The concept of the rate-determining step is very important to the optimization and understanding of many chemical processes such as catalysis and combustion.
Such a situation in which an intermediate (here NO3) forms an equilibrium with reactants prior to the rate-determining step is described as a pre-equilibrium[6] For the reaction of NO2 and CO, this hypothesis can be rejected, since it implies a rate equation that disagrees with experiment.
If the first step were at equilibrium, then its equilibrium constant expression permits calculation of the concentration of the intermediate NO3 in terms of more stable (and more easily measured) reactant and product species: The overall reaction rate would then be which disagrees with the experimental rate law given above, and so disproves the hypothesis that the second step is rate-determining for this reaction.
However, some other reactions are believed to involve rapid pre-equilibria prior to the rate-determining step, as shown below.
A useful rule in the determination of mechanism is that the concentration factors in the rate law indicate the composition and charge of the activated complex or transition state.
[7] The observed rate law is which implies an activated complex in which the reactants lose 2H+ + Cl− before the rate-determining step.
One possible mechanism in which the preliminary steps are assumed to be rapid pre-equilibria occurring prior to the transition state is[7] In a multistep reaction, the rate-determining step does not necessarily correspond to the highest Gibbs energy on the reaction coordinate diagram.
[8] In the previous examples the rate determining step was one of the sequential chemical reactions leading to a product.
This case is referred to as diffusion control and, in general, occurs when the formation of product from the activated complex is very rapid and thus the provision of the supply of reactants is rate-determining.