7.14 Free Energy of Dissolution

Dissolution as an equilibrium process

Dissolution is governed by ΔG = ΔH − TΔS. A solute dissolves spontaneously when ΔG < 0. Breaking solute-solute and solvent-solvent interactions requires energy (raising ΔH), while forming solute-solvent interactions releases energy. Mixing increases entropy (ΔS > 0) almost always, since dissolved particles are more disordered than in a pure solid or liquid.

Entropy usually drives dissolution

Even when dissolution is endothermic (ΔH > 0), it can still be spontaneous if the positive ΔS term is large enough to make TΔS outweigh ΔH, giving a negative ΔG. This is why many ionic salts dissolve in water despite absorbing heat — the increase in disorder favors dissolving.

Temperature and solubility connection

Because ΔG = ΔH − TΔS, temperature changes affect the sign of ΔG. For endothermic dissolution, increasing T makes the −TΔS term more negative, favoring dissolution (solubility increases with temperature). For exothermic dissolution, raising T can make ΔG less negative, sometimes decreasing solubility.

Key terms

ΔG = ?

ΔG = ΔH − TΔS, the equation for Gibbs free energy of a process.

Spontaneous dissolution condition

Occurs when ΔG < 0 (negative free energy change).

Why can endothermic dissolution still occur?

If the increase in entropy (ΔS) is large enough that TΔS > ΔH, ΔG becomes negative and dissolution is spontaneous.

Effect of dissolution on entropy

Entropy usually increases (ΔS > 0) because dissolved particles are more disordered/dispersed than in the pure solute or solvent.

Temperature's effect on endothermic dissolution

Raising temperature makes −TΔS more negative, favoring dissolution and often increasing solubility.

Try an AP-style question

Dissolving NH₄NO₃ in water is endothermic but happens on its own. What makes it favorable?

1. A catalyst in the water
2. A large increase in entropy
3. A large release of heat
4. A decrease in entropy