OCR A Jun 2016 Paper 2 Q4

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10 This question is about equilibrium and catalysts. (a) The equilibrium between NO2 and N2O4 gases is set up in a gas syringe at room temperature. The two gases are different in appearance. 2NO2(g) brown N2O4(g) colourless H = 58 kJ mol1 Using le Chateliers principle, predict and explain how the following changes would affect the appearance of the equilibrium mixture. (i) The gas mixture is compressed by pushing in the plunger of the gas syringe.[2] (ii) The gas syringe is placed in a warm water bath.[2] (b) When potassium chromate(VI), K2CrO4, is dissolved in water an equilibrium is set up. The position of equilibrium is well to the left and the solution is a yellow colour. 2(aq) + 2H+(aq) 2CrO4 yellow Cr2O7 orange 2(aq) + H2O(l) The addition of aqueous acid turns the solution an orange colour. Aqueous alkali is then added and the solution turns a yellow colour. Explain these observations in terms of le Chateliers principle.[2]OCR 2016<br />
 (c) Catalysts can be used to change the rate of some chemical reactions. 11 (i) Zinc and sulfuric acid react together to form a solution of zinc sulfate, ZnSO4, and hydrogen gas. The reaction is exothermic. The rate of the reaction increases when a catalyst is added.Complete the enthalpy profile diagram for this reaction using the formulae of the reactants and products. Label activation energies, Ea (without catalyst) and Ec (with catalyst). Label the enthalpy change of reaction, H. enthalpy progress of reaction [3]OCR 2016 Turn over<br />
 12 (ii) Using a Boltzmann distribution, explain how a catalyst increases the rate of a chemical reaction. Include a labelled sketch of your Boltzmann distribution on the grid below. Label the axes and any other important features. Your answer needs to be clear and well organised using the correct terminology.[4]OCR 2016<br />
 (d) The chemical industry uses catalysts for many of its reactions. 13 (i) State an example of a catalyst used by the chemical industry and write the equation for the reaction that is catalysed. catalystequation[1] (ii) State two ways that the use of catalysts helps chemical companies to make their processes more sustainable and less harmful to the environment.[2] (e) In the stratosphere, nitrogen oxides can catalyse the breakdown of ozone. (i) State two sources of nitrogen oxides in the stratosphere.[1] (ii) Write equations to show how nitrogen monoxide catalyses the breakdown of ozone.[2] [Total: 19]OCR 2016 Turn over<br />

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