A chemical reaction to occur, the reactions need to resolve obstacles for activation. The activation barrier is the sum of the energy needed to get the reaction going. An activation barrier is often considered canonically because during the reaction the molecule must climb over a hill. Since chemical reactions generically exhibit energetic barriers, they will generically possess barrier-type resonances.
Activation energy & Effective collision between reactant molecules are the two barriers for a chemical reaction to occur.
Activation energy – In chemistry, activation energy is the minimum amount of energy required to activate atoms or molecules to a condition in which they can undergo a chemical or physical transformation.