Britannica Sources used in this answer

How can kinetic measurements show whether a reaction occurs in one step or several steps?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A graph showing a potential energy curve for a chemical reaction, with the y-axis labeled potential energy and the x-axis labeled progress of reaction. The curve rises from the initial state (reactants) to a peak labeled transition state (activated complex), indicating activation energy, then drops to the final state (products). — chemical kinetics In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy required to transform reactants into products in a chemical reaction. On a potential energy curve, the value of the activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (known as the activated complex, or transition state) and particles of reactants in their initial state. The activation energy thus can be visualized as a barrier that must be overcome by reactants before products can be formed.

In chemical kinetics, scientists determine whether a reaction occurs in a single step (an elementary reaction) or multiple steps (a composite or complex reaction) by analyzing how the reaction rate depends on experimental conditions, such as reactant concentrations.12

Key methods for identifying these mechanisms include:

Kinetic Law Discrepancies: If the observed kinetic equation (the rate law) does not correspond to the stoichiometry of the balanced chemical equation, it is a strong indication that the reaction proceeds through a stepwise mechanism. For example, if a reaction's rate is proportional to concentrations in a way that differs from the balanced equation, it suggests a sequence of elementary steps rather than a single-step process.23
Detection of Intermediates: The identification of reaction intermediates—such as atoms, free radicals, or other unstable species—provides evidence for a composite mechanism. These can often be detected using spectroscopy.3
Mathematical Modeling: Researchers propose mechanisms and derive rate equations for each step. By applying approximations like the steady-state treatment for low-concentration intermediates, they compare the theoretical rate to experimental data. If the results align, the mechanism is accepted, though it can never be established with absolute certainty.13