Energy Profiles for Many Steps
Introduced| English | Meaning |
|---|---|
| intermediate/ˌɪntəˈmiːdɪət/ | A species formed in one step of a reaction mechanism and consumed in a later step. |
Measure each barrier from its own starting valley
- A two-step profile has reactants at 20, first peak at 80, an intermediate at −20, second peak at 60 and products at 0, all in kJ per mole.
- The forward barriers are 80−20=60 and 60−(−20)=80. The second peak is lower on the page, but the second step has the larger local barrier.
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. Compare the rise from each starting valley to its following peak.
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?
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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...
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Intermediates rest in the dips between peaks.
The highest barrier rules
- Each activation energy is a difference: transition-state energy minus the energy at the start of that step. In the simplified comparison used here, the larger local barrier identifies the slower step.
- Absolute peak height alone cannot identify that barrier.
Under the simplified barrier comparison, which step is expected to be slower?
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Compare transition state minus the starting state for each step. Absolute peak height is insufficient.
The rate-determining step is always the first step.
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Compare each local forward barrier. Neither first position nor highest absolute peak automatically identifies the slow step.
A two-step diagram has reactants at 20, peaks at 80 and 60, an intermediate at −20 and products at 0, all in $\dfrac{\text{kJ}}{\text{mol}}$.
- Step 1: $E_{a,1}=80-20=60$. Step 2: $E_{a,2}=60-(-20)=80$, in $\dfrac{\text{kJ}}{\text{mol}}$.
- The lower second peak has the larger local barrier, so step 2 is slower under the simplified barrier comparison.
Read a two-hump energy profile
Identify each feature on the energy profile of a two-step reaction.
Transition states are in the valleys of an energy profile, and intermediates are at the peaks.
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False. It is the other way round: transition states are at the peaks and intermediates sit in the valleys between them.
The overall $\Delta H$ depends only on the reactant and product energies, not the peaks.
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$\Delta H$ 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. Measure each forward barrier from its own starting valley; do not select a step from the highest absolute peak alone. And the overall $\Delta H$ is still just products minus reactants, ignoring the humps in between.
Carry the reasoning to a new case
- The overall enthalpy change is 0−20=−20 kJ per mole.
- Under the usual simplified barrier comparison, the second step is slower; actual mechanism rates also depend on concentrations and kinetic factors.
Using the profile values, calculate the second forward activation energy in kJ per mole.
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Measure from the intermediate at −20 to the second transition state at 60: 60−(−20)=80.
From reactants at 20 to products at 0, calculate the overall enthalpy change in kJ per mole.
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Products minus reactants: 0−20=−20. Peak heights do not enter this subtraction.
A multi-step energy profile has one peak per step and an intermediate resting in each valley between them. Compare local activation barriers, measured from each step's starting state; a lower absolute peak may have a larger barrier. The overall $\Delta H$ is still just the difference between the two ends.