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title: "A student investigates the thermodynamics of dissolution by comparing two soluble chloride salts at \\(298\\text{ K}\\). The student records the standard entropy of dissolution, \\(\\Delta S^\\circ_{\\text{soln}}\\), for each salt in the table below.  | Process | \\(\\Delta S^\\circ_{\\text{soln}}\\,[\\text{J}/(\\text{mol}\\cdot\\text{K})]\\) | | :— | :— | | \\(\\text{NaCl}(s) \\rightarrow \\text{Na}^+(aq) + \\text{Cl}^-(aq)\\) | \\(+43\\) | | \\(\\text{AlCl}_3(s) \\rightarrow \\text{Al}^{3+}(aq) + 3\\text{ Cl}^-(aq)\\) | \\(-263\\) |  Which of the following best explains why \\(\\Delta S^\\circ_{\\text{soln}}\\) is negative for the dissolution of \\(\\text{AlCl}_3(s)\\)?"
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url: "https://nerd-notes.com/ubq/123922/"
date_modified: "2026-09-28T12:32:13+00:00"
---

# A student investigates the thermodynamics of dissolution by comparing two soluble chloride salts at \(298\text{ K}\). The student records the standard entropy of dissolution, \(\Delta S^\circ_{\text{soln}}\), for each salt in the table below.

| Process | \(\Delta S^\circ_{\text{soln}}\,[\text{J}/(\text{mol}\cdot\text{K})]\) |
| :— | :— |
| \(\text{NaCl}(s) \rightarrow \text{Na}^+(aq) + \text{Cl}^-(aq)\) | \(+43\) |
| \(\text{AlCl}_3(s) \rightarrow \text{Al}^{3+}(aq) + 3\text{ Cl}^-(aq)\) | \(-263\) |

Which of the following best explains why \(\Delta S^\circ_{\text{soln}}\) is negative for the dissolution of \(\text{AlCl}_3(s)\)?

A student investigates the thermodynamics of dissolution by comparing two soluble chloride salts at \(298\text{ K}\). The student records the standard entropy of dissolution, \(\Delta S^\circ_{\text{soln}}\), for each salt in the table below.

| Process | \(\Delta S^\circ_{\text{soln}}\,[\text{J}/(\text{mol}\cdot\text{K})]\) |
| :--- | :--- |
| \(\text{NaCl}(s) \rightarrow \text{Na}^+(aq) + \text{Cl}^-(aq)\) | \(+43\) |
| \(\text{AlCl}_3(s) \rightarrow \text{Al}^{3+}(aq) + 3\text{ Cl}^-(aq)\) | \(-263\) |

Which of the following best explains why \(\Delta S^\circ_{\text{soln}}\) is negative for the dissolution of \(\text{AlCl}_3(s)\)?

- **A.** The entropy of the solution decreases because breaking the ionic bonds in \(\text{AlCl}_3(s)\) absorbs thermal energy from the solvent, lowering the average kinetic energy of the \(\text{H}_2\text{O}\) molecules.
- **B.** The entropy of the solution decreases because \(\text{Al}^{3+}\) has a larger ionic radius than \(\text{Na}^+\), which restricts the volume available for the random translation of solvent molecules.
- **C.** The entropy of the solution increases because the formation of \(4\text{ mol}\) of aqueous ions per mole of \(\text{AlCl}_3(s)\) creates more possible microstates than the \(2\text{ mol}\) of ions produced by \(\text{NaCl}(s)\).
- **D.** The entropy of the solution decreases because the high charge density of \(\text{Al}^{3+}\) establishes strong ion-dipole attractions that rigidly order surrounding \(\text{H}_2\text{O}\) molecules, which outweighs the increase in entropy from lattice breakdown.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123922/*