In fact, only a small fraction of collisions lead to products. In order for reaction to occur the reactant molecules must collide in the correct orientation and with enough energy to form products. The Arrhenius Equation A is called the frequency factor 14.5 Reaction Mechanisms reaction mechanism Elementary steps molecularity Unimolecular Bimolecular Termolecular intermediates . The rate laws of the elementary steps determine the overall rate law of the reaction. The rate law of an elementary step is determined by its molecularity.
Unimolecular processes are first order.
Bimolecular processes are second order. Termolecular processes are third order. Most reactions occur by mechanisms with more than one elementary step. Often one step is much slower than the others. The slow step limits the overall reaction rate.