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. This is called the rate-determining step of the reaction. This step governs the overall rate law for the overall reaction. 14.6 Catalysis catalyst homogeneous catalysis heterogeneous catalysis exists in a different phase than the reactants.