Define the rate-determining step.

The rate-determining step is the slowest step in a multi-step reaction mechanism, which controls the overall rate.

Why does the slowest step control the overall rate?

The whole sequence can proceed no faster than its slowest stage, just as a queue moves at the pace of its narrowest point.

Which species appear in the rate equation of a multi-step reaction?

Only the species involved in, or before, the rate-determining step appear in the rate equation.

What does it mean if a reactant in the overall equation is absent from the rate equation?

That reactant takes part only in a fast step after the rate-determining step, so its concentration does not affect the overall rate.

How is the order with respect to a species related to the rate-determining step?

The order equals the number of particles of that species involved in the rate-determining step, or in steps up to and including it.

A reaction has rate = k[A]²[B]. How many particles take part in the rate-determining step?

Three particles are involved in the steps up to and including the rate-determining step: two of A and one of B.

OCR A-Level Chemistry B H433

Development of Practical Skills in Chemistry

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Elements of Life

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The Chemical Industry

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The Chemical Industry

Rate-determining step and mechanism

4 things to know
Components 01, 02 and 03

Overview

Recall questions on what the rate-determining step is, what the orders imply about which species take part, and testing a proposed mechanism.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What is the rate-determining step, how many particles take part up to it when rate = k[A]²[B], and which species are involved in it when A + 2B + C → products has rate = k[A][B]?

Process

A reaction is found to be zero order with respect to one reactant. What does this reveal about the mechanism?

Process

Why is a step involving three particles colliding at once considered unlikely?

Process

Can kinetic evidence ever prove a mechanism is correct?

Questions from this subtopic

Each one with the model answer underneath.

1
Define the rate-determining step.
Model answer
The rate-determining step is the slowest step in a multi-step reaction mechanism, which controls the overall rate.
2
Why does the slowest step control the overall rate?
Model answer
The whole sequence can proceed no faster than its slowest stage, just as a queue moves at the pace of its narrowest point.
3
Which species appear in the rate equation of a multi-step reaction?
Model answer
Only the species involved in, or before, the rate-determining step appear in the rate equation.
4
What does it mean if a reactant in the overall equation is absent from the rate equation?
Model answer
That reactant takes part only in a fast step after the rate-determining step, so its concentration does not affect the overall rate.
5
How is the order with respect to a species related to the rate-determining step?
Model answer
The order equals the number of particles of that species involved in the rate-determining step, or in steps up to and including it.
6
A reaction has rate = k[A]²[B]. How many particles take part in the rate-determining step?
Model answer
Three particles are involved in the steps up to and including the rate-determining step: two of A and one of B.

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