Well, if Gibbs free energy is zero, then no energy is being produced in that reaction that actually can be used in any useful way. In fact, when Gibbs free energy is zero, that reaction is said to have reached equilibrium. And at that moment in time, the rate of the four reaction is equal to the rate of the reverse reaction. So if the Gibbs free energy is zero, the reaction has achieved equilibrium and is said to be neither spontaneous nor non spontaneous. In such a case, the rate of the four reaction going from reactants to products is equal to the rate of the reverse reaction going from products back to reactants. Now let's move on to Activation energy.