Chemistry 9701/43 — May/June 2012

The apparatus shown in the diagram can be used to measure the enthalpy change of formation of magnesium oxide, $\Delta H_f^\ominus(\text{MgO})$.

List the measurements you would need to make using this apparatus in order to calculate $\Delta H_f^\ominus(\text{MgO})$.

Use the following data, together with appropriate data from the Data Booklet, to calculate a value of $\Delta H_f^\ominus(\text{MgO})$.

$$\begin{aligned} \text{lattice energy of MgO(s)} &= -3791 \text{ kJ mol}^{-1} \\ \text{enthalpy change of atomisation of Mg} &= +148 \text{ kJ mol}^{-1} \\ \text{electron affinity of the oxygen atom} &= -141 \text{ kJ mol}^{-1} \\ \text{electron affinity of the oxygen anion, O}^- &= +798 \text{ kJ mol}^{-1} \end{aligned}$$ $$\Delta H_f^\ominus(\text{MgO}) = \text{.......................... kJ mol}^{-1}$$

Write equations, including state symbols, for the reactions, if any, of the following two oxides with water. Suggest values for the pH of the resulting solutions.