OCR A Jun 2015 Paper 5 Q2

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2 Hydrogen, H2, reacts with nitrogen monoxide, NO, as shown below: 2H2(g) + 2NO(g) N2(g) + 2H2O(g) (a) The rate equation for this reaction is: rate = k[H2(g)][NO(g)]2 The concentration of NO(g) is changed and a rateconcentration graph is plotted. initial rate / 104 mol dm3 s1 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 0.0 1.0 2.0 3.0 [NO(g)] / 104 mol dm3 4.0 5.0 6.0 7.0 The chemist uses H2(g) of concentration 2.0102 mol dm3. Using values from the graph, calculate the rate constant, k, for this reaction. Give your answer to two significant figures and in standard form. Show your working. k =units[4]OCR 2015<br />
 (b) A chemist investigates the effect of changing the concentration of H2(g) on the initial reaction rate at two different temperatures. The reaction is first order with respect to H2(g). (i) Using the axes below, sketch two graphs of the results. Label the graphs as follows:H for the higher temperature. L for the lower temperature initial rate 0,0 [H2(g)] (ii) State the effect of the higher temperature on the rate constant, k.[1] [2]OCR 2015 Turn over<br />
 (c) The reaction can also be shown as being first order with respect to H2(g) by continuous monitoring of [H2(g)] during the course of the reaction.Using the axes below, sketch a graph to show the results.State how you would use the graph to show this first order relationship for H2(g). [H2(g)] time[2] (d) The chemist proposes a three-step mechanism for the reaction: 2H2(g) + 2NO(g) N2(g) + 2H2O(g) (i) On the dotted line below, write the equation for step 3. step 1: step 2: 2NO N2O2 H2 + N2O2 N2O + H2O step 3:fast slow fast [1] (ii) Explain why this mechanism is consistent with the rate equation rate = k[H2(g)][NO(g)]2.[1] [Total: 11]OCR 2015<br />

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