RI H2 CHEM P2 Prelim

1   © Raffles Institution 2016 9647/02/S/16 [Turn Over RAFFLES INSTITUTION 2016 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY 9647/02 Paper 2 Structured Questions 14 September 2016 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and index number in the spaces provided at the top of this page. Write in dark blue or black pen in the spaces provided. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. A Data Booklet is provided. Do not write anything in it. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 / 12 2 / 13 3 / 13 4 / 18 5 / 16 Paper 1 Paper 2 Paper 3 Planning Penalty Total Grade / 40 / 60 / 80 / 12 / 100 This document consists of 24 printed pages.

2 © Raffles Institution 2016 9647/02/S/16 [Turn Over 1 Planning (P) For examiner’s use Copper plating is a method employed to coat an object’s surface with a thin layer of copper. The object to be plated is lowered into an electrolyte containing Cu 2+ ions, which are reduced on the surface of the object to form a thin layer of copper when an electric current is passed through the set-up. This set-up can also be used to determine the Faraday constant (F), which is the charge carried by 1 mol of electrons, as described below. A pre-weighed metal object and a pure copper rod are placed in a copper( II) sulfate solution, and then connected to an electrical source. After a sufficient duration, the mass of copper plated onto the metal object is determined by taking the difference between the object’s mass before and after plating. The mass of copper plated is dependent on the amount of electrical charge flowing through the circuit. The value of the Faraday constant may be determined graphically by repeating the experiment several times using different amounts of electrical charge, and plotting the collected data on a graph. (a) The amount of electrical charge flowing through the circuit is dependent on time and electrical current. Show that the relationship between the mass of copper plated onto the metal object and the time taken for the copper plating is given by: 63.5 2Fmt I [2] (b) The value of the Faraday constant (F) is widely accepted by chemists to be 96500 C mol–1. Using this value and given an electrical power source that supplies a constant current of 0.500 A, calculate the maximum time (to the nearest second) for which copper plating can be carried out so that the mass of copper plated onto the metal object will not exceed 1.00 g. maximum time = …..…..…..…..…..….. [1] where m = mass of copper plated in grams (g), I = current of circuit in amperes (A), t = time in seconds (s)

3 © Raffles Institution 2016 9647/02/S/16 [Turn Over (c) By considering your answers to (a) and (b), write a plan for determining the value of the Faraday constant in a laboratory. You may assume that you are provided with:  0.0500 mol dm –3 copper(II) sulfate solution,  5 pieces of pure copper, each weighing approximately 1.5 g,  5 pieces of metal objects to be plated, each weighing exactly 2.00 g,  an electrical power source that supplies a constant current of 0.500 A,  distilled water,  filter paper,  the equipment and materials normally found in a school or college laboratory. Your plan should include the following:  a clearly labelled diagram of the set-up,  details of how you would vary a variable of this experiment to collect 5 sets of data that will be used to calculate the Faraday constant,  a sketch of the graph you would expect to obtain using the data collected and how the graph can be used to calculate the Faraday constant,  the measures you would take to ens ure the accuracy of the experiment. For examiner’s use Diagram for experimental set-up: ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..…………………

4 © Raffles Institution 2016 9647/02/S/16 [Turn Over ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… ……………………………………………………………………………………..………………… [9] [Total: 12] For examiner’s use

5 © Raffles Institution 2016 9647/02/S/16 [Turn Over 2 (a) “Vitamin C” refers to a group of compounds which includes ascorbic acid and its salts. ascorbic acid compound A Compound A is the mirror image of ascorbic acid. For examiner’s use (i) Besides compound A, ascorbic acid has two other stereoisomers. Draw the structures of these two isomers. [2] (ii) Pure ascorbic acid rotates plane-polarised light clockwise by 20° (or +20°). Compound A was mixed with ascorbic acid, in an unknown proportion, in a beaker. Plane-polarised light was shone through the beaker and it was found that the light had rotated 10° anti-clockwise (or 10°). Calculate the percentage of ascorbic acid and compound A in the mixture. % ascorbic acid = ……….………; % compound A = ……….……… [2]

6 © Raffles Institution 2016 9647/02/S/16 [Turn Over (iii) Ascorbic acid is used as a food additive due to its solubility in water. Explain why ascorbic acid is soluble in water. ……………………………………………………………………………………..… ……………………………………………………………………………………..… ……………………………………………………………………………………..… ……………………………………………………………………………………..… [1] For examiner’s use (b) Ascorbic acid has antioxidant properties and it exists mainly as the ascorbate ion at the physiological pH of 7.4. An ascorbate ion reacts with a hydroxyl free radical in a 1:1 ratio to give product B and a radical anion C as shown below: (i) Explain what is meant by a free radical. ……………………………………………………………………………………..… ……………………………………………………………………………………..… [1] (ii) Suggest the identity of B. ……………………………………………………………………………………..… [1]

7 © Raffles Institution 2016 9647/02/S/16 [Turn Over (c) Asparagine and serine are important -amino acids in the synthesis of proteins. asparagine serine   Suggest, with reagents and conditions, a simple chemical test to distinguish between the two -amino acids. Reagents and conditions: ………………………………………………………………………………