Gibbs Free Energy

Definition

The energy available to do useful work, and the decider of spontaneity. If Gibbs energy drops, the process can happen on its own.

$\Delta G = \Delta H - T\Delta S$

Symbol: $G$ · Unit: kJ/mol

Technical definition

Gibbs free energy (G = H - TS) is the thermodynamic potential whose change at constant temperature and pressure determines spontaneity: negative means spontaneous, zero means equilibrium, positive means nonspontaneous. It links to the equilibrium constant through deltaG = -RT ln K.

Example

Ice melting at 25 C has negative deltaG (spontaneous) because the entropy gain outweighs the enthalpy cost. At -10 C the signs flip and freezing is spontaneous.

Common confusion

Spontaneous does not mean fast. Diamond turning to graphite has negative deltaG but takes millions of years: kinetics controls speed, thermodynamics controls possibility.