SJI 2021 Y4 Prelim Chemistry 6092 P1 & 2 Answers

1 St. Joseph’s Institution 2021 Year 4 OP Preliminary Exam Answer Scheme Paper 1 1 2 3 4 5 6 7 8 9 10 A C C D B A D D C B 11 12 13 14 15 16 17 18 19 20 B A D C D C B C D C 21 22 23 24 25 26 27 28 29 30 C A B D A B B C D C 31 32 33 34 35 36 37 38 39 40 A D B C C B C C C D Paper 2 Section A Qn Suggested answers Mark A1a 1 A1b To react with the colourless spots to make them visible. 1 A1ci X and Y 1 A1cii L, M, N and P 1 A1d Show measurements on diagram (W and Z) Rf value = W / Z = (answer to 2 d.p) 1 1 TOTAL 6 S W Z

2 Qn Suggested answers Mark A2ai 400 – 450 °C, 200 – 250 atm, finely divided iron catalyst 1 A2aii Calcium hydroxide reacts with ammonium-based fertilisers to form ammonia gas, which escapes into the atmosphere. This results in the loss of nitrogen from the fertilizer which is an important element for plant growth. 1 1 A2bi When aqueous ammonia is added dropwise then in excess, green precipitate formed which is insoluble in excess aqueous ammonia. 1 A2bii Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s) [1m – ionic equation, 1m – state symbols] 2 A2ci Ammonium chloride salt is thermally unstable and decomposes on heating to form hydrogen chloride and ammonia gas. 1 A2cii crystallisation 1 A2d calcium oxide / quicklime 1 TOTAL 9 A3ai An acid is a substance or compound that dissolves and dissociates in water to form hydrogen ions (as the only positive ions). 1 A3aii CH3COOH(aq) ⇌ CH3COO- (aq) + H+ (aq) [1m – equation with reversible sign, 1m – state symbols] 2 A3b Similarity: In both experiments, the colourless solution turns blue / solid dissolves to form a blue solution. Difference: In experiment 1, effervescence is observed which is not observed in experiment 2. 1 1 A3ci 4KI + 2CuCl2 → 2CuI + I2 + 4KCl 1 A3cii The oxidation number of copper decreases from +2 in CuCl2 to +1 in CuI. This is reduction / CuCl2 is reduced. The oxidation number of iodine increases from -1 in KI to 0 in I2. This is oxidation / KI is oxidised. Since oxidation and reduction occurs simultaneously, this is a redox reaction. 1 1 A3d step 1: Add excess zinc oxide to dilute hydrochloric acid. step 2: Filter the mixture. The filtrate is zinc chloride solution. step 3: Heat the filtrate to saturation, leave the solution to cool. Collect the crystals by filtration. step 4: Rinse the crystals with distilled water and dry between pieces of filter paper. 1 1 1 1 TOTAL 12

3 Qn Suggested answers Mark A4a Student A is wrong, Student B is correct. [must state but no mark] If concentration is decreased, the speed of reaction will decrease, however the volume of carbon dioxide formed will also decrease as dilute hydrochloric acid is the limiting reactant / zinc carbonate is in excess. If zinc carbonate granules are used, the total