Well, it is usually zero, but this requires that we specify which one...
The Gibbs' free energy, #G#, is a function of the temperature #T# and pressure #P#, i.e. #G = G(T,P)#. So, if the temperature and TOTAL pressure are constant, as they are in a chemical equilibrium in a closed system, #DeltaG = 0#.
The Helmholtz free energy, #A#, is a function of the temperature #T# and volume #V#, i.e. #A = A(T,V)#. So, if the temperature and volume are constant, as they (sometimes) are in a chemical equilibrium in a closed system, #DeltaA = 0#. (This is less often considered because if different mols of gas are formed than was started with, #DeltaV ne 0# and thus #DeltaA ne 0#.)
