2018 H2 Chem P1 Q8 — Thermochemistry Solution

The answer is C.
\(\Delta H^\circ_\text{solution}\) is the enthalpy change when one mole of the solid dissolves in water to give an infinitely dilute solution of its ions. \(\text{CaCl}_2(\text{s}) + \text{aq} \rightarrow \text{Ca}^{2+}(\text{aq}) + 2\text{Cl}^-(\text{aq})\) is exactly this for 1 mol of \(\text{CaCl}_2\).
• A is wrong: lattice energy is the enthalpy change when one mole of the ionic solid forms from its gaseous ions. The equation shown forms 1 mol of \(\text{Al}_2\text{O}_3(\text{s})\), so its enthalpy change is \(\Delta H^\circ_\text{lattice energy}(\text{Al}_2\text{O}_3)\) itself, not twice it.
• B is wrong: \(\Delta H^\circ_\text{neutralisation}\) is defined per mole of water formed. This equation forms 2 mol of \(\text{H}_2\text{O}\), so its enthalpy change is \(2\Delta H^\circ_\text{neutralisation}\).
• D is wrong: \(\Delta H^\circ_\text{formation}\) is for one mole of product formed from its elements in their standard states. This equation forms 8 mol of \(\text{SO}_2\), so its enthalpy change is \(8\Delta H^\circ_\text{formation}(\text{SO}_2)\).