Upon completion of this module, you should be able to define activation energy, explain the difference between effective and ineffective particle collisions, and use the Arrhenius equation to calculate a rate constant. The activation energy (Ea) is the minimum amount of energy needed to initiate a chemical reaction. In a chemical reaction, reactant particles need to collide with enough activation energy to form the products. Reactions are typically carried at a particular temperature. At a particular temperature reactant particles have average kinetic energy.