Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A multistep energy profile is the profiles of the elementary steps joined end to end.
- Each peak is a transition state. There is one peak for every elementary step.
- Each valley between peaks is an intermediate. It is a real species that lasts long enough to react again.
- The curve starts at the overall reactants and ends at the overall products.
- The axes are energy (kJ mol⁻¹) and reaction coordinate. The reaction coordinate is not time.
Key relationships
| Quantity | How to read it |
|---|---|
| Number of steps | Number of peaks |
| Number of intermediates | Valleys between peaks (steps − 1 for steps in a chain) |
| Ea of a step | Its peak − the valley where the step starts |
| ΔH of a step | Energy at its end − energy at its start |
| Overall ΔH | Products − reactants = sum of the steps’ ΔH |
| Reverse Ea of a step | Its peak − the valley where the step ends = Ea − ΔH |
| Rate-limiting step | The step with the largest Ea |
Shapes to recognise
- Slow first step: the first climb is the largest; the rate law comes from step 1.
- Fast pre-equilibrium, then slow step: a low first hump, an intermediate that can easily slide back, then a much larger climb (Topic 5.9).
Assumptions
- Every hump is one elementary step.
- A larger barrier means a smaller fraction of collisions succeed, so that step is slower.
Mistakes to avoid
- Counting valleys as steps. Count peaks.
- Measuring every Ea from the reactants. Use each step’s own starting valley.
- Calling a transition state an intermediate. Peaks are transition states; valleys are intermediates.
- “Biggest drop = slowest step.” A drop is ΔH, not a barrier.
- Changing overall ΔH when the route changes. It depends only on reactants and products.
Quick self-check
A two-step reaction has step 1: Ea = 40, ΔH = −10; step 2: Ea = 70, ΔH = +25 (kJ mol⁻¹). Reactants are at 0.
- What is the energy of the intermediate? (−10 kJ mol⁻¹)
- What is the energy of TS 2? (−10 + 70 = 60 kJ mol⁻¹)
- What is the overall ΔH? (+15 kJ mol⁻¹)
- Which step is rate-limiting? (Step 2: its barrier, 70, is larger)
Next: practice questions.