9701 · 23.4
Gibbs free energy change, ΔG
Gibbs free energy, ΔG, combines a reaction's heat change (ΔH) and disorder change (ΔS) into a single value that tells us if it will happen spontaneously. A negative ΔG means the reaction is feasible and can proceed without continuous energy input.
Need to know
What you need to know
- **ΔG < 0**: The reaction is **spontaneous** (feasible) in the forward direction.
- **ΔG > 0**: The reaction is **non-spontaneous**. The reverse reaction is spontaneous.
- **ΔG = 0**: The system is at **equilibrium**.
Explanation
The Decider: Will a Reaction Go?
- ΔG = ΔH − TΔS — spontaneous when ΔG < 0.
- ΔG° = −nFE° for electrochemical cells.
- Temperature can reverse feasibility if ΔH and ΔS oppose.
- ΔG and equilibrium: ΔG° = −RT ln K.