Copy of MGS Chemistry P2 (Answers)

1 METHODIST GIRLS’ SCHOOL SEC 4 CHEMISTRY PRELIM EXAM 2024 Answer Scheme Section A A1 (a)(i) F 1 (a)(ii) A 1 (a)(iii) E 1 (b) + 2 A2 (a)(i) Carbon dioxide / CO2 1 (a)(ii) Redox reaction or reduction 1 (a)(iii) CuO + C → Cu + CO2 1 (b)(i) At mass 0g, there is no theoretical yield of copper or If no copper carbonate is used, no copper is made Or No reaction 1 (b)(ii) 80g The theoretical yield is directly proportional to the mass of copper at the start / the graph has a positive gradient or; linearly related or; as mass of copper(II) carbonate doubles, mass of copper doubles so at 40g, the starting mass of copper will be 80g. or; when the yield is 20g, the mass of CuCO 3 is 40g. hence by proportion, when the yield is 40g the mass of CuCO3 is 80g. 1 1 H H N Key: x : electron of hydrogen • : electron of nitrogen H H

2 (b)(iii) Tick (✓) two boxes. She did not use enough copper carbonate. She did not dry the copper at the end. She did not heat the copper oxide for long enough. Her copper contains solid impurities. 2 (c) Students need to calculate out the % yield of both Nina and Kai and make a comparison. Kai’s % yield is higher than that of Nina’s, so he has extracted more copper than Nina. % yield for Kai = 4.8/5 x 100% = 96% % yield for Nina = 18 / 26 x 100% = 69.2% 1 1 A3 (a) Cathode: Mg2+ (l) + 2 e- → Mg (l) Anode: 2 Cl - (l) →Cl2 (g) + 2e- State symbols not required 2 (b) To prevent molten magnesium from (coming into contact and hence) reacting with oxygen in the air 1 (c) Magnesium chloride has a very high melting point. Lowering its melting point will reduce energy cost / less energy needed./ lower cost of electricity 1 (d) Fe (s) → Fe2+ (l) + 2 e- or Fe (s) → Fe3+ (l) + 3 e- The iron will react with the chlorine evolved to form iron(II)/ iron(III) chloride OR The iron anode, being a reactive anode, will be oxidized and shrink in size / become smaller / dissolve into the molten electrolyte, and no bubbles will be formed. 1 1

3 A4 (a) Propanone: In experiment 1 and 2, increases (rate)/doubling concentration doubles rate iodine: In experiment 2 and 3, no effect (on rate) Must state the experiments 1 1 (b) particles move faster/particles have more energy / KE more particles have (energy greater than) the activation energy/sufficient energy to overcome the energy barrier and leads to higher frequency of effective collisions 1 1 (c) 1 (d) isotope number of electrons number of neutrons Iodine−127 53 74 Iodine−130 54 77 2 A5