A mechanism describes in detail exactly what takes place at each stage of a chemical transformation. It describes the transition state and which bonds are broken, and in what order, which bonds are formed and in what order, and what the relative rates of the steps are. A complete mechanism must also account for all reactants used, the function of a catalyst, stereochemistry, all products formed and the amount each.
A reaction mechanism must also account for the order in which molecules react. Often what appears to be a single step conversion is in fact a multistep reaction. Consider the following example:
- CO + NO2 → CO2 + NO
In this reaction, it has been experimentally determined that this reaction takes place according to the rate law . Therefore, a possible mechanism by which this reaction takes place 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. All of the elementary steps must add up to the original reaction. There are four types of elementary steps: 1) Addition, 2) Elimination, 3) Substitution and 4) Rearrangement.
When determining the overall rate law for a reaction, the slow step is the step that determines the reaction rate. Because the first step is the slow step, it is the rate-determining step. Because it involves the collision of 2 NO2 molecules, it is a bimolecular reaction with a rate law of . If one were to cancel out all the molecules that appear on both sides of the reaction, you would be left with the original reaction.