Deducing the rate law from a postulated mechanism

An early objective in a mechanistic investigation is to establish the rate law (see Chapter 3) which is an algebraic equation describing the instantaneous dependence of the rate on concentrations of compounds or other properties proportional to concentrations (e.g. partial pressures). Rate laws cannot be rehably deduced from the stoichiometry of the overall balanced chemical equation-they have to be determined experimentally. The functional dependence of rates on concentrations maybe simple or complicated, and concentrations may be of reactants, products or even materials not appearing in the overall chemical equation, as in the case of catalysis (see Chapters 11 and 12) [3-7]. 

A credible mechanism and the rate law deduced from it must obey the following three general requirements. 

Sometimes, a complex mechanistic rate law is obtained which is not obviously in agreement with the experimental finding often, consideration of the experimental conditions and/or probable relative magnitudes of rate constants of individual steps will allow simplification of the mechanistic rate law, which leads to correspondence with what is observed experimentally. 

Of course, a mechanistic rate law which corresponds to the one determined experimentally (i.e. has exactly the same form) indicates no more than that the mechanism is not wrong - it is insufficient evidence that the mechanism is correct. Commonly, more than one mechanism is consistent with the observed rate equation, and further experimental work is required to allow rejection of the wrong ones. And, although only the overall chemical change is usually directly observed for most chemical reactions, kinetic experiments can sometimes be designed to detect reaction intermediates (see Chapter 9), and the possible sequence of steps in the overall proposed mechanism [3-7]. 

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