Reaction MechanismsStudies 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.

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1.5 Reaction Mechanisms and Catalysis

Intermediates and CatalystsIn any multi-step reaction mechanism the first collision, or decomposition of anenergized reactant, always produces some product that is consumed in a later step ofthe mechanism. Such a species is called a reaction intermediate.

CatalystA catalyst functions by “providing an alternate, lower-energy pathway” for a reaction to follow

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1.5 Reaction Mechanisms and Catalysis

Species Identification

Rate Determining StepThe 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

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1.5 Reaction Mechanisms and Catalysis

Reaction Mechanisms

Overall Activation Energy

Example: Notice that Figure 1.5.3 has three “peaks” corresponding to the three steps in the catalyzed reaction mechanism. The rate-determining step is the second one. This stepwould also determine Ea for the overall reaction.

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1.5 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 reactantsExample: Catalytic Converter

Catalysts which exist in the same phase as the rest of the reaction system, aresometimes called homogeneous catalysts.Example: metalloenzymes

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1.5 Reaction Mechanisms and Catalysis

Catalysts