18 (B410U20-1) © WJEC CBAC Ltd. 10. Hydrogen peroxide reacts with iodide ions in acid solution to produce iodine. The equation for the reaction is as follows. A student is asked to investigate the effect of changing the concentration of hydrogen peroxide on the rate of reaction by a “clock method”. She is told to carry out the following method. • Measure 10.0 cm3 of sulfuric acid, 10.0 cm3 of sodium thiosulfate, 15.0 cm3 of potassium iodide and 1.0 cm3 of starch solution into a conical flask. • Measure 5.0 cm3 of hydrogen peroxide and 9.0 cm3 of water into a boiling tube. • Add the peroxide solution to the other reagents in the flask and start a stopwatch at the same time. • Record the time taken, to the nearest second, for a blue-black colour to form in the reaction mixture. • Repeat the procedure five times, with each run differing only in the peroxide concentration in the mixture, ensuring that the reaction times are neither too short nor too long. She obtains the following results. H2O2(aq) + 2H+(aq) + 2I–(aq) I2(aq) + 2H2O(l) Experiment Volume H2O2 / cm3 Volume H2O / cm3 Time / s 1 Time / s–1 1 5.0 9.0 28 0.0357 2 6.0 8.0 3 7.0 7.0 17 0.0589 4 4.0 10.0 36 0.0278 5 3.0 11.0 54 0.0185 6 2.0 12.0 102 0.0098 (B410U20-1) 19 © WJEC CBAC Ltd. Examiner only Turn over. (a) The student says that it would be better to make up a single batch containing the acid, thiosulfate, iodide and starch solutions in the correct proportions before starting and use 36.0 cm3 of this in each experiment. Is she correct? Justify your answer. [1] (b) State two changes which could be made in order to improve the results in this experiment. Explain your reason in each case. [4] (c) Give a reason why the peroxide is measured into a boiling tube first and not added directly to the flask from a burette. [1] (d) Suggest a reason why the student did not carry out the procedure using 8.0 cm3 of hydrogen peroxide and 6.0 cm3 of water. [1] 20 (B410U20-1) © WJEC CBAC Ltd. (e) She plotted a graph of rate against concentration of peroxide. This is shown below. [ H2O2 ] / mol dm–3 1 Time / s–1 0 0.008 0.012 0.016 0.020 0.024 0.028 0.032 0.036 0.040 0.044 0.048 0.052 0.056 0 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.060 (B410U20-1) Turn over. 21 Examiner only © WJEC CBAC Ltd. (i) State how the rate depends on the concentration of peroxide. [1] (ii) Use the graph to find the time taken for the colour change to occur when she used 6.0 cm3 of hydrogen peroxide and 8.0 cm3 of water. [2] Time = ..................................................... s (f) Suggest another method, not using sodium thiosulfate, by which the rate of this oxidation reaction could be measured. [2] (g) Explain, using simple collision theory, why the rate of this reaction changes as the concentration of hydrogen peroxide changes. [2] END OF PAPER 14 22 (B410U20-1) Examiner only © WJEC CBAC Ltd. For continuation only. BLANK PAGE (B410U20-1) 23 © WJEC CBAC Ltd.