Steady-State Approximation
The assumption that the concentration of a reactive intermediate stays essentially constant because it is consumed as fast as it is formed.
Formula / equation
d[intermediate]/dt ≈ 0
Unit
mol/L
In depth
Setting the net rate of change of a short-lived intermediate to zero converts a set of differential equations into simple algebra, allowing a rate law to be derived from a proposed mechanism. The approximation is valid when the intermediate is highly reactive and never accumulates.
Examples in the real world
Deriving the rate law for the decomposition of N₂O₅ by assuming a steady concentration of NO₃ radicals.