10   5 Nitrogen monoxide reacts with oxygen to form nitrogen dioxide. 2NO(g) + O2(g) → 2NO2(g) The rate is proportional to the concentration of oxygen and to the square of the concentration of nitrogen monoxide. (a) The rate of this reaction can be determined by measuring the change in the total gas pressure. (i) Give a reason why this method can be used in this reaction. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) State two factors, other than initial amounts of reactants, that must be kept constant for this method to work. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 11   (b) The graph shows four lines of a quantity Y plotted against a quantity X. Y X P Q R S (i) Which line shows the relationship between the concentration of nitrogen monoxide (Y) and time (X)? (1) A line P B line Q C line R D line S (ii) Which line shows the relationship between rate (Y) and concentration of oxygen (X)? (1) A line P B line Q C line R D line S (c) The rate of this reaction is z mol dm–3 s–1 under certain conditions. The concentration of nitrogen monoxide is doubled and the concentration of oxygen is halved. All other conditions remain the same. What will be the new rate of reaction in mol dm–3 s–1? (1) A z/2 B z C 2z D 4z 12   (d) Nitrogen monoxide is formed in car engines. It is removed by the catalytic converter in the car exhaust. 2NO(g) + 2CO(g) → 2CO2(g) + N2(g) The reaction is exothermic and the most active catalyst is platinum. (i) Complete the labelled reaction profile for the catalysed reaction. (3) Enthalpy 2NO(g) + 2CO(g) Progress of reaction (ii) Catalysts, such as platinum, are very expensive. Explain an economic benefit of using a catalyst in an industrial process. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 5 = 10 marks) Turn over 13   6 Iodine reacts with propanone in acidic conditions. I2(aq) + CH3COCH3(aq) CH3COCH2I(aq) + H+(aq) + I–(aq) A student was asked to investigate the kinetics of this reaction. The student predicted that the rate equation for the reaction would be rate = k[I2(aq)][CH3COCH3(aq)][H+(aq)]0 because the balanced equation shows that one molecule of iodine reacts with one molecule of propanone and the acid is a catalyst. (a) The student first determined the order of reaction with respect to iodine by keeping the concentrations of propanone and acid constant. The student used the outline procedure shown. • mix 25 cm3 of aqueous propanone with 25 cm3 of dilute sulfuric acid in a conical flask • add 25 cm3 of aqueous iodine, immediately start a stopwatch and swirl the mixture in the conical flask • use a pipette to remove a 10.0 cm3 sample of the solution and place it in a clean conical flask • add a spatula measure of sodium hydrogencarbonate and note the exact time it is added • take four more 10.0 cm3 samples of the mixture and add sodium hydrogencarbonate to each of them at regular time intervals • titrate the unreacted iodine in the samples with sodium thiosulfate solution using starch indicator. (i) State how the student could ensure that the concentrations of propanone and acid are effectively constant throughout the experiment. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Explain why sodium hydrogencarbonate is added. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... H+(aq) 14   (b) The student obtained these results. Time / min 5 10 15 20 25 Volume of thiosulfate / cm3 15.0 13.8 12.6 11.4 10.2 (i) Give a reason why it is not necessary to calculate the concentration of iodine at each time to work out the order of reaction with respect to iodine. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Plot a graph to show that the order of reaction with respect to iodine is zero. (2) (iii) Give a reason why the graph shows that the order of reaction with respect to iodine is zero. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 15   (c) Further experiments showed that the correct overall rate equation is rate = k[CH3COCH3(aq)][H+(aq)][I2(aq)]0 (i) Deduce a possible rate determining step in the mechanism of this reaction. Curly arrows are not required. (2) (ii) Data from two experiments carried out at the same temperature are shown. Experiment [CH3COCH3(aq)] / mol dm–3 [H+(aq)] / mol dm–3 [I2(aq)] / mol dm–3 Rate / mol dm–3 s–1 1 3.0 0.4 0.02 3.36 × 10–5 2 4.0 0.2 0.04 What is the rate, in mol dm–3 s–1, in Experiment 2? (1) A 2.24 × 10–5 B 3.36 × 10–5 C 4.48 × 10–5 D 8.96 × 10–5 (iii) The experiment in (a) is repeated but using aqueous bromine instead of aqueous iodine. All other conditions are kept the same. Explain how you would expect the rate of reaction of bromination of propanone to compare with the rate of iodination of propanone. Assume that the reaction between bromine and propanone in acidic conditions has the same rate equation as that between iodine and propanone in acidic conditions. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 6 = 12 marks)