ACJC H2 Chem 2012 Prelim P2 Soln

1 © ACJC 2012 9647/02/Prelim/12 [Turn over ACJC H2 Prelim 2012 Paper 2 Answers 1 Planning (P) A student is interested to determine the Faraday co nstant. He proposed that the value of the Faraday constant can be determined exp erimentally using the electrolysis of aqueous nickel (II) sulfate. The student did some research and made the following notes. General information • Reaction at the cathode: Ni 2+ (aq) + 2e → Ni (s) • Reaction at the anode: Ni (s) → Ni 2+ (aq) + 2e • 1 mol of Ni (s) is deposited at the cathode by 2 m ol of electrons. • The Faraday constant is the charge in coulombs, C, carries by 1 mol of electrons. • The Faraday constant = 96500 C mol -1 Experimental setup The experimental setup for the electrolysis of nick el (II) sulfate solution is shown above. The student will electrolyse some nickel (II ) sulfate solution using nickel electrodes. Before the nickel cathode is placed int o the electrolyte, it will be cleaned and weighed. The current that passes through the electrolyte wil l be kept constant at 0.3 A for 40 min by adjusting a variable resistor in the experiment. At the end of 40 min, the nickel cathode will be weighed. The experiment will be repeated for another 7 times at 40 min intervals.

2 © ACJC 2012 9647/02/Prelim/12 [Turn over (a) Based on the information given under experimental setup, give a full description of the procedures you would use in Step A (Start of experiment) in the space provided. Step B (Rinsing and reweighing of cathode) is provided. [2] Experimental method Step A (Start of experiment) 1. The cathode is cleaned and weighed before being placed in the nickel (II) sulfate solution. 2. The circuit is switched on and the current is ma intained at 0.3 A by adjusting the variable resistor. 3. The current is maintained at 0.3 A for exactly 4 0 mins and the circuit is switched off. Step B (Rinsing and reweighing of cathode) 1. The cathode is removed from the solution and carefu lly washed with distilled water to remove any nickel (II) sulfate solution. 2. Distilled water is removed from the cathode by rins ing it with propanone in which the water dissolves. 3. The cathode is finally dried by allowing the propan one to evaporate from the surface. 4. The cathode is reweighed and placed back to the solution. 5. A constant current of 0.3 A is passed for a further 40 mins when the rinsing, drying and weighing are repeated. This procedure (Step A and B) is repeated for a further 6 times.

3 © ACJC 2012 9647/02/Prelim/12 [Turn over (b ) The student performed the experiment using the expe rimental setup