Reaction Mechanism - Examples

Examples

Consider the following reaction:

CO + NO₂ → CO₂ + NO

In this case, it has been experimentally determined that this reaction takes place according to the rate law . This form suggests that the rate-determining step is a reaction between two molecules of NO₂. A possible mechanism for the overall reaction which explains the rate law is:

2 NO₂ → NO₃ + NO (slow)

NO₃ + CO → NO₂ + CO₂ (fast)

Each step is called an elementary step, and each has its own rate law and molecularity. The elementary steps should add up to the original reaction.

When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step (in the above reaction) is the slowest step, it is the rate-determining step. Because it involves the collision of two NO₂ molecules, it is a bimolecular reaction with a rate law of . If we were to cancel out all the molecules that appear on both sides of the reaction, we would be left with the original reaction.

Other reactions may have mechanisms of several consecutive steps. In organic chemistry, one of the first reaction mechanisms proposed was that for the benzoin condensation, put forward in 1903 by A. J. Lapworth.

There are also more complex mechanisms such as chain reactions, in which the propagation steps of the chain form a closed cycle.