OCR A Jan 2011 Paper 5 Q4

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4 Redox reactions are used to generate electrical energy from electrochemical cells. (a) Table 4.1 shows three redox systems, and their standard redox potentials. 12 redox system Cu+(aq) + e Cr3+(aq) + 3e Sn4+(aq) + 2e Cu(s) Cr(s) Sn2+(aq) E o/V +0.52 0.74 +0.15 Table 4.1 (i) Draw a labelled diagram to show how the standard electrode potential of a Sn4+/Sn2+ redox system could be measured. (ii) Using the information in Table 4.1, write equations for the reactions that are feasible. Suggest two reasons why these reactions may not actually take place. [3][5]OCR 2011<br />
 (b) Modern fuel cells are being developed as an alternative to the direct use of fossil fuels. The fuel can be hydrogen but many other substances are being considered. In a methanol fuel cell, the overall reaction is the combustion of methanol. 13 As with all fuel cells, the fuel (methanol) is supplied at one electrode and the oxidant (oxygen) at the other electrode. Oxygen reacts at the negative electrode of a methanol fuel cell: O2 + 4H+ + 4e 2H2O (i) Write an equation for the complete combustion of methanol.[1] (ii) Deduce the half-equation for the reaction that takes place at the positive electrode in a methanol fuel cell.[1] (iii) State two advantages of vehicles using fuel cells compared with the combustion of conventional fossil fuels.[2] (iv) Suggest one advantage of using methanol, rather than hydrogen, in a fuel cell for vehicles. Justify your answer.[1] [Total: 13]OCR 2011 Turn over<br />

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