Edexcel Jun 2013 Paper 4 Q20

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 20 The ionic equation for the reaction of ammonium peroxodisulfate (persulfate), (NH4)2S2O8, with potassium iodide, KI, is S2O8 2(aq) + 2I(aq) o 2SO4 2(aq) + I2(aq) (a) In a series of experiments to determine the rate equation for this reaction, 10 cm3 of 0.0050 mol dm3 sodium thiosulfate was mixed with 20 cm3 of (NH4)2S2O8 solution and 5 drops of starch solution. 20 cm3 of KI solution was added with mixing and the time taken for the solution to darken was noted. The initial concentrations of the (NH4)2S2O8 and KI solutions and the times for the mixture to darken are shown below. Experiment Number 1 2 3 Initial concentration / mol dm3 2 S2O8 0.10 0.05 0.10 I 0.20 0.20 0.10 Time for solution to darken / s 35 69 70 (i) Explain the purpose of the sodium thiosulfate solution. (2)(ii) Use the data in the table to deduce the rate equation for the reaction between S2O8 at your answer. 2 and I ions. Explain, by referring to the data, how you arrived (3)20 *P41572A02024*<br /> (b) A further experiment was carried out to confirm the order of the reaction with respect to iodide ions. (NH4)2S2O8 was mixed with KI to form a solution in which the initial concentration of (NH4)2S2O8 was 2.0 mol dm3 and that of KI was 0.025 mol dm3. The concentration of iodine was measured at various times until the reaction was complete. (i) Outline a method, not involving sampling the mixture, which would be suitable for measuring the iodine concentrations in this experiment. Experimental details are not required but you should state how you would use your measurements to obtain iodine concentrations. (3)(ii) Explain why the initial concentration of (NH4)2S2O8 is much higher than that of KI. (1)(iii) State how the initial rate of reaction may be obtained from the results of this type of experiment. (2)*P41572A02124* 21 Turn over<br /> (iv) In such an experiment a student calculated the initial rate of reaction to be 8.75105 mol dm3 s1. Use this value, the initial concentrations in (b) and the rate equation that you obtained in (a)(ii), to calculate the rate constant for this reaction. Include units in your answer. (2) (c) Using the method outlined in (b), the rate constant for this reaction was determined at various temperatures. The data from these experiments are shown in the table below. Note that none of the temperatures corresponds to that used in (b) and that the rate constant is given in appropriate units. Temperature T / K 300 310 320 330 340 Rate constant 0.00513 0.00833 0.0128 0.0201 0.0301 ln k 5.27 4.79 4.36 3.91 3.50 1/T / K1 0.00333 0.00323 0.00313 0.00303 0.00294 22 *P41572A02224*<br /> (i) Use the data in the table to plot a graph of ln k (on the y axis) against 1/T (on the x axis) and draw a best fit line through the points. 0.0029 3.50 0.0030 0.0031 0.0032 0.0033 0.0034 1/T / K1 (2) (4) ln k 4.00 4.50 5.00 5.50 (ii) Determine the gradient of the best fit line in (c)(i) and use this value to calculate the activation energy, Ea, of the reaction, stating the units. The rate constant of a reaction, k, is related to the temperature, T, by the expression ln E=a R+ 1 T constant R = 8.31 J K1 mol1 (Total for Question 20 = 19 marks) TOTAL FOR SECTION C = 19 MARKS TOTAL FOR PAPER = 90 MARKS *P41572A02324* 23<br />

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