Intermediate Topic 437 of 1102 Kinetics & Equilibrium
Reaction Intermediate
A species formed in one step and consumed in a later step, not appearing in the overall equation.
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In-depth explanation of Reaction Intermediate
A reaction intermediate is a species that is formed in one elementary step and consumed in a later step, so it appears in the mechanism but not in the overall balanced equation. Intermediates are usually highly reactive and short-lived, and their concentrations stay very low throughout the reaction. They generally do not appear in the experimental rate law, although some can be detected by spectroscopy or trapped chemically. Intermediates differ from catalysts: a catalyst is consumed in one step and regenerated in a later step, whereas an intermediate is produced and then consumed. Recognizing intermediates is essential for constructing and testing mechanisms.
Going deeper: Reaction Intermediate
Key ideas: Intermediates appear on the product side of one step and the reactant side of another and cancel in the overall equation. On an energy diagram they sit in a local minimum between two transition states, with a finite although tiny lifetime. They can sometimes be observed by low temperature spectroscopy or trapped by a scavenger.
Common mistakes and connections: Students confuse intermediates with transition states, which sit at maxima and cannot be isolated, and with catalysts, which are consumed first. Also, intermediates must never appear in the final rate law, so they must be replaced using the steady state or pre-equilibrium. The concept links to the Hammond postulate and to radical chain mechanisms.
Visualization of Reaction Intermediate
An intermediate sits in an energy valley between two transition states: made in one step, consumed in the next, and absent from the overall equation. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In the chain mechanism for H2 + Br2 -> 2HBr, the bromine atom and the hydrogen atom are intermediates, each formed in one step and consumed in the next.
Frequently asked questions about Reaction Intermediate
What is Reaction Intermediate?
A species formed in one step and consumed in a later step, not appearing in the overall equation.
Is Reaction Intermediate a beginner, intermediate or advanced topic?
Reaction Intermediate is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Reaction Intermediate?
Study these earlier topics in Kinetics & Equilibrium first: Elementary Reactions, Reaction Mechanism, Rate-Determining Step.
What comes after Reaction Intermediate?
The next topics in Kinetics & Equilibrium are Steady State Approximation and Pre-equilibrium Approximation.
Can I practise Reaction Intermediate with quizzes?
Yes. The interactive lesson includes a self-check question on Reaction Intermediate, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Reaction Intermediate fits in Kinetics & Equilibrium
Reaction Intermediate is topic 18 of 91 in Kinetics & Equilibrium.