Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of reactions by which overall chemical change occurs.

Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism.

An overall description of how a reaction occurs. A mechanism describes in detail exactly what takes place at each stage of a chemical transformation. It describes 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, 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 <math>R = k[NO_2]^2<math>. 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 it’s own rate law and molecularity. All of the elementary steps must add up to the original reaction.

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 <math>R = k[NO_2]^2<math>. 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.