So what exactly is the activation energy? Well, any reaction has some activation energy and this is simply the amount of energy that we have to input for the reaction to take place to convert the reactants to the products or in reverse. Now let's suppose we go from reactants to products. In this case, our activation energy is simply this quantity here. It's the difference between the energy of the molecule found on this topmost portion of the hill and the energy of that reacting. This is the GIBS free energy given by Delta G with the symbol on top or simply Delta E A, where the A stands for activation.