Gibbs free energy (G) tells us whether a process can happen on its own at constant temperature and pressure.
- It combines energy (enthalpy, H) and disorder (entropy, S):
G = H – T × S
Spontaneity rules:
- ΔG < 0 → reaction happens naturally (spontaneous)
- ΔG > 0 → reaction does not happen naturally (non-spontaneous)
- ΔG = 0 → reaction is at equilibrium
Simple idea:
If a process releases free energy, it can occur on its own.
If it needs free energy, it won’t happen without help.
Example:
- Burning wood → ΔG < 0 → happens naturally
- Freezing water at 100 °C → ΔG > 0 → does not happen naturally