# What is the change in free energy of a system at chemical equilibrium?

The Gibbs' free energy, $G$, is a function of the temperature $T$ and pressure $P$, i.e. $G = G \left(T , P\right)$. So, if the temperature and TOTAL pressure are constant, as they are in a chemical equilibrium in a closed system, $\Delta G = 0$.
The Helmholtz free energy, $A$, is a function of the temperature $T$ and volume $V$, i.e. $A = A \left(T , V\right)$. So, if the temperature and volume are constant, as they (sometimes) are in a chemical equilibrium in a closed system, $\Delta A = 0$. (This is less often considered because if different mols of gas are formed than was started with, $\Delta V \ne 0$ and thus $\Delta A \ne 0$.)