Reaction mechanisms
Reaction mechanisms
- Most reactions happen in several steps.
- The slowest step controls the overall rate.
- The rate equation reveals the mechanism.
The rate-determining step
- The slowest step is the rate-determining step (RDS) — it controls the overall rate.
- Only the species involved up to and including the RDS appear in the rate equation.
- An intermediate is made in one step and used up in a later one — it is not in the overall equation.
Practice
The rate-determining step is:
The slowest step is the bottleneck, so it determines the overall rate.
Practice
Which species appear in the rate equation?
Species after the RDS do not affect the rate, so they are not in the rate equation.
Practice
An intermediate is a species that is:
Intermediates appear during the mechanism but cancel out of the overall equation.
Deducing the mechanism, and temperature
- Suggest a mechanism that fits both the rate equation and the overall equation.
- From initial-rate data you find the rate equation, and from that the mechanism.
- Raising the temperature increases the rate constant $k$ (more molecules pass the activation energy), so the rate rises.
Practice
Raising the temperature increases the rate because:
A higher temperature raises k, so the rate increases.
You've got it
Key idea
- the rate-determining step is the slowest step; it controls the rate
- only species up to and including the RDS appear in the rate equation
- an intermediate is made then used up (not in the overall equation)
- higher temperature raises k and so the rate