Edexcel Jan 2015 (IAL) Paper 4 Q21

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 (a) Iodine reacts with propanone, CH3COCH3, in the presence of a catalyst of dilute hydrochloric acid. CH3COCH3(aq) + I2(aq) (cid:111) CH3COCH2I(aq) + HI(aq) Students carried out a rate investigation of this reaction. In each set of experiments, the initial concentration of one substance was varied and the initial concentrations of the other two substances were kept constant. First set of experiments The initial concentration of propanone was varied. Second set of experiments The initial concentration of iodine was varied. Third set of experiments The initial concentration of hydrochloric acid was varied. The results of each set of experiments are shown in the graphs below. First set Second set Initial rate Initial rate [CH3COCH3(aq)] [I2(aq)] Initial rate Third set [H+(aq)] 10 *P45044A01028*<br />
 (i) For the second set of experiments, state a practical method for following the progress of this reaction. Indicate which substance is being monitored by your method. (2)*(ii) Use the graphs to deduce the orders of reaction with respect to propanone, iodine and H+ ions. Explain your reasoning. (4)(iii) Write the rate equation for the reaction. (1) *P45044A01128* 11 Turn over<br />
 (iv) In one of the experiments, the following data were collected: [CH3COCH3(aq)] = 0.667 mol dm3 [I2(aq)] = 1.67103 mol dm3 [H+(aq)] = 0.667 mol dm3 Initial rate = 8.80106 mol dm3 s1 Use the data to calculate the value for the rate constant. Include units in your answer. (v) Use the rate equation to suggest a possible rate-determining step in the mechanism for the reaction between iodine and propanone in the presence of dilute hydrochloric acid. Explain your reasoning. (2) (2)12 *P45044A01228*<br />
 (b) Iodine also forms hydrogen iodide by direct reaction with hydrogen. A graph of ln k against 1/T for this reaction is shown below. H2(g) + I2(g) (cid:111) 2HI(g) 1/T / K1 1.3103 1.4103 1.5103 1.6103 1.7103 1.8103 ln k 1012345678910(i) Calculate the gradient of the graph. Include a sign and units in your answer. (2) *P45044A01328* 13 Turn over<br />
 (ii) Use your value for the gradient of the graph to calculate the activation energy, Ea. Include units and give your answer to three significant figures. The Arrhenius equation is ln k =E a R 1 T + a constant [Gas constant, R = 8.31 J mol1 K1] (2) (Total for Question 21 = 15 marks) 14 *P45044A01428*<br />

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