CJC H2 Chem 2013 Prelim P3 Soln

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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 . Standard electrode potential , E o , is defined as the potential difference between a standard hydrogen electrode and a metal (the electrode) which is immersed in a solution containing metal ions at 1 mol dm –3 concentration at 25 °C and 1 atmospheric pressure. [values of standard conditions should be given] (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] E o /V Co2+(aq)+ 2e– Co(s) –0.28 [Co(NH3)6]2+ + 2e– Co + 6NH 3 –0.43 OR Ni2+(aq) + 2e– Ni(s) –0.25 [Ni(NH3)6]2+ + 2e– Ni + 6NH 3 –0.51 E o of [M(NH3)6]2+/M is more negative than that of [M(H2O)6]2+/M. This is because as H2O ligands in the aquated ions are replaced by the stronger NH3 ligands, the +2 oxidation state of M is stabilised and this makes the metal more reducing, or its ion less oxidising. (b) A student set up the following electrochemical cell. (i) What device is used to measure the e.m.f. of the cell? high-resistance voltmeter. salt-bridge + Ag(s) – AgNO3(aq) 0.010 mol dm–3 Ag(s) saturated AgCl(aq) AgCl(s) 448

3 9647/03/CJC JC2 Preliminary Exam 2013 (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. salt bridge can be prepared by using KNO3(aq) or NaNO3(aq). K+ (or Na +) ions leave the salt bridge into LHS half-cell (where Ag + is reduced to Ag) to 'balance' the loss of positively charged A