prelim Exam 6092_2025 Mark Scheme / P4/10 Question Answers Marks Total Comment • When copper electrodes are used, the concentration of Cu2+ ions in the solution remains unchanged. • When graphite electrodes are used, the concentration of Cu2+ ions decreases. 1 5 (a) element Fe O mass / g 4.2 1.8 Ar 56 16 no. of moles 0.075 0.1125 mole ratio 1 1.5 [1] for each correct no. of moles calculated empirical formula is Fe2O3 2 1 8 (b) (i) magnesium oxide – no visible change / solid remains white copper(II) oxide – black solid turns red-brown 1 1 (ii) Copper(II) oxide Copper is the least reactive compared to magnesium and lead. Copper(II) oxide is the least stable metal oxide and will undergo reduction at the fastest rate. 1 1 1 6 (a) • Compounds have the same general formula of CnH2n+1NH2 • Compounds have the same functional group −NH2 (may consider) • Successive members differ from one another by a −CH2− unit 1 1 7 Reject: Gradual increase in physical properties like boiling point down the series. (b) Every 2 points is 1 mark: • Particles in methylamine are very far apart in a disorderly manner • while those in propylamine are closely packed in a disorderly arrangement • Particles in methylamine are moving rapidly and randomly in all directions • while those in propylamine are sliding past one another. 2 Must show comparison of arrangement and movement. (c) (i) (CH3NH3)2SO4 1 (ii) Blue precipitate forms. Soluble in excess methylamine to form a dark blue solution. 1 1

CHIJ SECONDARY Sec 4 Chemistry Prelim Exam 6092_2025 Mark Scheme / P5/10 Question Answers Marks Total Comment 7 (a) It is a long-chain molecule / macromolecule with many ester linkages 1 9 Both ideas for 1M (b) Water is added to the polymer, with acid as a catalyst Which causes the polymer to break down into its monomers. 1 1 (c) 1 (d) [1] correct shape [1] correctly labelled activation energies [1] correctly labelled enthalpy change 3 (e) (i) 1 (ii) Non-biodegrable plastics leads to increase in built-up waste in landfills when disposed by burying, causing land pollution; or When burnt, plastics can release toxic gases into the atmosphere, causing air pollution. or When thrown into water bodies, plastics can cause marine organisms to mistake them as food and choke on them, causing death. 1 monomer A

CHIJ SECONDARY Sec 4 Chemistry Prelim Exam 6092_2025 Mark Scheme / P6/10 Answers Marks Total Comment 8 (a) (i) The higher the concentration, the faster the rate of reaction. (ORA) [1] The more reacting particles per unit volume, the higher frequency of effective collisions. [1] 2 12 Question was generally well done. A number of students mentioned activation energy, which was not relevant for concentration. (ii) When the concentration of C2H4 doubles from 0.10mol/dm3 in Experiment 1 to 0.20mol/dm3 in Experiment 3 / 2 to 4, the rate of reaction doubles from 0.10 moldm- 3s-1 to 0.20 moldm-3s-1. [1] When the concentration of Br2 doubles from 0.10mol/dm3 in Experiment 2 to 0.20mol/dm3 in Experiment 3, the rate of reaction doubles (from 0.10 moldm-3s-1 to 0.20 moldm-3s-1). [1] Hence the reaction is first order with respect to C2H4 and first order with respect to Br2. [1] 3 Students must either cite the experiments or concentrations/ rates they are making reference to clearly. Answers that calculated the constant k to show that it