Reaction Mechanisms: The Steady-State Approximation

Many reactions proceed through multiple elementary steps involving reactive intermediates. Adding these steps gives the overall reaction without intermediates.

Consider a reaction where reactant, R, forms product, P, through an intermediate, I.

Here, the first step is fast and reversible. The second step is a slow and rate-determining step that controls the overall rate.

Initially, the concentration of I rises quickly. It reaches a small maximum, then decreases and stabilizes at a low, nearly constant value. So, its concentration remains negligible compared to R and P.

Note that I is formed during the first step. However, it is consumed during the reverse of the first step and the second step of the reaction.

The steady-state approximation assumes that the concentration of I remains constant. So, the rate of formation of I equals its rate of consumption.

Solving for I and substituting it into the overall rate law yields the final rate expression with no intermediates. This rate law is useful for reactions that proceed through multiple elementary steps.