Chemistry · General chemistry II · Concept
Potential energy diagrams: exothermic, endothermic and catalyzed reactions
Read a potential energy diagram: where ΔH, the forward and reverse activation energies and the transition state are, how exothermic and endothermic reactions differ, and what a catalyst changes and what it cannot.
What the diagram shows
The horizontal axis is reaction progress: how far the atoms have moved from reactants toward products. It is not time. The vertical axis is potential energy per mole. Reactants and products sit on flat levels, and the path between them climbs over a hump. The top of the hump is the transition state, an arrangement with bonds partly broken and partly formed that cannot be isolated.
| Feature | Measured from | To |
|---|---|---|
| ΔH | Reactant level | Product level |
| Ea (forward) | Reactant level | Top of the hump |
| Ea (reverse) | Product level | Top of the hump |
| Transition state | — | The top of the hump |
| Intermediate | — | A valley between two humps |
Exothermic and endothermic reactions
If the products sit below the reactants, energy is released and ΔH is negative: exothermic. If the products sit above, energy is absorbed and ΔH is positive: endothermic. Both kinds of reaction must climb a barrier first, so even a strongly exothermic reaction can be slow at room temperature.
The reverse reaction climbs from the product level to the same transition state, so its barrier differs from the forward one by exactly ΔH. For an endothermic reaction, the forward Ea can never be smaller than ΔH, because the transition state must lie above the products too.
What a catalyst changes
A catalyst provides a different pathway with a lower transition state. It lowers the forward and reverse barriers by the same amount, so both directions speed up by the same factor. It does not move the reactant or product levels, so ΔH, the equilibrium constant K and the equilibrium position stay the same; equilibrium is simply reached sooner. The catalyst is used in one step and regenerated in a later one, so a catalyzed pathway often has more than one hump.
Several humps: mechanisms on the diagram
A mechanism with several elementary steps has one hump per step and a valley for each intermediate. When earlier steps are fast and reversible, the step whose transition state stands highest above the reactants limits the overall rate. That link between the diagram and the rate law is what the mechanisms page tests step by step.
Common mistakes
- Reading the horizontal axis as time.
- Measuring Ea from the product level for the forward reaction, or from the axis instead of the reactant level.
- Saying a catalyst makes a reaction more exothermic or changes ΔH.
- Saying a catalyst shifts equilibrium toward products.
- Drawing an endothermic reaction with Ea smaller than ΔH.
- Calling a transition state an intermediate.
Work through an example
A reaction has ΔH = −40 kJ/mol and Ea = 50 kJ/mol. A catalyst lowers the forward barrier to 35 kJ/mol. Find the reverse barriers with and without the catalyst, say what happens to ΔH and K, and estimate how much faster the catalyzed reaction is at 298 K.
Read an energy diagram with and without a catalyst →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
- Tro, Chemistry: A Molecular Approach, 4th ed., §14.5 The Effect of Temperature on Reaction Rate, pp. 642–647 (activation energy and the transition state, p. 643)
- Tro, Chemistry: A Molecular Approach, 4th ed., §14.7 Catalysis, pp. 653–657 (catalyzed and uncatalyzed pathways, pp. 653–654)
- OpenStax Chemistry 2e — Collision theory
- OpenStax Chemistry 2e — Catalysis