Sulfur trioxide, SO3, is manufactured from sulfur dioxide and oxygen by the Contact process. 2SO2(g) + O2(g) ⇌ 2SO3(g) ΔH = –196.0 kJ mol⁻¹

1. (a)(i)The enthalpy change of formation of SO2, ΔHƒ SO2(g), is –296.8 kJ mol⁻¹. Define the term enthalpy change of formation.[2]
2. (a)(ii)Use the data to calculate the enthalpy change of formation of SO3(g). ΔHƒ SO3(g) = __________ kJ mol⁻¹[2]
3. (b)(i)The Contact process is usually carried out at a temperature of approximately 700 K, a pressure of approximately 150 kPa and in the presence of a vanadium(V) oxide catalyst, V2O5. The Boltzmann distribution for a mixture of SO2 and O2 at 700K is shown. Eacat represents the activation energy for the reaction in the presence of the catalyst. [Figure: Boltzmann distribution curve with x-axis 'molecular energy' and y-axis 'proportion of molecules with a given energy'. Eacat is marked on the curve.] Add a labelled mark, Eauncat, to the diagram to indicate the activation energy in the absence of the catalyst.[1]
4. (b)(ii)State the benefit of using a catalyst in this reaction. Explain how it achieves this effect.[2]
5. (b)(iii)State and explain how an increase in pressure would affect both the rate of reaction and the yield of SO3 in the Contact process. rate __________ yield __________[4]
6. (c)(i)At a pressure of 1.50 × 10⁵ Pa, 1.00 mol of sulfur dioxide gas, SO2, was mixed with 1.00 mol of oxygen gas, O2. The final equilibrium mixture formed was found to contain 0.505 mol of O2. 2SO2(g) + O2(g) ⇌ 2SO3(g) Calculate the amount, in mol, of SO2 and SO3 in the equilibrium mixture. SO2 = __________ mol SO3 = __________ mol[1]
7. (c)(ii)Calculate the partial pressure of oxygen gas, pO2, in the equilibrium mixture. pO2 = __________ Pa[2]
8. (d)(i)In another equilibrium mixture formed from different starting amounts of SO2 and O2, the partial pressures of SO2, O2 and SO3 were as shown. pSO2 = 8.42 × 10² Pa pO2 = 6.00 × 10⁴ Pa pSO3 = 9.10 × 10⁴ Pa Write the expression for the equilibrium constant, Kp, for the production of SO3 from SO2 and O2. Kp = __________[1]
9. (d)(ii)Calculate the value of Kp for this reaction and state the units. Kp = __________ units = __________[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.