OCR A Jan 2012 Paper 5 Q5

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5 Redox reactions can be used to generate electrical energy from electrochemical cells. (a) A student carries out an investigation based on the redox systems shown in Table 5.1 below. 14 redox system 1 Ni2+(aq) + 2e Fe3+(aq) + e 2 3 Cr3+(aq) + 3e Ni(s) Fe2+(aq) Cr(s) Table 5.1 E o/ V 0.25 +0.77 0.74 The student sets up two standard cells to measure two standard cell potentials.Cell A is based on redox systems 1 and 2.Cell B is based on redox systems 1 and 3. (i) Draw a labelled diagram to show how the student could have set up Cell A, based on redox systems 1 and 2, to measure the standard cell potential. [3] (ii) For each standard cell below,what would be the standard cell potential?what would be the sign of the Ni electrode? Cell A based on redox system 1 and 2: standard cell potential =V sign of Ni electrode, + or=Cell B based on redox system 1 and 3: standard cell potential =V sign of Ni electrode, + or=[2]OCR 2012<br /> (b) The student left each cell in (a) connected for a length of time. 15 For each cell, the student weighed the nickel electrode before connecting the cell and after the cell had been disconnected. The student made the following observations.In Cell A, the nickel electrode lost mass. In Cell B, the nickel electrode gained mass. In both cells, the measured cell potential slowly changed. Explain these observations. Include equations in your answer.[3] (c) Nickel metal hydride cells (NiMH cells) are being developed for possible use in cars. In a NiMH cell, an alloy is used to absorb hydrogen as a metal hydride. For simplicity, the alloy can be represented as M and the metal hydride as MH. The overall cell reaction in a NiMH cell is shown below. MH + NiO(OH) M + Ni(OH)2 The half-equation at one electrode is shown below. NiO(OH) + H2O + e Ni(OH)2 + OH (i) Deduce the half-equation at the other electrode.[1] (ii) State a method, other than absorption, that is being developed to store hydrogen for possible use as a fuel in cars.[1] [Total: 10]OCR 2012 Turn over<br />

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