Why do some reactions not follow simple rate laws?

Some reactions do not follow simple rate laws due to the complexity of their reaction mechanisms.

Simple rate laws are typically applicable to elementary reactions, which are reactions that occur in a single step. However, many reactions in chemistry are not elementary but are instead complex reactions that occur in multiple steps. These reactions have a reaction mechanism, which is a series of elementary reactions that add up to the overall reaction. The rate law for a complex reaction is determined by its slowest step, also known as the rate-determining step. Therefore, the rate law for a complex reaction may not be directly related to the stoichiometry of the overall reaction, causing it not to follow a simple rate law.

Furthermore, some reactions involve intermediate species, which are formed in one step of the reaction and consumed in a subsequent step. These intermediates can complicate the rate law, as they may not appear in the overall balanced equation for the reaction, but they do influence the rate of the reaction.

In addition, some reactions may not follow simple rate laws due to the presence of a catalyst. Catalysts speed up reactions by providing an alternative reaction pathway with a lower activation energy. This can change the rate law of the reaction, as the rate may now depend on the concentration of the catalyst as well as the reactants.

Lastly, reversible reactions, where products can react to form reactants, also do not follow simple rate laws. The rate of a reversible reaction depends on the concentrations of both the reactants and the products, and the rate law must take into account the forward and reverse reaction rates.

In conclusion, the complexity of reaction mechanisms, the presence of intermediate species or catalysts, and the reversibility of reactions can all cause reactions not to follow simple rate laws.