The elements sodium to chlorine, in the third period, all form oxides.

1. (a)Draw a diagram to show the shape of the molecule of each of the oxides, SO3 and Cl₂O. Name each shape. In SO3 each oxygen atom forms a double bond with the sulfur atom.[4]
2. (b)(i)Explain why the melting point of MgO is higher than that of Na2O.[2]
3. (b)(ii)Explain why the melting point of SiO2 is much higher than that of SO3.[2]
4. (c)(i)SO3 is produced by the reaction between SO2 and O₂ in the Contact process. A dynamic equilibrium is established. 2SO2(g) + O2(g) ⇌ 2SO3(g) ΔΗ = –196 kJ mol⁻¹ Explain why increasing the total pressure, at constant temperature, increases the rate of production of SO3 and increases the yield of SO3.[4]
5. (c)(ii)The graph shows how the concentrations of all three species in the system change with time for a typical reaction mixture. The gradients of all three lines decrease with time and then level off in this dynamic equilibrium. Explain why the gradients of the SO2 and O2 lines decrease with time.[2]
6. (c)(iii)Explain why all three lines become horizontal.[1]
7. (c)(iv)Suggest a reason why the initial gradient of the SO2 line is steeper than that of the O2 line.[1]
8. (d)(i)2.00 moles of SO2(g) and 2.00 moles of O2(g) are sealed in a container with a suitable catalyst, at constant temperature and pressure. The resulting equilibrium mixture contains 1.98 moles of SO3(g). The total volume of the equilibrium mixture is 40.0 dm³. 2SO2(g) + O2(g) ⇌ 2SO3(g) Write the expression for the equilibrium constant, Kc, for the reaction between SO2(g) and O2(g) to produce SO3(g).[1]
9. (d)(ii)Calculate the amount, in moles, of SO2(g) and O2(g) in the equilibrium mixture.[2]
10. (d)(iii)Use your answers to (d)(i) and (d)(ii) to calculate the value of Kc for this equilibrium mixture. Give the units of Kc.[3]

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