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 and method above. The results of his experiment are recorded below. Time/ min Mass of cathode/ g Charge passed/ C Mass of Ni (s) deposited on the cathode/ g 0 115.74 0 0 40 115.97 720 0.23 80 116.22 1440 0.48 120 116.46 2160 0.72 160 116.70 2880 0.96 200 116.94 3600 1.20 240 117.19 4320 1.45 280 118.01 5040 2.27 320 117.67 5760 1.93 Calculate and record the charge passed and the mass of nickel deposited on the cathode at 160 min in the space given above. [1]

4 © ACJC 2012 9647/02/Prelim/12 [Turn over (c ) The student plotted the data on the graph of mass o f Ni (s) deposited on the cathode against charge passed. Draw a best-fit line on the graph below. [1] (d ) With reference to part (b ) and part (c ), state the anomalous point. Based on the procedures for the experiment, explain for the anomaly. [2] (5040C, 2.27g) There could be some residual liquid on the cathode when it is weighed. (e ) The weighing balance used by the student weighed to 2 decimal places. With reference to the results of the experiment, explain why it is not appropriate to use a weighing balance accurate to 2 decimal places. [1] As the mass of Ni (s) deposited is very small, the % error of the calculated value is large. (f ) The student calculated the value of the gradient in part (c ). Gradient of the line = 3.05 x 10 -4 g C -1 Using the information provided, calculate a numeric al value of the Faraday constant. [2] Ni 2+ (aq) + 2e → Ni (s) Mass of Ni (s) deposited = 3.05 x 10 -4 (charge passed) Mass of Ni (s) deposited by 1 mol of electron = 0.5 x 58.7 = 29.35g Charge passed to deposit 29.35 g of Ni (s) = 29.35/3.05 x 10 -4 = 96230 C Faraday constant = 96230 C mol -1 Mass of Ni(s) deposited on the cathode/ g 0 Charge passed/ C x x x x x x x x x

5 © ACJC 2012 9647/02/Prelim/12 [Turn over (g ) What other measurements could be made during the co urse of the experiment to provide alternative data to confirm the determined value of the Faraday constant? [1] The loss in mass of the Ni anode can also be measured and recorded to confirm the determined value of the Faraday constant. (h ) The same experiment was performed by another studen t. The plotted mass of nickel deposited against charge passed was obtained below. Suggest an explanation for the shape of this graph. Hence, comment on the accuracy of the value of th e Faraday constant obtained by this student. [2] The NiSO 4 solution is impure. It could be acidified NiSO 4 solution. Thus, the evolution of H 2 (g) could compete with the deposition of Ni (s) as H + and Ni 2+ could be simultaneously discharged. [Note: E o (H +/H 2) = 0.00V and E o (Ni 2+/ Ni) = -0.25V). Simultaneous reduction of H+ and Ni 2+ can occur although E o (H +/H 2) is more positive than E o (Ni 2+/ Ni) ]. Given the amount of charges pass through the circui t, less Ni(s) is deposited initially. The gradient becomes steeper w hen all the H + are completed reduced and only Ni 2+ is being discharged. The F constant is accurate if the student use the s teeper gradient in his calculation. OR The F constant is not accurate if t he student use the gentle gradient in his calculation. [Total: 12 marks] Charge passed/ C Mass of Ni(s) deposited on the cathode/ g 0

6 © ACJC 2012 9647/02/Prelim/12 [Turn over 2 Period 3 elements vary in their melting points and electrical conductivities. (a) (i) On the axes below, sketch these trends for the stated elements. [4] (ii ) Phosphorus reacts with chlorine to form mixtures of PC l3 and PC l5 which are commonly used in organic reaction synthesis. Dr aw and name the shapes of PC l3 and PC l5, and state clearly the bond angles. [4] P Cl Cl Cl P Cl