Advanced Topic 438 of 1102 Kinetics & Equilibrium
Steady State Approximation
Assumes intermediate concentration remains constant because it forms and reacts at equal rates.
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In-depth explanation of Steady State Approximation
The steady state approximation assumes that the concentration of a reactive intermediate stays nearly constant during most of the reaction, because it is formed and consumed at equal rates. Mathematically this means the net rate of change of the intermediate concentration is set to zero, d[I]/dt = 0, which allows an algebraic expression to be solved for [I]. Substituting this into the rate of the overall mechanism gives a rate law that can be compared with experiment. The approximation is valid when the intermediate is much more reactive than the reactants and products, so its concentration remains small.
Going deeper: Steady State Approximation
Key ideas: Procedure: write the rate of formation and loss of the intermediate, set d[I]/dt = 0, solve for [I], and substitute into the rate of product formation. It works when the intermediate is consumed much faster than it forms, for instance k2 much larger than k1. The result often gives a rate law with a denominator that simplifies at high or low concentration.
Common mistakes and connections: Students apply it to species that build up appreciably, such as a stable intermediate, and so get errors. Another mistake is claiming d[I]/dt = 0 means [I] is zero, when it just means constant and small. It differs from the pre-equilibrium approximation, which needs a fast reversible step. It is widely used for chain reactions, Lindemann theory and enzymes.
Visualization of Steady State Approximation
For A to I to P with fast consumption of I, the intermediate (plotted x5) stays small and nearly constant while A and P change, so d[I]/dt is about zero. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The Michaelis-Menten equation for enzymes is derived by applying the steady state approximation to the enzyme-substrate complex.
Frequently asked questions about Steady State Approximation
What is Steady State Approximation?
Assumes intermediate concentration remains constant because it forms and reacts at equal rates.
Is Steady State Approximation a beginner, intermediate or advanced topic?
Steady State Approximation is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Steady State Approximation?
Study these earlier topics in Kinetics & Equilibrium first: Reaction Mechanism, Rate-Determining Step, Reaction Intermediate.
What comes after Steady State Approximation?
The next topics in Kinetics & Equilibrium are Pre-equilibrium Approximation and Collision Theory.
Can I practise Steady State Approximation with quizzes?
Yes. The interactive lesson includes a self-check question on Steady State Approximation, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Steady State Approximation fits in Kinetics & Equilibrium
Steady State Approximation is topic 19 of 91 in Kinetics & Equilibrium.