Question

Consider the spontaneous dissolution of NaCl in water: NaCl(s) → Na+(aq) + Cl −(aq)

Predict whether the entropy change for this process is positive or negative and explain your answer. Then, predict whether the free energy change for the process is positive or negative and explain your answer.

Answer 1

Entropy change (ΔS): positive.

- Reason: In the solid, Na+ and Cl− are locked into a highly ordered crystal lattice. When NaCl dissolves the ions become free to move throughout the solvent and the number of accessible microstates increases, so overall disorder increases. Although water molecules form partially ordered hydration shells around the ions (which lowers the solvent entropy locally), the increase in disorder from separating and dispersing the ions outweighs that ordering. Net ΔS > 0.

Free energy change (ΔG): negative (spontaneous under ordinary conditions).

- Reason: ΔG = ΔH − TΔS. Because ΔS is positive the −TΔS term is favorable. For NaCl the enthalpy of solution is small (slightly endothermic), so the TΔS contribution makes ΔG negative at room temperature, and dissolution proceeds spontaneously (until the solution becomes saturated).