Reaction Mechanisms Studies have shown that most reactions occur by a series of steps called the reaction mechanism. The individual steps in such a series are called elementary processes. These steps are themselves individual balanced chemical reactions that may be added together algebraically to give the overall balanced chemical equation. Text Page Reaction Mechanisms and Catalysis

Intermediates and Catalysts In any multi-step reaction mechanism the first collision, or decomposition of an energized reactant, always produces some product that is consumed in a later step of the mechanism. Such a species is called a reaction intermediate. Catalyst A catalyst functions by “providing an alternate, lower-energy pathway” for a reaction to follow Text Pages Reaction Mechanisms and Catalysis Species Identification

Rate Determining Step The overall reaction rate cannot exceed the rate of the slowest elementary process. Because this slow step limits the overall reaction rate it is called the rate-determining step. A variety of conditions may cause a particular step to limit the overall reaction rate. These include the step having: complex collision geometry a high activation energy low concentrations of reactants a termolecular collision Text Pages Reaction Mechanisms and Catalysis Reaction Mechanisms

Overall Activation Energy Example: Notice that Figure has three “peaks” corresponding to the three steps in the catalyzed reaction mechanism. The rate-determining step is the second one. This step would also determine Ea for the overall reaction. Text Pages Reaction Mechanisms and Catalysis Potential Energy Diagrams

Heterogeneous and Homogeneous Catalysts Heterogeneous catalysts are those in which reactions are limited to their surface area only. The form of catalysis in which the phase of the catalyst differs from that of the reactants Example: Catalytic Converter Catalysts which exist in the same phase as the rest of the reaction system, are sometimes called homogeneous catalysts. Example: metalloenzymes Text Pages Reaction Mechanisms and Catalysis Catalysts