ACJC H2 Chem 2012 Prelim P2 QP

© ACJC 2012 9647/02/Prelim/12 [Turn over Index No. Name Form Class Tutorial Class Subject Tutor ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY 9647/02 Higher 2 Paper 2 Structured Questions 22 August 2012 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, index number, form class, tutorial cl ass and subject tutor’s name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or co rrection fluid. Answer all questions. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work se curely together including the cover page. For Examiner's Use Question no. Marks 1 2 3 4 5 6 7 TOTAL This document consists of 22 printed pages and 1 blank page 9647/02/Prelim/12 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2012 Department of Chemistry [Turn over

2 © ACJC 2012 9647/02/Prelim/12 [Turn over 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 experimentally 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 nick el (II) sulfate solution using nickel electrodes. Before the nickel cathode is placed into 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. For Examiner’s use

3 © ACJC 2012 9647/02/Prelim/12 [Turn over (a) Based on the information given under experimental s etup, 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. Experimental method Step A (Start of experiment) …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………… ……………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………. [2] 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 rinsing 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. For Examiner’s use

4 © 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 0.72 160 116.70 200 116.94 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] For Examiner’s use

5 © 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] For Examiner’s use (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] (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] Charge passed/ C Mass of Ni(s) deposited on the cathode/ g 0 x x x x x x x x x

6 © ACJC 2012 9647/02/Prelim/12 [Turn over (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] For Examiner’s use (g ) What other measurements could be made during the co urse of the experiment to provide alternative data to confirm the determin ed value of the Faraday constant? …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [1]

7 © ACJC 2012 9647/02/Prelim/12 [Turn over (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 the Faraday constant obtained by this student. …………………………………………………………………………………………… …………………………………………………………………………………………… ……………………………………………………………………………………… …… …………………………………………………………………………………………… [2] For Examiner’s use [Total: 12 marks] Charge passed/ C Mass of Ni(s) deposited on the cathode/ g 0

8 © ACJC 2012 9647/02/P