# How is gibbs free energy related to enthalpy and entropy?

##### 2 Answers

Gibbs free energy combines enthalpy and entropy into a single value.

Gibbs free energy is the energy associated with a chemical reaction that can do useful work. It equals the enthalpy minus the product of the temperature and entropy of the system.

*G* = *H* – *TS*

At constant temperature

Δ*G* = Δ*H* – *TΔS*

Δ*G* predicts the direction of a chemical reaction. If Δ*G* is negative, then the reaction is spontaneous. If Δ*G* is positive, then the reaction is non-spontaneous.

The change in Gibbs Free Energy (ΔG) for any process is related to changes in enthalpy (ΔH) and entropy (ΔS) by the relationship

ΔG = ΔH - TΔS

where T is absolute temperature in Kelvins.

When ΔG is less than zero, there is a thermodynamic driving force for the reaction or process in the forward direction (as written). When ΔG is positive, then reactants are favored instead. When ΔG=0 the system is at equilibrium.