On dissolving a substance in water, it can produce either an ionic or molecular solution. From the f — Ionic Equilibrium Chemistry Question
Question
On dissolving a substance in water, it can produce either an ionic or molecular solution. From the following data, predict what type of solution is likely to be formed by $\text{CaCl}_2$ and $\text{HgCl}_2$ at $25^\circ\text{C}$.<br><br>[Born-Haber cycle data provided in source image: $\text{CaCl}_2$ ionic dissolution $\Delta H = -100.3 \text{ kJ}$, molecular dissolution $\Delta H = +175.7 \text{ kJ}$; $\text{HgCl}_2$ ionic dissolution $\Delta H = +83.8 \text{ kJ}$, molecular dissolution $\Delta H = +16.8 \text{ kJ}$].<br>The maximum contribution of $T\Delta S$ in dissolution process is $30 \text{ kJ}$ at $25^\circ\text{C}$
💡 Solution & Explanation
Using $\Delta G = \Delta H - T\Delta S$, the process with $\Delta G < 0$ is spontaneous. For $\text{CaCl}_2$ ionic: $\Delta H = 209.2 + 1004.2 + 1715.4 - 719.6 - 1598.3 - 711.2 = -100.3 \text{ kJ/mol}$, meaning $\Delta G$ is highly negative (favourable). For $\text{HgCl}_2$ ionic: $\Delta H = 83.7 + 460.2 + 2815.8 - 719.6 - 1845.1 - 711.2 = +83.8 \text{ kJ/mol}$, making $\Delta G \ge 53.8 \text{ kJ/mol}$ (unfavourable). For $\text{HgCl}_2$ molecular: $\Delta H = 83.7 - 66.9 = +16.8 \text{ kJ/mol}$, allowing $\Delta G$ to be negative ($-13.2 \text{ kJ/mol}$) since $T\Delta S$ can be up to 30 kJ. Thus, $\text{CaCl}_2$ forms an ionic solution and $\text{HgCl}_2$ forms a molecular solution. Therefore, correct answer is C.