Gibbs free energy (ΔG) is very useful for understanding phase transitions (changes between solid, liquid, and gas) because it tells us which phase is stable under given conditions.
1. Spontaneity of phase changes
- A phase transition occurs spontaneously when the Gibbs free energy of the new phase is lower than the old phase.
- At equilibrium, the Gibbs free energy of both phases is equal (ΔG = 0).
2. Examples of phase transitions
- Melting (solid → liquid):
- Ice melts when ΔG for liquid water < ΔG for solid ice.
- At 0°C, ΔG = 0 → ice and water coexist in equilibrium.
- Boiling (liquid → gas):
- Water boils when ΔG for vapor < ΔG for liquid.
- At 100°C and 1 atm, ΔG = 0 → liquid and vapor coexist.
- Sublimation (solid → gas):
- Occurs spontaneously when ΔG for gas < ΔG for solid.
3. Role of temperature and pressure
- ΔG depends on temperature (T) and pressure (P).
- Increasing temperature or reducing pressure can make ΔG negative for transitions like melting or boiling, making them spontaneous.
Summary
- ΔG < 0 → phase change is spontaneous
- ΔG = 0 → phases coexist at equilibrium
- ΔG > 0 → phase change does not occur spontaneously
In short, Gibbs free energy predicts which phase is stable, which direction a phase change will occur, and at what conditions equilibrium happens.