Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- Dissolving a salt, MX(s) ⇌ M⁺(aq) + X⁻(aq), has its own ΔH°, ΔS° and ΔG°, and its K is K_sp.
- Factor 1, breaking up the solid: ion–ion attractions overcome. ΔH > 0 (large); ΔS > 0 (ions freed).
- Factor 2, reorganising the solvent: some water–water attractions broken; water molecules ordered around ions. ΔH usually > 0; ΔS usually < 0.
- Factor 3, ion–solvent attractions: ion–dipole attractions form. ΔH < 0 (large).
- Small or highly charged ions order more water, so ΔS° of dissolving is smaller or even negative.
- The total is a small difference of large terms, so predicting ΔG° is hard. The model explains results better than it predicts them.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| ΔG° = ΔH° − TΔS° | free energy of dissolution from its enthalpy and entropy | decide whether dissolving is favoured |
| ΔG° = −RT ln K_sp | ΔG° < 0 ⇔ K_sp > 1 (very soluble); ΔG° > 0 ⇔ K_sp < 1 | convert between ΔG° and K_sp |
| ΔH° > 0: K_sp rises with T | endothermic dissolving | predict the effect of heating |
| ΔH° < 0: K_sp falls with T | exothermic dissolving | predict the effect of heating |
Assumptions behind the reasoning
- ΔH° and ΔS° of dissolution stay roughly constant over the temperature range used.
- K_sp is written for the dissolving equation as given, with ideal behaviour of the dissolved ions.
- The three factors are a model: they are imaginary steps used to explain, not separate events you could watch.
Mistakes to avoid
- “Endothermic, so insoluble.” A large positive ΔS° can make ΔG° negative.
- “Exothermic, so soluble.” A strongly negative ΔS° can make ΔG° positive.
- “Dissolving always increases entropy.” Ordered water shells lower it.
- “Separating ions releases energy.” It always needs energy.
- kJ and J mixed in ΔH° − TΔS° or in −RT ln K_sp.
- Only one factor named. A full explanation compares the opposing contributions.
Quick self-check
- Which factor has a negative ΔH contribution? (Factor 3, ion–solvent attractions)
- ΔH° = +8.0 kJ mol⁻¹ and ΔS° = +20 J mol⁻¹ K⁻¹ for an invented salt. Is dissolving favoured at 298 K? (No: ΔG° = +2.04 kJ mol⁻¹, K_sp = 0.44)
- Dissolving a salt is exothermic. Does warming the water increase or decrease K_sp? (Decrease)
Next: practice questions.