An Aside Some Basic Ideas on Reaction Kinetics

1 An Aside Some Basic Ideas on Reaction Kinetics 

Most photochemical reactions are composite reactions that is, they can be regarded as being made up of several elementary steps, constituting the mechanism, each of which involves a simple number of species. The 

A valid mechanism for a reaction should be based on sound experimental evidence acquired through a study of the way in which the various intermediates involved affect the kinetics of the reaction. Experimentally-determined rate laws provide us with a means of classifying reactions according to their kinetics. For example, for a reaction X + Y — Z, if the experimentally-determined rate law allows us to determine the order of the reaction  

The reaction is first-order with respect to X, second-order with respect to Y and third-order overall k is the rate constant. Note that the order of a reaction does not necessarily have to have integral values. 

Simplification of a rate law can be achieved by use of the isolation method, in which all reactants except one are present in large excess. If a reactant is present in large excess, its concentration may be considered to be constant throughout the course of the reaction. For example, for the rate law  

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