Intermediate Topic 435 of 1102 Kinetics & Equilibrium
Reaction Mechanism
The sequence of elementary steps that make up an overall reaction.
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In-depth explanation of Reaction Mechanism
A reaction mechanism is the sequence of elementary steps that together describe how an overall reaction occurs at the molecular level. A valid mechanism must add up to the balanced overall equation, and it must predict a rate law that matches experimental observations. Mechanisms include reactive intermediates, which are formed in one step and consumed in another, and sometimes catalysts. Several mechanisms can often be consistent with the same data, so a mechanism can be supported by evidence but never proven with certainty. Proposed mechanisms are tested by further experiments such as isotope labelling and spectroscopic detection of intermediates.
Going deeper: Reaction Mechanism
Key ideas: To test a mechanism, add the steps and cancel intermediates to recover the overall equation, then derive the rate law from the slowest step, using steady state or pre-equilibrium to remove intermediates. Compare predicted orders with experiment. Rate laws with fractional orders often point to chain mechanisms.
Common mistakes and connections: Students think that a mechanism that fits the rate law is proven; it is only consistent. Another mistake is including intermediates in the final rate law or ignoring the possibility of a fast step before the slow one. A third is writing a termolecular step when two sequential bimolecular steps are more probable. Related tools are isotope labelling and detecting intermediates.
Visualization of Reaction Mechanism
A mechanism is a chain of elementary steps; each barrier is one step and the valley between them is an intermediate. The steps must add up to the overall reaction. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The reaction H2 + I2 -> 2HI was long treated as a simple one-step process, but detailed studies showed that its mechanism is more involved.
Frequently asked questions about Reaction Mechanism
What is Reaction Mechanism?
The sequence of elementary steps that make up an overall reaction.
Is Reaction Mechanism a beginner, intermediate or advanced topic?
Reaction Mechanism is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Reaction Mechanism?
Study these earlier topics in Kinetics & Equilibrium first: Integrated Rate Laws, Molecularity, Elementary Reactions.
What comes after Reaction Mechanism?
The next topics in Kinetics & Equilibrium are Rate-Determining Step and Reaction Intermediate.
Can I practise Reaction Mechanism with quizzes?
Yes. The interactive lesson includes a self-check question on Reaction Mechanism, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Reaction Mechanism fits in Kinetics & Equilibrium
Reaction Mechanism is topic 16 of 91 in Kinetics & Equilibrium.