The Sequence of Steps
Introduced| English |
|---|
| rate-determining step/reɪt dɪˈtɜːmɪnɪŋ step/ |
The slowest link sets the pace
- A chain of tasks is only as fast as its slowest one.
- A reaction's steps are the same -- one drags its feet.
- That slowpoke controls how fast the whole thing goes.
- Speed up the fast steps and nothing changes.
The rate-determining step
- The rate-determining step 决速步 is the slowest elementary step.
- It is the bottleneck of the whole mechanism.
- The overall rate can go no faster than this step.
The rate-determining step is the...
The slowest step is the bottleneck controlling the overall rate.
The overall reaction can go no faster than its ____ step.
The slowest (rate-determining) step caps the overall rate.
The rate law follows it
- The overall rate law comes from the slowest step's reactants.
- A step's own coefficients are its orders, unlike the overall reaction.
- So the mechanism predicts the experimental rate law.
For a slow step $A + A \to C$, the rate law is...
Two A's collide in the slow step, giving second order in A.
For an elementary step, the coefficients can be used as the reaction orders.
This is only valid for elementary steps, not overall reactions.
Fast steps barely matter
- Steps after the slow one do not affect the rate.
- Speeding up a fast step leaves the overall rate unchanged.
- Only the bottleneck matters.
The rate-determining step
Follow a two-step mechanism and spot the slow step that controls the rate.
Speeding up a fast step that is not the bottleneck changes the overall rate.
Only the slowest step controls the rate.
Slow step: $A + A \to C$. Fast step: $C + B \to D$.
- The slow step's reactants are two A's.
- So $\text{rate} = k[A]^2$, set entirely by the slow step.
The experimental rate law is best predicted from...
The slowest step's reactants set the rate law.
The rate law comes from the rate-determining (slowest) step, not from the overall balanced equation. A step's coefficients give its orders only because it is elementary. And speeding up a step that is not the bottleneck will not speed the reaction.
The rate-determining step is the slowest elementary step, and it sets the overall rate and rate law. Because it is elementary, its coefficients are its orders. Faster steps before or after it do not change the pace -- only the bottleneck does.