Parameter in thermodynamics which describes the 'favorableness' of a process.

The change in energy going from one state to another is expressed as:

deltaG = deltaH - TdeltaS

deltaG - change in free energy
deltaH - change in enthalpy
deltaS - change entropy
T - temperature

If deltaG is greater than zero, then the process is unfavorable and wont occur spontaneously. If less than zero, then the process will be spontaneous. If equal to zero, then the process is at equilibrium.

The change in free energy for a process where the equilibrium constant (K) between the two state is known is expressed as:

deltaG = -RT ln(K)

R - gas constant (8.31441 J K-1 mol-1)