Activation

For the reaction to occur, the colliding reactant molecules must possess a certain minimum kinetic energy. This minimum kinetic energy is the activation energy. The reaction would occur only if the colliding molecules possess kinetic energies equal to or greater than the activation energy.

The energy in question comes from the thermal motion of the molecules themselves. A pair of molecules that collides with less than the activation energy simply rebounds unchanged — the collision is elastic, and no bonds are broken or formed. Only the energetic minority of collisions can push the reacting atoms up and over the energy barrie …