Page 197 - Instant notes

Formulation of rate laws 183

Rate determining step

The rate determining step is the slowest reaction in a reaction mechanism and
consequently controls the rate of overall reaction to form products. In the rate law derived
in the preceding section for the oxidation of NO:

the rate of reaction of N 2O 2 with O 2 is assumed to be much slower than the rate of N 2O 2
decomposition (i.e. ). In other words, reaction between N 2O 2 and O 2 is the
rate determining step in this case.
Under these conditions the overall rate of reaction is controlled by the rate constant k 2
and the concentration of O 2. However, if the concentration of O 2 in the reaction mixture
is sufficiently large that the expression for the derived rate law becomes:

and the reaction is now second order in NO only. The concentration of O 2 is no longer
important once [O 2] exceeds a certain value because essentially all molecules of N 2O 2
that form react with O 2 to produce NO 2 before they have a chance to decompose back
into NO. Consequently the rate of reaction is independent of [O 2] and determined solely
by the rate at which N 2O 2 is formed through reaction 1. These two scenarios illustrate that
rate determining step is not necessarily a fixed entity but may switch from one
elementary step to another for different experimental conditions.