Short on time? This page is the recap. For the labelled figure and worked examples, use the full study guide.
Recap
- In an elementary step, some bonds break and new bonds form, at overlapping times.
- The reaction coordinate (x-axis) tracks progress through that rearrangement. It is not time.
- The energy profile goes from the reactant level, up to one peak, the transition state, then down to the product level.
- The transition state is the highest-energy arrangement, with partial bonds. It cannot be isolated.
- Activation energy is the energy needed to get from reactants to the transition state. Rate depends on the fraction of collisions that can reach it.
Key relationships
| Quantity | How to read it | Sign |
|---|---|---|
| Eₐ(forward) | E(transition state) − E(reactants) | always positive |
| Eₐ(reverse) | E(transition state) − E(products) | always positive |
| ΔH (overall energy change) | E(products) − E(reactants) | negative: exothermic; positive: endothermic |
| Link | Eₐ(reverse) = Eₐ(forward) − ΔH | |
| Arrhenius equation | k = A e^(−Eₐ/RT) | qualitative use only |
In the Arrhenius equation, e^(−Eₐ/RT) is linked to the fraction of collisions that can reach the transition state; A holds collision frequency and orientation. T is in kelvin.
Assumptions
- A single elementary step has one peak and no dips between reactants and products.
- The profile is a property of the reaction: it does not change with temperature or concentration.
- Only energy differences matter; the zero on the energy axis is arbitrary.
Mistakes to avoid
- Reading Eₐ as the peak’s axis value instead of the gap from reactants.
- Linking ΔH to speed. An exothermic step can be slow.
- Calling the transition state an intermediate.
- Drawing a lower peak at higher temperature. The peak is fixed.
- Getting the sign of ΔH backwards. Products below reactants: ΔH negative.
- Drawing two humps for one elementary step.
Quick self-check
- An elementary step has Eₐ(forward) = 75 kJ mol⁻¹ and ΔH = −30 kJ mol⁻¹. What is Eₐ(reverse)? (105 kJ mol⁻¹)
- Where on the profile is the transition state, and what are its bonds like? (At the single highest point; old bonds partly broken, new bonds partly formed)
- Two steps have similar A. Which one’s rate rises by the larger factor when heated? (The one with the larger Eₐ)
Next: practice questions.