Gibbs Free Energy
Definition and meaning of Gibbs Free Energy in chemistry.
Gibbs free energy (G) is a thermodynamic quantity that combines a system's enthalpy and entropy to predict whether a process will occur spontaneously at constant temperature and pressure.
In more detail
Gibbs free energy is defined as G = H − TS, so for a process at constant T and P, ΔG = ΔH − TΔS. A negative ΔG means the process is spontaneous (exergonic) as written; a positive ΔG means it is nonspontaneous and requires energy input; ΔG = 0 marks equilibrium. Because ΔG weighs the enthalpy change against the entropy change scaled by temperature, it explains why some endothermic processes (like ice melting above 0°C) still occur spontaneously, the TΔS term outweighs a positive ΔH. ΔG also links directly to the equilibrium constant via ΔG° = −RT ln K.
Key facts
| Symbol | G (change: ΔG) |
|---|---|
| Equation | ΔG = ΔH − TΔS |
| Units | joules or kilojoules per mole (kJ/mol) |
| Field | Physical Chemistry |
For ice melting at 1 atm: below 0°C, ΔG > 0 and melting is nonspontaneous; above 0°C, ΔG < 0 and melting proceeds spontaneously; at exactly 0°C, ΔG = 0 and ice and liquid water coexist at equilibrium.
Frequently asked questions
What does a negative ΔG mean for a reaction?
It means the reaction is spontaneous (exergonic) under the given temperature and pressure, releasing usable energy as it proceeds toward equilibrium.
How is ΔG related to the equilibrium constant K?
They are linked by ΔG° = −RT ln K, so a large negative standard free energy change corresponds to a large equilibrium constant favoring products.