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?
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
- 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?
Compare transition state minus the starting state for each step. Absolute peak height is insufficient.
The rate-determining step is always the first step.
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 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. 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.
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.
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.