2022 HYSS Chem Prelims Ans

Marking scheme Chemistry Prelims 2022 Paper 1: 1 2 3 4 5 6 7 8 9 10 D A C A A B A B C A 11 12 13 14 15 16 17 18 19 20 A B D B C B C D C A 21 22 23 24 25 26 27 28 29 30 D D C A B C D D B B 31 32 33 34 35 36 37 38 39 40 C C B B D D A C D C

Marking scheme Chemistry Prelims 2022 Paper 1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Paper 2: A1 (a) Fe (b) Pt (c) Zn (d) C (e) C / N (f) K / Ca 1m each A2(a) The molecules can roll and glide over one another and can move throughout the liquid. They are arranged closed together in a disorderly arrangement. 1m 1m A2(b) 1m A2(c) Bromine is more reactive than iodine and gains electrons more readily. Hence it will displace iodine from KI to give KBr and I2. Br2 (l) + 2 I- (aq) → 2 Br- (aq) + I2 (s/aq) 1m 1m 1m A3(a) (i) no change/(solution remains pink) (ii) grey solid; forms in green solution 1m 1/2m, 1/2m A3(b) CuO + 2 E → E2O + Cu 1m formulae 1m balanced A3(c) Electrolysis 1m A4(a) Total amount of energy absorbed during bonding breaking = (413 × 8) + (390 × 6) + (3 × 498) = 7138 kJ Total amount of energy released during bonding forming = (413 × 2) + (891 × 2) + (464 × 12) = 8176 kJ Total amount of energy released is greater than total amount of energy absorbed OR Enthalpy change of energy is negative 1m 1m 1m A4(b) Platinum is a catalyst. It provides an alternative pathway with a lower activation energy. The proportion of particles with sufficient energy to overcome the activation energy increases. As a result, the reacting molecules/ particles (reject ions) collide with each other more often causing the frequency of effective collisions to increase and therefore the rate of reaction. 1m 1m 1m Br Br

A4(c) The reaction is reversible so hydrogen cyanide can decompose back to its reactants as it is formed. 1m A5(a) F: ammonium sulfate G: barium sulfate L: calcium carbonate 1m 1m 1m A5(b) Blue precipitate formed which dissolves in excess ammonia solution to give a dark blue solution 1m 1m A6(a) 1. Purple solution turned colourless. 2. Effervescence observed. 1m 1m A6(b) The oxidation state of C increases from +3 in C2O42- to +4 in CO2. C2O42- is oxidised to form CO2. C2O42- is the reducing agent. OR The oxidation state of Mn decreases from +7 in MnO4- to +2 in Mn2+. MnO4- is reduced to Mn2+. C2O42- is the reducing agent. 1m 1m A6(c) No. of moles of KMnO4 used = 250/1000 x 0.5 = 0.125 mol No. of moles of CO2 produced = 0.125 x (10/2) = 0.625 mol Volume of CO2 produced = 0.625 x 24 = 15.0 dm3 1m 1m 1m A7(a) (i) electrons (ii) ions 1m 1m A7(b) Apparatus A: A1 Apparatus B: B1 1/2m 1/2m A7(c) Copper(II) ions (Cu2+) gain electrons more easily than hydrogen ions (H+) to form copper solid. OR Copper(II) ions (Cu2+) are preferentially