Britannica Sources used in this answer

Why can the steady-state approximation be used for short-lived atoms and free radicals?

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.

The steady-state approximation is a valuable tool in chemical kinetics used to derive approximate rate equations for complex reaction mechanisms.1

This approximation can be applied to short-lived atoms and free radicals because these species are highly reactive and typically exist only at very low concentrations during a reaction. Because they are consumed as rapidly as they are formed, their net rate of change in concentration is negligible throughout the course of the reaction. Consequently, it is a valid and reliable approximation to assume that the rate of change of their concentration is zero, meaning the intermediate exists in a "steady state".1

By setting the rate of change for these intermediates to zero, chemists can simplify complex mathematical equations to obtain a reliable expression for the overall reaction rate in terms of reactant concentrations. This method is generally effective for most mechanisms, provided the reaction is not an explosion, where the concentration of intermediates rises rapidly rather than remaining steady.1