# How does the rate determining step affect the observed rate law?

Feb 19, 2017

For the concentration of a reactant to appear in the rate law equation, that substance must appear in the reaction mechanism during or prior to the rate determining step.

#### Explanation:

One of the crucial tests for judging a reaction mechanism as acceptable, is that it account for the rate law. This essentially means that one must see correlation between the coefficients of the reactants that appear in the mechanism and the exponents in the rate law.

Here is a very simple example. Suppose the mechanism of a reaction was

$A + B \rightarrow A B$
$A B + C \rightarrow D + E$

If the rate law is rate=k[A][B], we expect that the first step is the rate-determining step. The reaction rate is independent of [C].

On the other hand, if the rate law is rate=k[A]{B]{C], then we expect the second step to be the rate-determining step.

As a point of clarification, we know the rate law from experimentation. It is what it is. What chemists must do is try to piece together a mechanism that is consistent with the rate law, including a decision on which is the rate-determining step. So, the rate law determines the type of mechanism we judge to be possible, not the other way around!