A theoretical model relating the kinetic, mechanistic, and thermodynamics aspects of a chemical reaction is presented. The model is based upon the consistency between the principle of maximum hardness; and the Hammond postulate, and it is used to obtain insights about the reaction mechanisms and to define rate constants in terms bf the chemical potential and hardness of activation. An important result is that when both electronic properties are allowed to vary along the reaction coordinate, it is found that the activation process is controlled by the change in chemical potential while the relaxation one is controlled by the change in hardness.