16 (B410U20-1) 12. A student carried out an experiment to study the reaction between magnesium and hydrochloric acid. Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) ΔH = –467 kJ mol–1 He used the following apparatus to measure the volume of hydrogen produced over time. © WJEC CBAC Ltd. gas syringe hydrochloric acid magnesium strip The experiment was carried out at a temperature of 25 °C and 1 atm pressure. The amount of acid used was sufficient to react with all the magnesium. These are the results obtained. Time / s Volume of hydrogen / cm3 0 0 10 32 20 50 30 64 40 75 60 88 80 92 100 100 120 100 (B410U20-1) Turn over. 17 Examiner only (a) Plot the results for the experiment and draw a line of best fit. Label it A. [4] © WJEC CBAC Ltd. (b) Use the graph to calculate the rate of reaction at 20 seconds in cm3 s–1. [2] Rate = ........................................................ cm3 s–1 (c) When he repeated the experiment, it took him 8 seconds to replace the bung in the tube and start the stopwatch after adding the magnesium. On the graph, sketch the curve that would be obtained if the results of this experiment were plotted. Label it B. [2] 18 (B410U20-1) Examiner only (d) Calculate the mass of the magnesium strip used in the experiment. [2] Mass = ........................................................ g (e) The rates of some reactions can be determined from the loss of mass over a period of time. However, the student said that he could not use this method as he only had a two decimal place balance. Is he correct? Justify your answer. [2] (f) He repeated the experiment using the same mass of magnesium and the same volume and concentration of acid, in order to collect 100 cm3 of hydrogen, but over a longer period of time. State one method of slowing down the reaction and use collision theory to explain this change of rate. [3] © WJEC CBAC Ltd. (B410U20-1) Turn over. 19 Examiner only © WJEC CBAC Ltd. Energy Extent of reaction 17 END OF PAPER (g) On the axes below, sketch the energy profile for this reaction, labelling the enthalpy change of reaction, ΔrH. [2] (B410U20-1) 20 © WJEC CBAC Ltd. Additional page. Examiner only (B410U20-1) 21 Examiner only © WJEC CBAC Ltd. Additional page. BLANK PAGE (B410U20-1) 22 © WJEC CBAC Ltd. BLANK PAGE (B410U20-1) 23 © WJEC CBAC Ltd.