OCR A Jun 2011 Paper 5 Q7

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7 Nickelcadmium cells (NiCd cells) have been extensively used as rechargeable storage cells. NiCd cells have been a popular choice for many electrical and electronic applications because they are very durable, reliable, easy-to-use and economical. 20 The electrolyte in NiCd cells is aqueous KOH. The standard electrode potentials for the redox systems that take place in NiCd cells are shown below. Cd(OH)2 + 2e NiO(OH) + H2O + e Cd + 2OH E o = 0.80 V Ni(OH)2 + OH E o = +0.45 V (a) Define the term standard electrode potential, including all standard conditions in your answer.[2] (b) What is the standard cell potential of a NiCd cell? answer = V [1] (c) When a NiCd cell is being used for electrical energy, it is being discharged. (i) Construct the overall cell reaction that takes place during discharge of a NiCd cell.[2] (ii) Using oxidation numbers, show the species that have been oxidised and reduced during discharge of a NiCd cell. oxidationreduction[2]OCR 2011<br />
 (d) NiCd cells are recharged using a battery charger. 21 (i) Suggest the reactions that take place in the NiCd cell during the recharging process.[1] (ii) As the cell approaches full charge, the aqueous KOH electrolyte starts to decompose, forming hydrogen gas at one electrode and oxygen gas at the other electrode. Predict half-equations that might take place at each electrode for the decomposition of the electrolyte to form hydrogen and oxygen.[2] [Total: 10]OCR 2011 Turn over<br />

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