Gibbs free energy

In thermodynamics, the Gibbs free energy, G, is the energy associated with a chemical reaction that can be used to do work at a constant temperature and pressure. The unit of Gibbs free energy, G, is joules, J.

Gibbs free energy is calculated by subtracting temperature, T, times entropy, S, from the enthalpy, H:

G = H - TS

The change in Gibbs free energy, ΔG, for a reaction at a certain temperature determines the spontaneity of the reaction at that temperature and is given by the equation

ΔG = ΔH - TΔS

If ΔG is:

• Negative: the process is spontaneous and may proceed as written. A reaction with a negative ΔG value is called an exergonic reaction.

• Positive: the process is non-spontaneous as written, but it may move spontaneously in the reverse direction. A reaction with a positive ΔG value is called an endergonic reaction.

• Zero: the process is at equilibrium.

Remember that ΔG will have different values, or even different signs, at different temperatures. You can read more about the temperature dependence of the Gibbs free energy.