OCR A Jun 2016 Paper 5 Q2

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2 Hydrogen peroxide reacts with iodide ions in acid conditions, as shown below. H2O2(aq) + 2I(aq) + 2H+(aq) I2(aq) + 2H2O(l) A student investigates the rate of this reaction by carrying out four experiments at the same temperature. The students results are shown below. Experiment [H2O2(aq)] /mol dm3 [I(aq)] /mol dm3 [H+(aq)] /mol dm3 0.0010 0.0020 0.0020 0.0040 0.20 0.20 0.20 0.40 0.10 0.10 0.20 0.10 Initial rate /mol dm3 s1 5.70106 1.14105 1.14105 4.56105 (a) The rate equation is: rate = k [H2O2(aq)] [I(aq)] Show that the students results support this rate equation.Calculate the rate constant, k, for this reaction. Give your answer to two significant figures, in standard form and with units. In your answer you should make clear how the experimental results provide evidence for the rate equation.[6]OCR 2016<br />
 (b) The student concluded that H+(aq) ions act as a catalyst. Explain why the students conclusion is not correct.[1] (c) A four-step mechanism has been proposed for this reaction. The rate-determining step is the first step. (i) State what is meant by the term rate-determining step.[1] (ii) The equation for Step 3 in the four-step mechanism is shown below. Suggest equations for the other three steps. State symbols are not required. Step 1:Step 2:Step 3: HIO + I I2 + OH Step 4:[3] [Total: 11]OCR 2016 Turn over<br />

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