Free Energy
Definition and meaning of Free Energy in chemistry.
Free energy is a thermodynamic quantity that measures the maximum useful work obtainable from a system and predicts whether a process will occur spontaneously at constant temperature and pressure (or volume).
In more detail
The most widely used form, Gibbs free energy (G), combines enthalpy (H) and entropy (S) through ΔG = ΔH − TΔS, where T is the absolute temperature. A negative ΔG signals a spontaneous process, a positive ΔG signals a nonspontaneous one, and ΔG = 0 marks equilibrium. Because it weighs both heat change and disorder change together, ΔG resolves cases where enthalpy and entropy pull in opposite directions. Helmholtz free energy (A = U − TS) is the analogous quantity for processes at constant volume and temperature rather than constant pressure.
Key facts
| Formula | ΔG = ΔH − TΔS |
|---|---|
| Field | Physical Chemistry |
| Units | kJ/mol (or J/mol) |
| Also known as | Gibbs free energy (Gibbs energy) |
For N2(g) + 3H2(g) → 2NH3(g) at 25°C, ΔG° = −33.0 kJ/mol, showing the forward reaction is thermodynamically favorable under standard conditions, even though a catalyst is needed for it to proceed at a practical rate.
Frequently asked questions
Does a negative ΔG mean a reaction happens fast?
No. ΔG predicts thermodynamic spontaneity (whether a process is favorable), not its rate; reaction rate depends on kinetics, particularly activation energy.
How does free energy relate to the equilibrium constant?
The standard free energy change is linked to the equilibrium constant K by ΔG° = −RT ln K, so a more negative ΔG° corresponds to a larger K.