Reaction Mechanisms A reaction mechanism is a sequence of molecular events, or reaction steps, that defines the pathway from reactants to products.

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Presentation transcript:

Reaction Mechanisms A reaction mechanism is a sequence of molecular events, or reaction steps, that defines the pathway from reactants to products.

Reaction Mechanisms Single steps in a mechanism are called elementary steps (reactions). An elementary step describes the behavior of individual molecules. An overall reaction describes the reaction stoichiometry.

Reaction Mechanisms NO2(g) + CO(g)  NO(g) + CO2(g) Overall 1) NO2(g) + NO2(g)  NO(g) + NO3(g) Elementary 2) NO3(g) + CO(g)  NO2(g) + CO2(g) Elementary The chemical equation for an elementary reaction is a description of an individual molecular event that involves the breaking and/or making of chemical bonds.

Reaction Mechanisms Molecularity: is the number of molecules (or atoms) on the reactant side of the chemical equation. Unimolecular: Single reactant molecule.

Reaction Mechanisms Bimolecular: Two reactant molecules. Termolecular: Three reactant molecules.

Reaction Mechanisms Determine the overall reaction, the reaction intermediates, and the molecularity of each individual elementary step.

Rate Laws and Reaction Mechanisms Rate law for an overall reaction must be determined experimentally. Rate law for elementary step follows from its molecularity.

Rate Laws and Reaction Mechanisms The rate law of each elementary step follows its molecularity. The overall reaction is a sequence of elementary steps called the reaction mechanism. Therefore, the experimentally observed rate law for an overall reaction must depend on the reaction mechanism.

Rate Laws and Reaction Mechanisms The slowest elementary step in a multistep reaction is called the rate-determining step. The overall reaction cannot occur faster than the speed of the rate-determining step. The rate of the overall reaction is therefore determined by the rate of the rate-determining step.

Rate Laws and Reaction Mechanisms

Rate Laws and Reaction Mechanisms The following reaction has a second-order rate law: H2(g) + 2 ICl(g)  I2(g) + 2 HCl(g) Rate = k[H2][ICl] Devise a possible mechanism. The following substitution reaction has a first-order rate law: Co(CN)5(H2O)2–(aq) + I–  Co(CN)5I3–(aq) + H2O(l) Rate = k[Co(CN)5(H2O)2–] Suggest a mechanism in accord with the rate law.