StellarA reaction mechanism is the step-by-step sequence that shows how an overall chemical reaction actually happens. While an overall balanced equation tells you the starting substances and final products, it does not show the path the reaction follows. Many reactions do not occur in one single collision. Instead, they take place through a series of smaller elementary steps, each describing one molecular event. When all of these elementary steps are added together, they must reproduce the overall reaction.
Reaction mechanisms are important because they explain why a reaction has a certain rate law and how reactants are converted into products at the particle level. In a multistep mechanism, some substances may be formed in one step and used up in a later step. These are called intermediates. Intermediates do not appear in the final overall reaction because they cancel out when all the steps are added together. A mechanism may also include a rate-determining step, which is the slowest step and often controls the overall reaction rate.
One possible mechanism is
Here, NO_3 is an intermediate.
Example 2: Suppose a reaction occurs in two steps:
The species C is produced and then consumed, so it is an intermediate and should not appear in the final overall reaction.