If a reacting system is not in equilibrium, the force used to attain the equilibrium is defined as free energy change, ΔG. Standard free energy ΔG is defined as the force required to attain equilibrium at standard condition (298K = 25°C) and when the reactants and products are present at 1M concentration at 1 atm.

At constant temperature and pressure, ΔG is dependent on actual concentration of the reactants and products.