VJC_H2_CHEM_P3_QN Prelim

 VJC 2014 9647/03/PRELIM/14 [Turn over VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Candidates answer on separate paper. 22 September 2014 2 hours Additional Materials: Answer Paper Data Booklet Graph Paper READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Write in dark blue or black pen on both sides of the paper. 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 any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear explanation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 11 printed pages and 1 blank page.

2    VJC 2014 9647/03/PRELIM/14   Answer any four questions. 1 (a) Use of the Data Booklet is relevant in this question. The following cell has been used to light up a light bulb. (i) Explain how the cell potential changes when the current has passed through for a while. The electrode potential of the cell is related to the equilibrium constant by the following equation, E o= ln Kc where T is the temperature measured in Kelvin, z is the number of moles of electrons transferred during the redox reaction and F is the Faraday constant. (ii) Determine the equilibrium constant Kc for the reaction between Cu2+ and Mg. (iii) Hence, determine the ratio [Mg 2+]:[Cu2+], when there is no current flowing in this cell. (iv) Suggest the significance of the magnitude of your answer in (a)(iii). [5] (b) Copper minerals often contain suifides of magnesium and silver as impurity. After minerals are reduced with carbon, the solid impure copper is purified by electrolysis. (i) Describe the electrode reactions that take place during the electrolysis and explain in detail how each of the two impurity metals is removed from copper. An impure copper rod is purified by electrolysis using a constant current. After 1 hour, mass of one electrode decreased by 19.0 g while mass of the other electrode increased by 17.8 g. The electrolyte was further analysed and found that the amount of Cu 2+ ions is reduced by 0.020 mol. (ii) Calculate the current used for this process. (iii) Determine the percentage composition by mass of the three elements. (iv) Suggest how magnesium can be recovered as magnesium oxide near the end of the electrolytic process. Write balanced equations for any reactions involving magnesium that might occur. [11] salt bridge Cu electrode Cu2+(aq), 1 mol dm–3 li