21 (A410U10-1) Examiner only 13. Ammonia, NH3, and hydrazine, NH2NH2, are both compounds containing only nitrogen and hydrogen. (a) The production of ammonia in the Haber process uses nitrogen and hydrogen gases as starting materials, a pressure of 200 atm and a temperature of 400 °C. The reaction occurring is shown below. N2(g) + 3H2(g) [ 2NH3(g) ΔH = −94 kJ mol−1 (i) Explain fully why a pressure of 200 atm is used for this reaction. [3] (ii) The enthalpy change value given above is not the standard enthalpy change of formation for ammonia. Give one reason why this is not the standard enthalpy change of formation. [1] (iii) Write an expression for the equilibrium constant, Kc, for this reaction. [1] (iv) State the effect (if any) of increasing temperature on the value of Kc. Give a reason for your answer. [2] © WJEC CBAC Ltd. Turn over. (A410U10-1) 22 Examiner only (v) A mixture of nitrogen and hydrogen has an initial concentration of 0.020 mol dm−3 of each gas. The mixture is allowed to come to equilibrium in a fixed volume. In the equilibrium mixture 20 % of the nitrogen gas had been converted into ammonia. Calculate the value of Kc under these conditions. [3] Kc = ....................................................... (b) The standard enthalpy change of formation for ammonia is −46 kJ mol−1 and for hydrazine it is +51 kJ mol−1. (i) State what information these values provide about the stability of these molecules. [1] (ii) One method of producing hydrazine is to oxidise ammonia using an appropriate oxidising agent, such as hydrogen peroxide. 2NH3(g) + H2O2(l) NH2NH2(l) + 2H2O(l) ΔH θ = −241 kJ mol−1 © WJEC CBAC Ltd. Substance Standard enthalpy change of formation, ΔfH θ / kJ mol−1 Standard entropy, S θ / J K−1 mol−1 NH3(g) −46 193 NH2NH2(l) +51 122 H2O2(l) 102 H2O(l) −286 70 I. Calculate the standard enthalpy change of formation of hydrogen peroxide, H2O2. [2] ΔfH θ = ....................................................... kJ mol−1 II. Calculate the temperature at which the value of ∆Gθ is equal to zero. [3] T = ....................................................... K III. A student states that the temperature calculated in part II is the minimum temperature required for the reaction to occur. Is the student correct? Give a reason for your answer. [2] (iii) An alternative route for producing hydrazine starts with the molecule urea, which is produced in biological systems. (NH2)2CO + NaOCl + 2NaOH N2H4 + H2O + NaCl + Na2CO3 Give one disadvantage of this route over the production of hydrazine from ammonia. [1] (A410U10-1) Turn over. 23 Examiner only © WJEC CBAC Ltd. 24 (A410U10-1) Examiner only © WJEC CBAC Ltd. (iv) Hydrazine can undergo both oxidation and reduction reactions. Electrochemical potentials for both processes are included in the table below. Standard electrode potential, E θ / V Co3+(aq) + e– [ Co2+(aq) +1.82 Fe3+(aq) + e– [ Fe2+(aq) +0.77 N2H4(aq) + 4H2O(l) + 2e– [ 2NH4 +(aq) + 4OH–(aq) +0.11 V3+(aq) + e– [ V 2+(aq) –0.26 Cr3+(aq) + e– [ Cr2+(aq) –0.42 N2(g) + 4H2O(l) + 4e– [ N2H4(aq) + 4OH–(aq) –1.15 I. Suggest whether addition of sodium hydroxide to a hydrazine solution will favour its use as a reducing agent. Give a reason for your answer. [2] II. Identify which of these four M3+ ions (if any) can be reduced by hydrazine under standard conditions. Give a reason for your answer. [2] 23 (A410U10-1) 25 © WJEC CBAC Ltd. BLANK PAGE Turn over. Turn over for Q.14