2022 SASS A-Z Chem Prelims Ans Pt5

1 A – Z School’s Preliminary Papers 4 Express Chemistry ANSWERS: Part 5 FINAL U YUying Secondary School V Victoria School W West Spring Secondary School X Xinmin Secondary School Y Yuhua Secondary School Z Zhenghua Secondary School YUYING SECONDARY SCHOOL Paper 1 1 B 11 D 21 A 31 D 2 A 12 D 22 B 32 A 3 A 13 C 23 C 33 C 4 C 14 B 24 C 34 D 5 A 15 D 25 D 35 D 6 A 16 B 26 C 36 C 7 C 17 B 27 A 37 C 8 D 18 C 28 C 38 C 9 A 19 D 29 C 39 B 10 C 20 A 30 D 40 A Paper 2 A1a) B b) C c) C d) A e) E A2) before – solid structure of unshaded magnesium atoms, regularly arranged. [1] After – alternating solid structure of unshaded magnesium atoms and shaded oxygen atoms in the 1:1 ratio, regularly arranged. [1] A3a) X marked a the intersection of the two curves. [1] b) Increase the pressure to 50 atm and above to obtain liquid carbon dioxide. [1] c) 1 m double bond, 1 m all other electrons correct. d) Acidic oxide [1]. It reacts with bases to form salts / dissolves in water to form acid. [1] SASChemistry 2022edition The finallap towards ❷❺OCT⓿❽NOVP2 P1

2 A4a) If only water is used, yellow dye is insoluble [1]. If only ethanol is used, red dye is insoluble [1] b) At 80% ethanol, all the dyes have the same solubility, so there will be only one spot formed on the chromatogram. c) blue: 0.5, red: 0.68, yellow: 0.16 2 marks for all 3 correct, 1 mark for 2 correct, ½ mark for 1 correct. A5a) All the 3 valence electrons of boron are used in the forming of boron nitride.[1] It has no delocalized electrons to move about to conduct electricity. [1] b) There are weak van der Waals forces [½] between the hexagonal rings [½] of boron nitride. These forces are easily overcome [½], hence boron nitride is soft and slippery [½]. It is suitable to be used for lipsticks. c) Very high melting point A6a) True, False, True (1m each) b) Add excess water to the mixture. [½] Stir thoroughly to dissolve all the copper(II) halide. [½] Filter the mixture to remove the copper as residue. [½] Wash the copper with distilled water and dry between pieces of filter paper. [½] c) Mole of cu = 96/64 = 1.5 mols [½] mole of CuCl2 = 1.5 mols [½] mass of CuCl2 = 1.5 x 135 [½] = 202.5 g [½] A7a) Q, copper, P, S, R [1] b) 1. The blue [½] solution decolourises. [½] 2. Pinkish brown deposits of copper formed [1] c) The voltage difference between zinc and copper is more than the voltage difference between P and copper [½]. P is a more reactive metal than copper, but less reactive than zinc [½]. Hence, P is extracted from it’s oxide by reduction with carbon. [1] d) metals have a ‘sea of delocalized electrons’ which are free to move about to conduct electricity. A8a) test: add aqueous silver nitrate. [1] With NaCl: a white precipitate is seen. [½] With Na2SO4 : no precipitate seen. [½] Or: can use aqueous barium nitrate and answers given accordingly. b) Add excess dilute nitric acid to dissolve both carbonates. [1] Add aq. NaOH (or aq. ammonia) [1] With Na2CO3 : no precip