Gibbs free energy The thermodynamic function Gibbs free energy, G is defined as.

G = H - TS

where H is the enthalpy, S the entropy and T the thermodynamic temperature. Replacing the enthalpy, H ( º U + pV )

G = U + pV - TS

dG = dU + pdV + Vdp - TdS - SdT

As

dU = dQ - pdV

=TdS-pdV

for a reversible change.

dG = Vdp - SdT

For dp = 0 and dT = 0

dG = 0

i.e. if a reversible change takes place at constant temperature and pressure the Gibbs free energy is constant.