20 (A410U10-1) Examiner only 16. The reaction between ethoxide ions (C2H5O−) and iodoethane produces ethoxyethane. The reaction was studied at various temperatures and the percentage of products formed every 10 s was measured for the first 50 s. (a) The initial rate of reaction can be calculated approximately using the formula: rate = percentage conversion time taken (i) One student chose to use the data at 50 s to calculate the rate at 40 °C. Suggest and explain why this will not give an appropriate value of initial rate. [2] (ii) A second student chose to use the data at 10 s to calculate the rate. Give a reason why the percentage error at lower temperatures may be more significant. [2] © WJEC CBAC Ltd. Temperature / °C Percentage conversion of reactants to products 10 s 20 s 30 s 40 s 50 s 10 2 % 3 % 5 % 6 % 7 % 20 5 % 10 % 15 % 19 % 24 % 30 16 % 30 % 41 % 50 % 58 % 40 41 % 65 % 79 % 88 % 93 % 50 77 % 95 % 99 % 100 % 100 % (A410U10-1) 21 Examiner only (b) The rate constant for the reaction in solution at 30 °C is 1.752 × 10−2 dm3 mol−1 s−1. (i) Give the overall order of the reaction. [1] (ii) Explain what information about the mechanism can be found from: • the overall order of reaction • the orders with respect to each reactant [3] (iii) State the Arrhenius equation. [1] (iv) The frequency factor for the reaction under these conditions is found to be 1.49 × 1011 dm3 mol−1 s−1. Use this value to calculate the activation energy of the reaction in kJ mol–1. [3] Activation energy = ..................................................... kJ mol–1 END OF PAPER © WJEC CBAC Ltd. 12 (A410U10-1) 22 Examiner only © WJEC CBAC Ltd. For continuation only. Examiner only (A410U10-1) 23 © WJEC CBAC Ltd.