OCR A Jun 2014 Paper 5 Q4

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4 A student carries out an initial rates investigation on the reaction below. 3I2(aq) + 3H2O(l) From the results, the student determines the rate equation for this reaction: 5I(aq) + IO3 (aq) + 6H+(aq) rate = k [ I(aq)]2 [ IO3 (aq)] [H+(aq)]2 (a) (i) What is the overall order of reaction?[1] (ii) A proposed mechanism for this reaction takes place in several steps. Suggest two reasons why it is unlikely that this reaction could take place in one step.[2] (b) On the rateconcentration graphs below, sketch lines to show the relationship between initial rate and concentration for IO3 (aq) and H+(aq). initial rate / mol dm3 s1 initial rate / mol dm3 s1 [ IO3 (aq) ] / mol dm3 [ H+(aq) ] / mol dm3 [2]OCR 2014<br /> (c) The table below shows some of the students results. (i) Complete the table by adding the missing initial rates in the boxes. [ I(aq)] / mol dm3 [ IO3 (aq)] / mol dm3 [H+(aq)] / mol dm3 Initial rate / mol dm3 s1 Experiment 1 Experiment 2 Experiment 3 0.015 0.045 0.060 0.010 0.010 0.040 0.020 0.020 0.080 0.60 [2] (ii) Calculate the rate constant, k, for this reaction. Include units. Give your answer to two significant figures. k =units [3] (iii) The student repeats Experiment 1 using 0.020 mol dm3 methanoic acid, HCOOH(aq) (pKa = 3.75), instead of 0.020 mol dm3 HCl (aq) as a source of H+(aq). Determine the initial rate in this experiment. Show your working. initial rate =mol dm3 s1 [3] [Total: 13] Turn overOCR 2014<br />

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