Woodlands Prelim P2 Ans

1 WOODLANDS SECONDARY SCHOOL PRELIMINARY EXAMINATION 2024 MARKING SCHEME FOR PAPER 2 Level: Secondary Four Marks: 80 Stream: Express Date: 20 August 2024 Subject: Pure Chemistry Day: Tuesday Section A – Structured Questions (70 marks) 1 Total = 5 (a) Magnesium oxide, zinc oxide 1 (b) Silicon dioxide 1 (c) Zinc oxide 1 (d) Sulfur dioxide, carbon dioxide 1 (e) Carbon monoxide 1 2 Total = 4 (a) Aqueous ammonia and concentrated hydrochloric acid evaporate respectively to form ammonia gas and hydrogen chloride gas. The gases diffuse towards each other to form ammonium chloride (white fume) nearer to the watch-glass of hydrochloric acid. This is because ammonia has a lower relative molecular mass of 17 as compared to hydrogen chloride with a relative molecular mass of 36.5 1 1 1 max 2 (b) 2

2 3 Total = 8 (a) Hydrogen fluoride has a low melting and boiling point / does not conduct electricity in any states. 1 (b) HF ⇌ H+ + F– Hydrofluoric acid dissociates partially in water to form a low concentration of hydrogen ions. 1 1 (c)(i) 2HF + Mg → MgF2 + H2 1 (c)(ii) No. of moles of HF = 0.200 / (1 + 19) = 0.01 mol Compare mole ratio of HF : H2 = 2 : 1 = 0.01 mol : 0.005 mol Volume of hydrogen gas = 0.005 x 24 = 0.12 dm3 1 1 (d) Magnesium fluoride is an ionic compound with a giant ionic crystal lattice structure held by strong electrostatic forces of attraction between the positive and negative ions / oppositely charged ions which require a large amount of energy to overcome. Hence, it has a high melting point. 1 1 4 Total = 6 (a)(i) Hydrogen gas / H2 1 (a)(ii) Ammonia gas / NH3 1 (a)(iii) Iron 1 (b) Add excess zinc oxide to dilute nitric acid and stir until no more zinc oxide can dissolve. Filter the mixture to remove the excess unreacted zinc oxide as the residue. Collect the filtrate (aqueous zinc nitrate). Heat the filtrate to evaporate most of the water to obtain a saturated solution of zinc nitrate. Allow the saturated solution to cool so that the salt can crystallise. Filter to collect the crystals as the residue. Wash the crystals with a little cold distilled water to remove any water-soluble impurities. Dry the crystals between a few sheets of filter paper. 1 1 1

3 5 Total = 4 (a) The colourless solution will turn brown. Chlorine is more reactive than iodine. Hence, it will displace iodine from aqueous potassium iodide, forming potassium chloride and iodine solution (brown). 1 1 (b) The oxidation state of oxygen increases from –1 in H2O2 to 0 in O2. Hence, oxidation takes place. The oxidation state of chlorine decreases from 0 in Cl2 to –1 in HCl. Hence, reduction takes place. Since both o