03_2024_CCKSS_4E_Prelim_CHEM_MS

2023 / S4 / SCI CHEM / PRELIM / MS Secondary 4E Chemistry Preliminary Examination 2024 Mark Scheme Paper 1 (40 marks) 1 B 11 D 21 A 31 D 2 D 12 D 22 C 32 A 3 B 13 D 23 D 33 C 4 A 14 B 24 C 34 A 5 B 15 A 25 C 35 D 6 B 16 C 26 D 36 D 7 B 17 A 27 A 37 D 8 C 18 B 28 B 38 A 9 B 19 C 29 C 39 B 10 C 20 A 30 C 40 C A – 9, B – 10 , C – 11 , D – 10 Paper 2 Section A (70 marks) 1(a) Potassium carbonate 1 1(b) Potassium iodide 1 1(c) Ammonium chloride 1 1(d) Potassium iodide 1 1(e) Lead(II) fluoride, sodium chloride, ammonium chloride 1 1(f) Barium sulfate, lead(II) fluoride 1 Total: 6 2(a) Element K Cu Cl Mass 31.5 25.6 100-31.5-25.6 = 42.9; Ar 39 64 35.5 Mol 31.5/39 = 0.80769 25.6/64 = 0.4 42.9/35.5 = 1.2084; ;implest ratio 0.80769/0.4 = 2.0192 = 2 0.4/0.4 = 1 1.2084/0.4 = 3.02 = 3 Empirical formula is K2CuCl3 1 1 1 2(bi) Protons: 29; Neutrons: 37; Electrons: 27; 1 2(bii) Add aqueous sodium hydroxide / add aqueous ammonia 1 Blue ppt insoluble in excess / blue ppt soluble in excess to form a dark blue solution 1

2023 / S4 / SCI CHEM / PRELIM / MS Total: 6 3(a) Calcium oxide is an ionic compound with giant ionic lattice structure. 1 At high temperatures, sufficient/large amount of energy is provided to 1 to overcome strong electrostatic force of attraction between oppositely charged ions 1 3(b) CaO(s) + 3C(S) à CaC2(s) + CO(g) 1m balanced chemical equation 1m state symbols 2 3(c) Number of moles of CO present = 2400 / 24000 = 0.1 mol 1 Mole ratio, CaO/CO = 1/1 Number of moles of CO = 0.1 mol 1 Mass of CaO = 0.1 x 56 = 5.6g Percentage purity = 5.6 / 8 X 100% = 70% 1 Total: 8 4(a) Reaction is endothermic/takes in heat from the surroundings/water. 1 Causing water to freeze/cool to below 0 °C OR temperature of water to decrease below melting/freezing point (of 0 °C). 1 4(b) Single-head arrow ΔH – 1m Single-head arrow Ea – 1m Correct energy levels + reactants and products (accept names or formula, no need state symbols) – 1m 3 Total: 5 5(a) No. of moles of MgO: 1 / 40 = 0.025mol No of moles of HCl = [25/1000] x 0.1 = 0.0025 mol MgO: HCl 1: 2 Therefore, HCl is the LR 1 No of moles of MgCl2= 0.0025/2 = 0.00125 mol 1 Mass of MgCl2 = 0.00125 x [24 + 2(35.5)]= 0.119g 1

2023 / S4 / SCI CHEM / PRELIM / MS 5(b) A: Before adding, pH= 1 thus presence of strong acid As MgO is added, pH increases gradually as H+ ions is removed by OH-/ HCl reacting with MgO B: At pH=4, the reaction is complete/neutralisation C: pH remains at 7 as excess MgO is insoluble in water/ no increase in concentration of OH- ions 6pts – 4m 4-5pts – 3m 2-3 pts – 2m 1pt – 1m 4 Total:7 6(a)(i) The high temperature of the engine causes the nitrogen and oxygen in air to react and form nitrogen oxides. 1 Incomplete combustion of fuels 1 6(a)(ii) Eye/ lung irritation, causes breathing difficulties 1 Prevents transport of oxygen to the body, leading to breathing d