2021 SNGS Prelim Answers

2021 SNGS Prelim MARKING SCHEME PAPER 1 1 B 11 D 21 A 31 C 2 D 12 D 22 B 32 C 3 B 13 D 23 D 33 D 4 B 14 D 24 C 34 A 5 C 15 A 25 C 35 D 6 D 16 A 26 C 36 C 7 C 17 B 27 A 37 B 8 A 18 B 28 C 38 A 9 B 19 B 29 C 39 B 10 C 20 B 30 B 40 C PAPER 2 A1 (a) (i) B has Weak intermolecular force of attraction [0.5] between simple molecules[0.5] Little amount of energy required to overcome forces of attraction[0.5] A has strong covalent bonds between the atoms [0.5]which extends throughout the vast network[0.5] A lot of energy required[0.5] [3] (ii) Giant metallic structure [0.5] E has strong electrostatic forces of attraction between cations and sea of delocalised electrons.[0.5] Mobile electrons[0.5] Giant ionic structure [0.5] solid D has opposite charged ions held fixed in position in solid state[0.5] no mobile ions.[0.5] [3] (b) Any group I [1] (c) iron [1]

2 A2 (a) Filter the mixture [0.5] to obtain residue[0.5] Wash residue with distilled water[0.5], completely dry the residue with sheets of filter paper[0.5] [2] (b) The universal indicator will turn blue[1] [1] (c) Silver ion has lower Ar/Mr than dichromate ion Silver ion diffuse faster than dichromate ion [1] Red solid formed nearer to ammonium dichromate[1] 2Ag+ + Cr2O72- → Ag2Cr2O7 [1] [3] (d) White solid [0.5] , silver chloride [0.5] occurs near to silver nitrate than ammonium chloride[0.5] Silver ion has higher Ar than chloride ion [0.5] [2] A3 (a) liquid hydroxypropanoic acid does not dissociate [0.5] to form mobile ions.[0.5] OR liquid hydroxypropanoic acid exists as simple molecules. Molecules are electrically neutral [0.5] No mobile ion[0.5] [1] (b) hydroxypropanoic acid : Universal indicator turns from green to orange/yellow.[0.5] hydroxypropanoic acid is weak acid. hydroxypropanoic acid partially dissociate to form H+[1] sulphuric acid : Universal indicator turns from green to red. [0.5] sulfuric acid is strong acid. sulfuric acid completely dissociate to form H+[1] [3] (c) Mole of NaOH= Mole of acid = 0.0232 x 0.1 = 0.00232 mol [1] Mr of acid = 0.172/0.00232 = 74.1 [1] [2] A4 (a) carbon dioxide[0.5], carbon monoxide[0.5]. [1] (b) Iron more reactive than hydrogen[1] hydrogen ions are preferentially discharge/reduced than iron (III) ion[1] [2]

3 (c) Anode : 2O2- → O2 + 4e [1] Cathode : Fe3+ +3e →Fe [1] [2] (d) 2Fe2O3 → 4Fe + 3O2 Mole of Fe2O3 = 10x 106/160= 62500 mol [1] Mole of Fe = 62500 x 2 =125000 mol[1] Mass of Fe = 125000x 56 = 7 tonnes.[1] [3] (e) Aluminium oxide is very stable.[1] Alumium oxide cannot be reduced by carbon/hydrogen/carbon monoxide[1] [2] A5 (a) Beaker Y as the electrodes are metal of different reactivity[1] Metals are connected in an electrolyte[1] [2] (b) [2] (c) Electrode B : Effervescence, colourless and odourless gas at the anode.[1] Electrode A: Cathode become bigger/ reddish brown solid c