Gibbs Free Energy
The thermodynamic potential whose change determines whether a process is spontaneous at constant temperature and pressure.
Formula / equation
ΔG = ΔH − TΔS
Unit
kJ/mol
In depth
Gibbs energy folds the entropy change of the surroundings into a single system property, so a negative value alone signals spontaneity. It also equals the maximum non-expansion work obtainable, and its sign can be reversed by temperature whenever ΔH and ΔS share a sign.
Examples in the real world
Ice melting has ΔG < 0 above 0 °C and ΔG > 0 below it, with ΔG exactly zero at the melting point.