Potential energy axisThe vertical axis: the potential energy of the reacting system.
Reaction progress axisThe horizontal axis: how far the reaction has gone. From reactants to products.
ReactantsThe energy of the starting substances.
ProductsThe energy of the substances made. Lower than the reactants when energy is released.
Transition state (activated complex)The highest-energy arrangement on the way from reactants to products.
Activation energyThe energy needed to reach the transition state, measured from the reactants.
Energy change (ΔH)The difference between reactants and products. Energy given out in an exothermic reaction, taken in in an endothermic one.
Other 2
Catalyzed pathThe route with a catalyst. It has a lower activation energy, here in two steps.
IntermediateA short-lived substance formed between the two steps of the catalysed path.
How this list was compiled: Read 2026-09-26. (1) OpenStax Chemistry 2e, 12.5 Collision Theory, https://openstax.org/books/chemistry-2e/pages/12-5-collision-theory : reaction diagrams; activation energy; the activated complex or transition state; exothermic reactions; reactants and products. (2) OpenStax Chemistry 2e, 12.7 Catalysis, https://openstax.org/books/chemistry-2e/pages/12-7-catalysis : reaction diagrams for an endothermic process with and without a catalyst; the catalysed path has a lower activation energy, two transition states and an intermediate in the valley between them. (3) Wikipedia, Energy profile (chemistry), https://en.wikipedia.org/wiki/Energy_profile_(chemistry) : potential energy against the reaction coordinate; reactants, products, transition state, activation energy, intermediates, enthalpy change, catalysts. Licence: own work
The drawing: Drawn for this site by script, 2026-09-26. Own work, nothing traced. Licence: own work
How it works
This potential energy diagram labeled part by part shows two reactions: an exothermic one, where the products end lower than the reactants, and an endothermic one, with the lower path a catalyst opens.
Read the full guide
Reading the curve
The curve starts at the reactants' energy, climbs to the transition state at the top of the hump, and falls to the products. The activation energy is the climb from the reactants to the top.
Energy change and catalysts
The energy change, ΔH, is the gap between reactants and products: negative when energy is given out, positive when it is taken in. A catalyst opens a lower path, here in two steps with an intermediate between them.
Activation energy is always measured from the reactants to the top of the hump, not from the bottom of the graph.
Questions and answers
What is activation energy on a potential energy diagram?
The height of the hump above the reactants: the energy the reactants need to reach the transition state before they can become products.
How can you tell if a reaction is exothermic from the diagram?
If the products end lower than the reactants, energy was given out and the reaction is exothermic. If they end higher, it is endothermic.
What does a catalyst do to the diagram?
It opens a different path with a lower activation energy, often with more than one step. The reactants and products stay at the same energy, so ΔH does not change.