Reaction mechanism
A mechanism is the series of simple steps (elementary steps) that make up an overall reaction. Intermediates form in one step and are used in another. The slowest step, the rate-determining step, controls the overall rate.
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Analogy
A relay race: the slowest runner decides the team's finishing time, however fast the others are.
Example
Step 1 (slow): NO₂ + F₂ → NO₂F + FStep 2 (fast): NO₂ + F → NO₂FRate = k[NO₂][F₂]Intermediate: F
In practice
For 2NO₂ + F₂ → 2NO₂F, the observed rate law is first order in each reactant, which points to a slow first step involving one of each.
Practice
Flashcards
- What is the rate-determining step?The slowest elementary step in a mechanism.
Interview questions
- Describe how a rate-determining step relates to the rate law of a multi-step reaction.
- Overall rate set by the slowest step
- Rate law follows the slow step's reactants
- Intermediates are replaced using earlier fast steps
- Mechanism must match the experiment
Questions people ask
What is the molecularity of an elementary step?
Molecularity is the number of molecules colliding in a single elementary step. One molecule is unimolecular, two is bimolecular, and three is termolecular. For an elementary step only, the rate law can be written directly from the coefficients. For a multi-step reaction, it cannot.
What is an intermediate and how is it different from a catalyst?
An intermediate is made in one step and used up in a later step, so it does not appear in the overall equation. A catalyst is used in an early step and then regenerated, and is present at the start. Both are missing from the overall equation, but a catalyst enters first.