2020 SNGS Prelim Answers

2020 SNGS Sec 4 OP Prelim 2020 Sec 4 Prelim P1 Answers 1 D 21 C 2 A 22 B 3 D 23 B 4 D 24 C 5 C 25 B 6 C 26 A 7 B 27 C 8 A 28 B 9 D 29 B 10 D 30 B 11 B 31 D 12 C 32 C 13 C 33 A 14 D 34 C 15 D 35 C 16 B 36 D 17 B 37 B 18 B 38 D 19 C 39 C 20 A 40 C

2 2020 Sec 4 Prelim P2 Answers Section A A1 (a) (i) brass [1] (ii) nitrogen monoxide [1] (iii) Aluminium/ Ammonium chloride [1] (iv) Ammonium chloride [1] (v) water [1] (b) Mix aqueous sodium carbonate and aqueous silver nitrate. [1] Filter the mixture [0.5] to obtain residue.[0.5] Wash with plenty of water [0.5] and dry between sheets of filter paper. [0.5] Wrong reagent max 1m [3] A2 (a) Closely together , disorderly [0.5] Slide over each other [0.5] Unacceptable : closely packed [1] (b) R : Group II [1] Q : Group VI [1] [2] (c) RCl2 has a giant ionic structure [0.5] The elec trostatic forces of attraction between oppositely chargely io ns are weakened [1] The ions are mobile for the conduction of electricity.[0.5] QCl2 has a simple covalent/molecular structure [0.5m] consisting discrete molecules [0.5m] which are electrically neutral. [0.5m] There are no mobile electrons/ions for the conduction of electricity. [4] (d) Dilute/aqueous hydrochloric acid [1] [1] (e) Any macromolcules [1]

3 A3 (a) name of halogen Melting point /oC Boiling point /oC bromine -7.2 58.8 chlorine -100.9 -34.7 Iodine 113.8 184.5 [1] (b) Reactivity decreases down the group [0.5] atomic size / electron shell /radius increases down the Group [0.5] OR distance between nucleus and valence shell increases [0.5] electrostatic forces of attraction between nucleus and electron decreases [0.5] gains electron less easily[0.5] [2] (c) (i) seawater turns reddish brown/orange[1] Chlorine is more reactive than bromine. [0.5] Chlorine displace bromine from its solution [0.5] [2] (ii) Add dilute nitric acid followed by aqueous barium nitrate [1] white precipitate observed.[1] [2] (d) Chlorine is oxidised [0.5] as the oxidation state of chlorine increases from 0 in Cl2 to +3 in ClF3. [1m] reducing agent [0.5] [2] A4 (a) carbon dioxide / CO2 [1] (b) Zinc ion [0.5] and copper(II) ion[0.5] When aqueous ammonia is added, precipitate is formed and dissolves in excess aqueous ammonia.[1] [2] (c) ZnCO3→ ZnO + CO2 Or CuCO3 → CuO +CO2 [1] (d) Ag+ +Cl- → AgCl [1] [1] (e) All the chloride ion have been precipiated/reacted[1] [1] A5 (a) Concentrated [0.5] copper(II) bromide [0.5]/ Concentrated CuBr2 [1] (b) (i) Electrode C : 2H+ +2e → H2 [1] Electrode D : 4OH- → O2 + 2H2O + 4e [1] [2] (ii) Haber process/ rocket fuel/fuel cell [1] (c) Mole of H2 = 0.084/24 = 0.0035 mol [0.5] Mol of copper = 0.0035 mol[0.5] Mass of copper = 0.0035x64= 0.224g [1] [2]

4 (d) Universal indicator change from green to blue/purple [1] Chloride ions are selectively discharged [0.5] Potassium hydroxide is formed whic