A reaction mechanism describes the step-by-step sequence of elementary steps that lead to an overall chemical change. Each elementary step represents a specific stage in the reaction process at the molecular level. For instance, consider two elementary steps in a reaction mechanism:

Elementary Step 1:
Cl(g) + O₃(g) → ClO(g) + O₂(g)

Elementary Step 2:
ClO(g) + ClO(g) → Cl(g) + ClO₂(g)

When combined, these two elementary steps form the complete reaction mechanism. To derive the overall reaction from this mechanism, it is essential to identify and cancel out the catalyst and reaction intermediates. A catalyst is a substance that appears as a reactant in the first elementary step and as a product in the final step. In this example, Cl is the catalyst.

On the other hand, a reaction intermediate is a compound that is produced in one elementary step and consumed in a subsequent step. In this case, ClO is the reaction intermediate. By canceling out the catalyst and intermediates, the remaining species yield the overall reaction.

For a more complex mechanism with multiple elementary steps, the first step is always the initial step, while the last step represents the final outcome. Understanding these concepts is crucial when analyzing any reaction mechanism, as they help clarify how individual steps contribute to the overall chemical transformation.