Lecture 29-Reaction mechanism - Reaction mechanisms A...

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Reaction mechanisms A reaction mechanism is a step-by-step description of a chemical reaction at molecular levelEach step in a mechanism is called an elementary reaction Reaction mechanisms must agree with the overall stoichiometric equation for the reaction Reaction mechanism should agree with the experimentally determined rate lawsfor the reaction Reaction mechanisms are proposed based on experimental dataIt is very difficult, if not impossible, to infer a reaction mechanism from the overall chemical equation alone
Elementary reactions Elementary reactions are classified as: Unimolecular: A → products (dissociation)Bimolecular: A + B → products (bimolecular collision) Termolecular:A + B + C → products (very rare)For elementary reactions, the order of the reactionwith respect to a given reagent equals the stoichiometric coefficientof the reagent: aA + bB → productsr= k[A]a[B]b
Elementary reactions Some elementary process are reversibleand, therefore, can be treated using the laws of chemical equilibriumalready covered in this course Intermediatesare chemical species produced in one elementary reaction that are consumed in another elementary reaction Intermediates should not appear in the overall stoichiometric equation nor in the final reaction rate law The rate determining stepis the slowest elementary reaction in the mechanism; it may determine the rate of the overall reaction

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