Energy Profiles for Many Steps
| English | Chinese | Pinyin |
|---|---|---|
| intermediate | 中间体 | zhōng jiān tǐ |
A mountain range, not a single hill
- A one-step reaction has a single hump to climb.
- A multi-step one has a whole ridge of peaks.
- Each peak is a step; each dip is an intermediate resting.
- The tallest peak decides the pace.
One hump per step
- A multi-step reaction shows one peak for each elementary step.
- Between the peaks lie small valleys.
- More steps means more humps.
A three-step reaction shows how many peaks on its energy profile?
One peak per elementary step, so three peaks.
Intermediates rest in the dips
- Each valley between two peaks holds an intermediate 中间体.
- It is more stable than the peaks around it, but not the final product.
- It is briefly formed, then consumed by the next step.
An intermediate appears on the profile as a...
Intermediates rest in the dips between peaks.
The highest barrier rules
- The tallest peak is the biggest activation energy.
- That step is the rate-determining, slowest one.
- It sets how fast the whole reaction can go.
The rate-determining step corresponds to the...
The highest barrier is the slowest, rate-determining step.
The rate-determining step is always the first step.
It is whichever step has the tallest peak, not necessarily the first.
A two-step diagram has peaks of $80\ \text{kJ}$ and $50\ \text{kJ}$.
- The $80\ \text{kJ}$ peak is the higher barrier.
- So that step, with the tall peak, is rate-determining.
Read a two-hump energy profile
Identify each feature on the energy profile of a two-step reaction.
Transition states are at the peaks, not in the valleys.
Peaks are transition states; valleys are intermediates.
The overall $\Delta H$ depends only on the reactant and product energies, not the peaks.
$\Delta H$ is products minus reactants, ignoring the barriers.
The intermediate sits in a valley (a dip), while the transition states sit at the peaks -- do not mix them up. The rate-determining step is the one with the tallest peak, not necessarily the first. And the overall $\Delta H$ is still just products minus reactants, ignoring the humps in between.
A multi-step energy profile has one peak per step and an intermediate resting in each valley between them. The tallest peak is the largest activation energy, marking the rate-determining step. The overall $\Delta H$ is still just the difference between the two ends.