The observed rate equation (or rate expression) is:
As for many reactions, the rate equation does not simply reflect the stoichiometric coefficients in the overall reaction: It is third order overall: first order in H2and second order in NO, although the stoichiometric coefficients of both reactants are equal to 2.
In chemical kinetics, the overall reaction rate is often explained using a mechanism consisting of a number of elementary steps. Not all of these steps affect the rate of reaction; normally the slowest elementary step controls the reaction rate. For this example, a possible mechanism is:
1. (fast equilibrium)
2. (slow)
3. (fast)
Reactions 1 and 3 are very rapid compared to the second, so the slow reaction 2 is the rate determining step. This is a bimolecular elementary reaction whose rate is given by the second order equation : , where k2 is the rate constant for the second step.
However N2O2 is an unstable intermediate whose concentration is determined by the fact that the first step is in equilibrium, so that : , where K1 is the equilibrium constant of the first step. Substitution of this equation in the previous equation leads to a rate equation expressed in terms of the original reactants
This agrees with the form of the observed rate equation if it is assumed that . In practice the rate equation is used to suggest possible mechanisms which predict a rate equation in agreement with experiment.