Gibbs free energy of a reaction ( Δ G ) is the change in free energy of a system that undergoes the chemical reaction. It is the energy associated with the reaction, which is available to do some useful work. If ΔG<0 , then the reaction can be utilized to do some useful work. If ΔG>0 , then work has to be done on the system or external energy is required to make the reaction happen. ΔG=0 when the reaction is at equilibrium and there is no net change taking place in the system.