CJC H2 Chem 2013 Prelim P3 QP

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2 9647/03/CJC JC2 Preliminary Exam 2013 Answer any four questions. 1. Use of the Data Booklet is relevant to this question. (a) (i) Define the term standard electrode potential, E o . (ii) For the element M (where M is Co or Ni), suggest an explanation for the difference in the E o values between M 2+ + 2e– M and [M (NH3)6]2+ + 2e– M + 6NH3 [4] (b) A student set up the following electrochemical cell. (i) What device is used to measure the e.m.f. of the cell? (ii) Name a compound that could be used to prepare a workable salt-bridge for this cell and explain how it helps to maintain electrical neutrality. (iii) Amongst other factors, the electrode potential is dependent on the temperature and the concentration of the ions. At 298 K, the reduction potential, E, of Ag +(aq)/Ag(s) can be obtained from the equation E = E o + 0.059 log10 [Ag+(aq)] The student found that the e.m.f. of the cell was +0.17 V at 298 K. Calculate the reduction potential, E, of each half-cell and hence the concentration of Ag +(aq) ions in the saturated AgCl(aq). [Given: E o of Ag+(aq)/Ag(s) = +0.80 V at 298 K] (iv) State and explain the effect on the cell e.m.f. of adding a small volume of NH3(aq) to the Ag+(aq)/Ag(s) half-cell. [9] salt-bridge + Ag(s) – AgNO3(aq) 0.010 mol dm–3 Ag(s) saturated AgCl(aq) AgCl(s) 434

3 9647/03/CJC JC2 Preliminary Exam 2013 (c) Putrescine (1,4-diaminobutane) is a foul-smelling compound found in rotting flesh and in "bad breath". It is produced by the breakdown of amino acids in living and dead organisms. Like ammonia, putrescine is a weak base. While ammonia has a p Kb value of 4.74, putrescine has two p Kb values which are found to be 3.20 (p Kb1) and 4.65 (p Kb2) respectively. (i) Explain why putrescine is a strong