Define activation energy.

Activation energy is defined as the minimum energy required for a chemical reaction to occur. It represents the energy barrier that reactants must overcome for the reaction to proceed. In the context of collision theory, not all collisions between particles result in a reaction because many do not have sufficient energy to surpass this activation energy threshold. Therefore, only a fraction of the total collisions lead to successful reactions, which is crucial for understanding reaction rates and the influence of temperature and catalysts on these rates.