16 (A410U10-1) Examiner only 14. (a) The usual method for measuring the standard electrode potential of a half-cell is to connect it to a standard hydrogen electrode using a high resistance voltmeter and a salt bridge. (i) State the function of the salt bridge. [1] (ii) Draw a labelled diagram of the standard hydrogen electrode. [3] (iii) When measuring the standard electrode potential for the Zn2+(aq) │ Zn(s) system a piece of zinc metal is placed in an aqueous solution containing Zn2+(aq). Explain why a similar method would not be appropriate for the Li+(aq) │ Li(s) system. [1] (b) The values for some standard electrode potentials are listed in the table below. © WJEC CBAC Ltd. Standard electrode potential, E θ / V Li+(aq) + e− s Li(s) –3.04 Na+(aq) + e− s Na(s) –2.71 Zn2+(aq) + 2e− s Zn(s) –0.76 Fe3+(aq) + 3e− s Fe(s) –0.04 I2(s) + 2e− s 2I− (aq) +0.54 Fe3+(aq) + e− s Fe2+ (aq) +0.77 Br2(l) + 2e− s 2Br − (aq) +1.09 Cl2(g) + 2e− s 2Cl− (aq) +1.36 (A410U10-1) Turn over. 17 Examiner only (i) Give the formula of the strongest reducing agent in the table. [1] (ii) Calculate the EMF of the cell formed when the Br2(l) │ Br −(aq) half-cell is connected to the Zn2+(aq) │ Zn(s) half-cell. [2] EMF = ...................................................... V (iii) Use the values in the table to explain why the reaction between gaseous iodine and heated iron wire produces iron(II) iodide whilst chlorine gas and iron wire produce iron(III) chloride. Your answer should include: • An explanation of the products formed in the two reactions using the standard electrode potentials listed. • Chemical equations for both reactions. • A prediction of the product that would be formed when bromine gas reacts with heated iron wire, including a reason. • An explanation of why the prediction made using the standard electrode potentials above may not be correct. [6 QER] © WJEC CBAC Ltd. 14