Gibbs Free Energy
Definition
The 'will it happen on its own?' energy — if Gibbs free energy drops, the process can run spontaneously.
Symbol: · Unit: joule (J)
Technical definition
G = H − TS; ΔG < 0 means spontaneous at constant temperature and pressure. It combines energy and entropy into a single spontaneity test.
Example
Ice melting at room temperature: ΔH is positive (it needs heat) but TΔS wins, so ΔG < 0 — the puddle is thermodynamically inevitable.
Common confusion
Negative ΔG means spontaneous, not fast. Diamond turning to graphite has ΔG < 0 but takes geological time — thermodynamics says 'allowed', kinetics says 'when'.