Gibbs Function

The Gibbs function is also known as the Gibbs free energy and is defined in terms of temperature, T, the enthalpy, H, and entropy, S, such that :
G = H - TS. 

In a thermodynamic process with no composition change, the change in Gibbs function is given by:
dG = dH - TdS - SdT. 

Using the combined first and second laws, dU = TdS - PdV and dH = TdS + VdP gives:

dG = VdP -SdT 

For a constant pressure process: (dG/dT)_p = - S, and for a constant temperature process:(dG/dP)_T = V, where V is the volume of the system.