12 (1410U30-1) 12 © WJEC CBAC Ltd. 11. Hydrogen peroxide, H2O2, decomposes slowly at 300 K to form water and oxygen gas. 2H2O2(l) 2H2O(l) + O2(g) The reaction profile for this reaction is shown below. Energy / kJ mol–1 Reaction path 0 20 40 60 80 100 120 140 160 180 200 220 240 75 kJ mol–1 98 kJ mol–1 (1410U30-1) Turn over. 13 13 © WJEC CBAC Ltd. Examiner only (a) The rate of decomposition of hydrogen peroxide can be influenced by a range of catalysts. The activation energy when using MnO2 as a catalyst is 58 kJ mol–1. Draw the reaction profile for the catalysed reaction on the grid opposite. [1] (b) Another catalyst was used and this gave a value for the rate constant, k, of 1.68 mol–1 dm3 s–1 and the frequency factor, A, of 1.41 × 104 mol–1 dm3 s–1 at a temperature of 300 K. (i) State the Arrhenius equation. [1] (ii) Calculate the activation energy using this catalyst and hence state whether it is a more effective catalyst than MnO2. [3] Activation energy = .................................................... kJ mol–1 (c) The standard enthalpy change of formation, ΔfHθ, of water is –286 kJ mol–1. Use this information and the graph to calculate the standard enthalpy change of formation of hydrogen peroxide. [2] ΔfHθ(H2O2) = ..................................................... kJ mol–1 (d) State whether you would expect the entropy change for the decomposition of hydrogen peroxide to be positive or negative. Give a reason for your answer. [1] V 14 (1410U30-1) 14 © WJEC CBAC Ltd. Examiner only (e) One way of assessing whether a reaction is feasible is to use standard electrode potentials. Standard electrode potential / V H2O2 + 2H+ + 2e– a 2H2O +1.77 Cr2O7 2– + 14H+ + 6e– a 2Cr3+ + 7H2O +1.33 (i) The apparatus below can be used to measure the standard electrode potential for the Cr2O7 2–/Cr3+ half-cell. B C A hydrogen, 1 atm an acidified mixture of 1 mol dm–3 Cr3+(aq) and 1 mol dm–3 Cr2O7 2– (aq) I. State what is represented by A and B on the diagram. [1] A ..................................................................................... B ..................................................................................... II. On the diagram show the direction of flow of electrons in the external circuit. [1] (1410U30-1) Turn over. 15 15 © WJEC CBAC Ltd. Examiner only III. State what is represented by C on the diagram and state its function. [1] IV. The concentrations of both Cr2O7 2– and Cr3+ ions are 1 mol dm–3. State and explain how the value shown on the high resistance voltmeter would change if the concentration of the Cr3+ ions were increased whilst the concentration of the Cr2O7 2– was left unchanged. [2] (ii) It is suggested that hydrogen peroxide could be used to oxidise Cr3+ ions in acidic solution to form dichromate ions. I. Write an equation for this proposed reaction. [1] II. Use the standard electrode potential values given to predict whether this reaction is feasible. [2] III. Another method of finding whether a reaction is feasible is to use the Gibbs free energy calculated from standard enthalpy of formation and standard entropy values. State, giving a reason, whether Gibbs free energies or electrochemical methods are more appropriate for finding whether this reaction is feasible. [2] 18