Edexcel Jun 2017 (IAL) Paper 4 Q19

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 19 The kinetics of the reaction between hydrogen peroxide and iodide ions in the presence of sulfuric acid is investigated. H2O2(aq) + 2H+(aq) + 2I(aq) (cid:111) 2H2O(l) + I2(aq) (a) First, the concentration of hydrogen peroxide is measured at different times, while keeping the concentrations of iodide and hydrogen ions constant. The following results are obtained. t / 103 s [H2O2] / mol dm3 10 0.20 0.14 0.09 0.06 0.04 0.03 (i) Plot a graph of [H2O2] / mol dm3 against t / 103 s . (2) 20 *P48385A02028* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> A E R A S I H T N I E T I R W T O N O D (ii) Find two successive half-lives from your graph. Show your working on your graph, together with their values. (iii) Deduce the order of the reaction with respect to hydrogen peroxide. Justify your answer. (2) (2)A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48385A02128* 21 Turn over<br /> (b) The experiment is repeated using an initial rate method. Mixtures are prepared using 0.10 mol dm3 solutions of each reactant, 2 cm3 of sodium thiosulfate solution mixed with starch and varying amounts of water so that the total volume is always 12 cm3. The time for the mixtures to change colour is recorded and the initial rate calculated. Run 1 2 3 4 5 Volume KI /cm3 3.0 2.0 1.0 3.0 3.0 Volume H2O2 /cm3 3.0 3.0 3.0 3.0 3.0 Volume H2SO4 /cm3 3.0 3.0 3.0 2.0 1.0 Volume of water /cm3 1.0 2.0 3.0 2.0 3.0 Initial rate /mol dm3 s1 1.06104 7.00105 3.50105 1.08104 1.05104 (i) Explain why it is necessary to keep the total volume of each mixture the same. (1)(ii) The reciprocal of time can be used as an approximate measure of rate. What assumption does this approximation depend on? (1)(iii) Use the results in the table to deduce the order of reaction with respect to iodide ions and hydrogen ions. Justify each answer by referring to relevant data from the table. (3) Iodide ionsHydrogen ions22 *P48385A02228* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (iv) Write the overall rate equation for this reaction using your answers to (a)(iii) and (b)(iii). (v) Calculate the actual concentrations of hydrogen peroxide and iodide ions in the mixture used in Run 1 from the table in (b). A E R A S I H T N I E T I R W T O N O D (vi) Calculate a value for the rate constant using Run 1 from the table in (b) and your answers to parts (b)(iv) and (b)(v). Include units for the rate constant. A E R A S I H T N (1) (1) (2) I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48385A02328* 23 Turn over<br /> (c) (i) The activation energy for this reaction is found by keeping the concentrations of reactants constant and repeating the reaction at different temperatures. A graph of ln(rate) of the reaction against reciprocal of temperature is given below. 3.0103 (cid:237)2.0 3.1103 3.2103 3.3103 3.4103 1 T K1 In (rate) 2.5 (cid:237)3.0 (cid:237)3.5 (cid:237)4.0 (cid:237)4.5 Calculate the gradient of the graph. Use your value of the gradient and the equation below to calculate the activation energy of the reaction. ln (rate) = E a1 R T + constant [R = 8.31 J K1 mol1] Include a sign and units with your answer. (3) 24 *P48385A02428* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> *(ii) If the same reaction is carried out in the presence of a catalyst of ammonium molybdate, the activation energy is found to be much lower. Sketch a Maxwell-Boltzmann distribution of molecular energies. Use your sketch to explain why this reduction in activation energy increases the rate of the reaction. (3) Explanation(Total for Question 19 = 21 marks) TOTAL FOR SECTION C = 21 MARKS TOTAL FOR PAPER = 90 MARKS A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48385A02528* 25<br />

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