CIE Jun 2015 v3 Paper 5 Q2

Answers available below

At high temperatures a mixture of iodine and hydrogen gases reacts to form an equilibrium with gaseous hydrogen iodide. H2(g) + I2(g) 2HI(g) (a) (i) Write an expression for the equilibrium constant, Kc, based on concentration, for this reaction. (ii) If the starting concentration of both iodine and hydrogen was a mol dm3 and it was found that 2y mol dm3 of hydrogen iodide had formed once equilibrium had been established, write Kc in terms of a and y. [1] (b) The expression for the equilibrium constant from (a)(ii) can be re-written as shown below. [1] y = a + 2 K c K c In an experiment, air was removed from a 1 dm3ask and amounts of hydrogen and iodine gases were mixed together such that their initial concentrations were both a mol dm3. This mixture was allowed to come to equilibrium at 760 K in theask. The equilibrium concentration of iodine, (ay) mol dm3, was then measured. The experiment was repeated for various initial concentrations, a mol dm3, and the results were recorded in the table below. (i) Complete the table to give the values of y mol dm3 to three decimal places. a mol dm3 (ay) mol dm3 y mol dm3 0.178 0.200 0.500 0.800 1.000 1.500 2.100 2.800 3.400 3.800 4.200 4.900 0.022 0.050 0.252 0.200 0.365 0.570 0.652 0.700 0.867 0.868 1.150 [2]UCLES 2015 9701/53/M/J/15<br /> (ii) Plot a graph to show how y mol dm3 varies with initial concentrations of hydrogen and iodine, a mol dm3. 5.0 4.0 3.0 3m d o m 2.0 1.0 1.0 2.0 a / mol dm3 3.0 4.0 5.0 (iii) Use your points to draw a line of bestt. [1] [1]UCLES 2015 9701/53/M/J/15 [Turn over<br /> (c) (i) Determine the slope of your graph. State the co-ordinates of both points you used for your calculation. Record the value of the slope to three signi cantgures. co-ordinates of both points used(ii) Use the value of your slope and the equation in (b) to calculate the value of Kc. Your working must be shown. slope =[2] [2] (d) Explain why, for safety reasons, it is necessary to remove air from the 1 dm3ask.[1] (e) One of the experiments in (b) was repeated in a 500 cm3ask instead of the 1 dm3ask. What effect, if any, would this have on the rate of reaction and the value of Kc measured?[2]UCLES 2015 9701/53/M/J/15<br /> (f) The reaction of hydrogen and iodine to form hydrogen iodide is exothermic. H2(g) + I2(g) 2HI(g) H = 9.6 kJ mol1 (i) On your graph, draw and label the line you would expect if the experiment was performed at 1000 K instead of 760 K. [1] (ii) What effect, if any, would the higher temperature have on the value of Kc?[1] [Total: 15]UCLES 2015 9701/53/M/J/15<br />

Show answer