2022 SASS A-Z Chem Prelims AnsPt2

1 A – Z School’s Preliminary Papers 4 Express Chemistry ANSWERS: Part 2 F Fuhua Secondary School G Geylang Methodist School (Secondary) H Hua Yi Secondary School I Holy Innocents’ High School J Juying Secondary School FUHUA SECONDARY SCHOOL Paper 1 Paper 2 2022 Secondary 4 Pure Chemistry (6092) Prelim Paper 1 Suggested Answers 1 2 3 4 5 6 7 8 9 10 D A A B B B B A C C 11 12 13 14 15 16 17 18 19 20 B B D B C B D A C B 21 22 23 24 25 26 27 28 29 30 A B C C B D C B B D 31 32 33 34 35 36 37 38 39 40 B A D A B A C B C C 1 FUHUA SECONDARY SCHOOL Sec 4E Chemistry 6092 Preliminary Examinations 2022 – Mark Scheme PAPER 2 Section A [50 marks] Q Answer Mark Remarks 1a Rb and F 1 bi Na and S 1 bii two ions of Na+ [1] and one ion of S2- [1] 2 e.c.f from bi c Al and Zn 1 d Meting point decreases and reactivity increases down Group I. [1] Melting point increases and reactivity decreases down Group VII. [1] 2 e Ca + 2H2O → Ca(OH)2 + H2 1 Total 8 marks 2ai 6NO + 4NH3 → 5N2 + 6H2O 1 ii Oxidation state of nitrogen increases from -3 in NH3 to 0 in N2. Hence ammonia is oxidised. [1] Oxidation state of nitrogen decreases from +2 in NO to 0 in N2. Hence NO is reduced. [1] 2 b NO will react with oxygen to form NO2, which reacts with water and oxygen to form nitric acid which dissolves in water; to form acid rain which erodes limestone buildings and corrodes metallic structures / kills aquatic life by lowering the pH of water bodies.[1] 1 1 c Any one of the following • Producing ammonia on-site is easier as minimum storage and transportation of ammonia is required. • Not producing ammonia on-site means ammonia, being a gas, must be stored in thick pressurised containers which are heavy and difficult to transport. • Do not need to burn fuels during transportation so less greenhouse gas pollution. 1 REJECT gas escapes/ recycled gases as other plants also can. time used for transportation d At high temperature, the urea and water have more kinetic energy and collide more frequently [;] More urea and water particles collide with energy greater than or equal to the activation energy [;] Leading to higher frequency of effective collisions between urea and water. [;] 3; [2] 1-2; [1] e Ammonia has a simple molecular structure [;] with weak intermolecular forces of attraction between molecules [;] hence little amount of energy taken in to overcome the forces. [;] 2 3; [2] 1-2; [1] Total 10 marks 3a 1000 1 b Add aqueous bromine to samples of each polymer separately.[1] If reddish-brown aqueous bromine decolourises rapidly, sample is A. If aqueous bromine remains reddish-brown, then sample is C. [1] 2 c False, True, True, False 2 4  : [2] 2-3  : [1] d 2 Must show all bonds SASChemistry 2022edition

2 1 FUHUA SECONDARY SCHOOL Sec 4E Chemistry 6092 Preliminary Examinations 2022 – Mark Scheme PAPER 2 Section A [50 marks] Q Answer Mark Remarks 1a Rb and F 1 bi Na and S 1 bii two ions of Na+ [1] and one ion of S2- [1] 2 e.c.f from bi c Al and Zn 1 d Meting point decreases and reactivity increases down Group I. [1] Melting point increases and reactivity decreases down Group VII. [1] 2 e Ca + 2H2O → Ca(OH)2 + H2 1 Total 8 marks 2ai 6NO + 4NH3 → 5N2 + 6H2O 1 ii Oxidation state of nitrogen increases from -3 in NH3 to 0 in N2. Hence ammonia is oxidised. [1] Oxidation state of nitrogen decreases from +2 in NO to 0 in N2. Hence NO is reduced. [1] 2 b NO will react with oxygen to form NO2, which reacts with water and oxygen to form nitric acid which dissolves in water; to form acid rain which erodes limestone buildings and corrodes metallic structures / kills aquatic life by lowering the pH of water bodies.[1] 1 1 c Any one of the following • Producing ammonia on-site is easier as minimum storage and transportation of ammonia is required. • Not producing ammonia on-site means ammonia, being a gas, must be stored in thick pressurised containers which are heavy and difficult to transport. • Do not need to burn fuels during transportation so less greenhouse gas pollution. 1 REJECT gas escapes/ recycled gases as other plants also can. time used for transportation d At high temperature, the urea and water have more kinetic energy and collide more frequently [;] More urea and water particles collide with energy greater than or equal to the activation energy [;] Leading to higher frequency of effective collisions between urea and water. [;] 3; [2] 1-2; [1] e Ammonia has a simple molecular structure [;] with weak intermolecular forces of attraction between molecules [;] hence little amount of energy taken in to overcome the forces. [;] 2 3; [2] 1-2; [1] Total 10 marks 3a 1000 1 b Add aqueous bromine to samples of each polymer separately.[1] If reddish-brown aqueous bromine decolourises rapidly, sample is A. If aqueous bromine remains reddish-brown, then sample is C. [1] 2 c False, True, True, False 2 4  : [2] 2-3  : [1] d 2 Must show all bonds

3 2 ei 1 eii Methanol, CH3OH 1 eiii I disagree. Butanoic acid formula is C3H7COOH but this acid formula is C3H5COOH OR butanoic acid does not have C=C but this acid contains a C=C. 1 f Disadvantage: Polymer can only be disposed off by burying in landfills which will lead to land or air pollution.[1] Advantage: Polymer is durable and resistant to corrosion. [1] 2 Total 12 marks 4a Magnesium is less reactive than calcium and sodium. Hence magnesium cannot displace calcium and sodium from the corresponding salt solutions and there is no reaction. 1 bi > 14.0 C and < 32.0 C 1 ii Mg (s) + Fe2+(aq) → Mg2+(aq) + Fe(s) 1 iii Less than 19.0 C [;] Since ethanoic acid is monobasic, same concentration and volume of ethanoic acid gives half the no of moles of H+ ions[;] compared to H2SO4 which is dibasic[;], Ethanoic acid is a weak acid and some heat energy is taken in to dissociate the ethanoic acid molecules that are reformed. amount of heat energy released is less than half that for sulfuric acid. [;] 3 4; [3] 2-3 [2] 1; [1] c activation energy (arrowhead upwards) H(arrowhead downwards wards) Label reactants & products 2 3;[2] 1-2; [1] Total 8 marks 5ai X: -ve, Y: +ve 1 ii 2H2O → 2H2 + O2 1 iii Similarity: At R, Colourless and odourless hydrogen gas would be produced. [1] Difference: At S, oxygen gas would be produced for dilute magnesium chloride while chlorine gas would be produced for concentrated magnesium chloride. [1] 2 bi electrode P: M2+(aq) + 2e → M(s) [1] electrode Q: M (s) → M2+(aq) + 2e [1] 1 ecf from ai ii Reddish brown solid deposited at P. 2 ecf from ai iii Volume of hydrogen gas = 300 cm3 Moles of hydrogen gas = 300 24000=0.0125 [1] 2H+(aq) + 2e → H2(g) Cu(s) → Cu2+(aq) + 2e Change in mass of Q = 0.0125 × 64 = 0.800 g [1] 2

4 3 c Cell 1 Cell 2 pH 7 7 reaso ning Cu anode is selectively oxidised while Cu2+ ions are reduced at the cathode. Concentration of H+ and OH- remained unchanged. [;] OH- and H+ ions are discharged at anode and cathode respectively. Volume of water decreases, however concentration of H+ = concentration of OH-. [;] 3 4; [3] 2-3;[2] 1; [1] Total 12 marks Section B [30 marks] Q Answer M Remarks 6ai H+ ion has one proton but no neutron or electron. 1 ii HNO3 + H2SO4 ⇌ H2NO3+ + HSO4− CH3NH2 + H2O ⇌ CH3NH3+ + OH− 2 2 boxes 1 mark b Ka increases from 6.4 × 10 −4 for HF acid to 3.2 × 10 9 for HI acid. Hence strength of acid increases from HF to HI. [1] The Σ decreases from 1.8 for H-F bond to 1.3 for H-I bond, indicating polarity of the bond decreases. [1] Hence, the more polar the H-X bond, the weaker the acid. Student 1 is incorrect. The bond energy decreases from 566 kJ/mol for H-F to 298 kJ/mol for H-I bond, less energy taken in to break the H-X bond. [1] Hence, the lesser ener