RGS_Chem_Y4_2022_PPA2_Ans

Raffles Girls’ School (Secondary) 2022 Year 4 Chemistry PPA 2 Answer Scheme (Student) Section A: MCQ (10 Marks) *1 C 6 B *2 D 7 B *3 C 8 D 4 D 9 A *5 B *10 C Section B: Structured Questions (30 marks) Qn Answer Marks 11 *(a) Reddish brown/Pink solid is formed. 1 (b) No. of moles of Sn = No. of moles of CuSO4 = 2.0 x (250 / 1000) = 0.500 mol Mass of Sn = 0.500 x 119 = 59.5 g 1 (c) No. of moles of SnSO4 = No. of moles of CuSO4 = 0.500 mol Theoretical yield of SnSO4.H2O = 0.500 x [119 + 32 + 4(16) + 18] = 0.500 x 233 = 116.5 g % yield of SnSO4.H2O = (111.6 / 116.5) x 100 = 95.8 % 2 *12 (a) P : calcium, Ca Q : iodine, I2 R : calcium iodide, CaI2 S : silver iodide, AgI 4 (b) 2I-(aq) + Cl2(aq)  2Cl-(aq) + I2(aq) 1 (c) Oxidising agent / To oxidise I- to I2 1 13 (a) A: anode / positive electrode B: cathode / negative electrode Direction of electron flow: → clockwise 2 (b) A: 2H2O(l) → O2(g) + 4H+(aq) + 4e− B: 2H2O(l) + 2e− → H2(g) + 2OH−(aq) 2 (c) Effervescence is observed. Colourless and odourless gas evolved relights a glowing splint. 2 (d) Concentration of zinc sulfate will increase over time due to decomposition of water to form oxygen and hydrogen gas. 2

(e) The product formed at the anode will be the same. But, the product formed at the cathode would be copper metal [1] as Cu2+ ion is below H+ ion in the electrochemical series and will be preferentially discharged. 3 14 *(a) Aluminium, zinc, X, iron 1 *(b) Aluminium oxidises readily / reacts readily with oxygen in air to form a protective layer of aluminium oxide around the aluminium metal. Very slow reaction is observed initially as the reaction is between the acid and the oxide layer. Once the oxide layer has reacted away, the acid is able to react with the underneath aluminium metal , resulting in the sudden rapid and violent reaction. 2 *(c) The charge of the metal X ion formed is +3. For the same number of moles of metal used, X produced the same total volume of hydrogen gas as aluminium. Hence X ion must have the same charge as aluminium ion. 2 (di) Zn + 2HCl → ZnCl2 + H2 1 (dii) Zn is oxidised as the oxidation state of Zn increases from 0 in Zn to +2 in ZnCl2 / Zn2+ ions. HCl / H+ ions is reduced as the oxidation state of H decreases from +1 in HCl / H+ ions to 0 in H2. Hence, the reaction is a redox reaction. 2 (diii) No. of moles of Zn = No. of moles of H2 = 96 / 24 000 = 0.00400 mol Mass of Zn = 0.00400 x 65 = 0.260 g 1