Reaction mechanisms
| English | Chinese | Pinyin |
|---|---|---|
| mechanism | 机理 | jī lǐ |
| rate-determining step | 决速步骤 | jué sù bù zhòu |
| intermediate | 中间体 | zhōng jiān tǐ |
| activation energy | 活化能 | huó huà néng |
The slowest step rules
- Most reactions happen in several steps.
- The slowest step controls the overall rate.
- The rate equation reveals the mechanism 机理.
Organic mechanism route
Trace electron-pair movement from reagent to product.
The slowest step of a reaction mechanism is the rate-______ step.
Only species up to and including it appear in the rate equation.
The rate-determining step 决速步骤
- 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 中间体 is made in one step and used up in a later one — it is not in the overall equation.
The rate-determining step is the slowest step in a multi-step reaction
The rate-determining step is:
The slowest step is the bottleneck, so it determines the overall rate.
Which species appear in the rate equation?
Species after the RDS do not affect the rate, so they are not in the rate equation.
An intermediate is a species that is:
Intermediates appear during the mechanism but cancel out of the overall equation.
Match each mechanism idea.
Each item links the term to its correct meaning.
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 活化能), so the rate rises.
Raising the temperature increases the rate because:
A higher temperature raises k, so the rate increases.
Linking mechanism to rate
- Only species in the rate-determining step (and earlier) appear in the rate equation.
- A proposed mechanism must agree with the experimental rate equation.
You've got it
- 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