OCR A Jan 2013 Paper 5 Q4

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Iodide ions, I, react with S2O8 10 2 ions as shown in the equation below. 2I(aq) + S2O8 2(aq) I2(aq) + 2SO4 2(aq) A student investigates the rate of this reaction using the initial rates method. The student measures the time taken for a certain amount of iodine to be produced. (a) Outline a series of experiments that the student could have carried out using the initial rates method. How could the results be used to show that the reaction is first-order with respect to both I and S2O8 2? In your answer you should make clear how the results are related to the initial rates.[4] (b) In one of the experiments, the student reacts together:8.0102 mol dm3 I(aq) 4.0103 mol dm3 S2O8 2(aq). The initial rate of this reaction is 1.2103 mol dm3 s1. The reaction is first-order with respect to I and first-order with respect to S2O8 Calculate the rate constant, k, for this reaction. State the units, if any. 2.OCR 2013 k =units[3]<br /> 11 (c) This reaction between I ions and S2O8 2 ions can be catalysed by either Fe2+(aq) ions or Fe3+(aq) ions. Standard electrode potentials are shown below. 2(aq) + 2e S2O8 Fe3+(aq) + e I2(aq) + 2e (i) Using this information, write two equations to show how the reaction of I ions and 2(aq) E o = +2.01 V E o = +0.77 V E o = +0.54 V 2SO4 Fe2+(aq) 2I(aq) S2O8 2 ions can be catalysed by Fe2+ ions. equation 1:equation 2:[2] (ii) Suggest why the reaction of I ions and S2O8 2 ions is also catalysed by Fe3+ ions.[1] [Total: 10]OCR 2013 Turn over<br />

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