2022 EFSS Chem Prelims P2 Ans

1 EDGEFIELD SECONDARY SCHOOL 2022 PRELIMINARY EXAM Secondary 4 Express CHEMISTRY 6092 August 2022 Answer scheme Paper 2 (80 marks) Qn. Answers Marks A1 a) A 1 5 b) D 1 c) B 1 d) F 1 e) I 1 A2 a) water and salts have different boiling points (1) water evaporates AND salts / residues / impurities / solids left in flask (1) water condenses / turns to liquid in the condenser (1) 3 10 b) i) Mg2+ and Cl– ii) 1.26/95 = 0.01325 mol (1) 0.01325 x 2 = 0.0265 mol/ dm3 (1) iii) white ppt/ solid/ deposit 2 c) i) Ba2+(aq) + SO42–(aq) à BaSO4(s) (1) ii) conc of SO42– = 1.24/96 = 0.0129 mol/dm3 (1) mol of SO42– = 0.129 x 50/1000 = 6.46 x 10–4 mol (1) mol of BaSO4 = 6.46 x 10–4 mol mass of BaSO4 = 6.46 x 10–4 x 233 = 0.150 (1) mass of SO42– in 50 cm3 seawater = 1.24 x 50/1000 = 0.062 g (1) mol of SO42– = 0.062/96 = 6.46 x 10–4 mol (1) mol of BaSO4 = 6.46 x 10–4 mol mass of BaSO4 = 6.46 x 10–4 x 233 = 0.150 (1) - Many students failed to notice that the concentration is given in g/dm3 so attempted to find moles of SO42– by multiplying 1.24 x 50/1000. 3

2 A3 a) 2.8.4 1 8 b) Si + 2Cl2 → SiCl4 - Many students attempted to give the state symbols but got it wrong (not penalised) - Do note that state symbols are not required unless stated in qns. Wrong state symbols may be penalised. 2 c) does not conduct electricity (1) low melting point / low boiling point (1) - Low density is not accepted since density depends on the actual state of the substance i.e. solid, liquid or gas. e.g. iodine has SMS but density is not low. 2 d) minus one mark for any mistake: missing legend/ non-bonding electrons 2 A4 a) Dividing % by mass by atomic mass N = 12.0/14 H = 3.4/1 O = 41.0/16 V = 43.6 /51 Dividing correctly by smallest to give correct ratio: N = 0.857 H = 3.4 O = 2.56 V = 0.855 Empirical formula: H 4NO3V (order of elements not considered in marking) 1 1 1 8 b) coloured Accept: can act as catalysts 1 c) NH4+ (1) VO3– (1) 2 d) (Y is an) oxidising agent (1) the oxidation number of iodine increases / iodide loses electrons / Y gains electrons (1) Reject: Y is an OA since it reacts with KI which is an RA (need to elaborate) Y is an OA since it oxidises KI to I2 (how do you know?) 2

3 e) i) Ammonia (1) ii) 2NH4VO3 → V2O5 + H2O + 2NH3 (1) 2 A5 a) C2H4O (1) Reject C12H24O6 1 9 b) 1 c) 2Na + Cl2 → 2NaCl - Many students attempted to give the state symbols but got it wrong (not penalised) - Do note that state symbols are not required unless stated in qns. Wrong state symbols may be penalised. d) sodium chloride has giant ionic lattice structure (1) a lot of energy required to overcome the strong electrostatic forces of attraction between oppositely charged ions. (1) chlorine has simple molecular structure (1) a small amount of energy is required to overcome the weak intermolecul