What distinguishes nonelementary reactions from elementary reactions?

Nonelementary reactions are characterized by a lack of direct correspondence between the rate law and the stoichiometric equation. This is because these reactions often involve multiple steps or mechanisms, meaning that the overall reaction can encompass intermediate species that do not appear in the final balanced equation. Consequently, the observed rate law can be more complicated and does not directly reflect the stoichiometric coefficients of the overall reaction, leading to a scenario where the reaction rate is influenced by factors like reaction intermediates and the specific mechanism, rather than simply the number of molecules of each reactant involved.

Conversely, elementary reactions feature a direct relationship between their stoichiometric equation and their rate law, where the exponents in the rate law correspond to the coefficients in the balanced equation, reflecting a single step process. Thus, this fundamental contrast underscores the complexity of nonelementary reactions in comparison to the simplicity of elementary reactions.