26 (A410U10-1) 14. It is possible to study the concentration dependence of rate by finding how the rate of a reaction changes over time. This is because the concentrations of the reactants change over time. The reaction below occurs in non-aqueous solution in the presence of a small amount of water. C2H5Br + OH– + H2O C2H5OH + Br – + H2O Three students carried out experiments to find how the concentration of each reactant affects the rate. Each one changed the concentration of a different reactant. They used the initial concentrations shown below and an automated sampling device to take measurements every 10 minutes for 6 hours. © WJEC CBAC Ltd. Initial concentration of each reactant / mol dm–3 [C2H5Br] [OH–] [H2O] George’s experiment: Finding the effect of [C2H5Br] on rate 2.00 × 10–3 2.00 2.00 Hannah’s experiment: Finding the effect of [H2O] on rate 2.00 2.00 2.00 × 10–3 Jamal’s experiment: Finding the effect of [OH–] on rate 2.00 2.00 × 10–3 2.00 (a) The results obtained in George’s experiment are shown on the graph below. 0 0.5 1.0 1.5 2.0 2.5 0 5 10 15 20 25 Time / 103 seconds Concentration of C2H5Br / 10−3 mol dm−3 (A410U10-1) Turn over. 27 Examiner only (i) Calculate the initial rate for the reaction, stating its unit. [3] Initial rate = ....................................................... Unit ....................................................... (ii) Use the graph to show that the reaction is first order with respect to C2H5Br. [2] (b) Suggest why this method uses much lower concentrations of the reactants being studied than those of the other reactants involved. [1] © WJEC CBAC Ltd. 28 (A410U10-1) Examiner only (c) Hannah finds that the concentration of water does not change during her experiment. (i) Give a reason why the concentration of water does not change. [1] (ii) The order of the reaction with respect to water is zero. Suggest how Hannah could confirm this. [1] (d) Jamal carried out his experiment at a slightly different temperature from George. He found that the reaction is first order with respect to hydroxide ions. The final rate equation is therefore as follows. rate = k[C2H5Br][OH–] The value of the rate constant is 4.07 × 10–5. (i) Give the unit of the rate constant. [1] ....................................................................................................... (ii) The activation energy for this reaction is 89.5 kJ mol–1 and its frequency factor, A, has a value of 4.30 × 1011. Calculate the temperature used for Jamal’s experiment. You must show your working. [3] Temperature = ....................................................... K END OF PAPER © WJEC CBAC Ltd. 12 (A410U10-1) 29 Examiner only © WJEC CBAC Ltd. Additional page. Turn over. (A410U10-1) 30 Examiner only © WJEC CBAC Ltd. Additional page. BLANK PAGE (A410U10-1) 31 © WJEC CBAC Ltd.