Drawing and reading energy profile diagrams

An energy profile diagram is a picture of a reaction's energy from start to finish. Read it correctly and you can tell at a glance whether a reaction is exothermic or endothermic, how much energy it needs to start, and what a catalyst changes.

The axes

Energy goes on the vertical axis and the progress of the reaction on the horizontal. You draw a level for the reactants on the left, a level for the products on the right, and a curve rising over a hump between them. The hump is the energy barrier the reaction must climb.

Exothermic vs endothermic

If the products sit lower than the reactants, energy was released — the reaction is exothermic. If the products sit higher, energy was absorbed — endothermic. The vertical gap between reactant and product levels is the overall energy change, ΔH (negative for exothermic, positive for endothermic).

Activation energy

The height of the hump above the reactant level is the activation energy — the minimum energy a collision needs for the reaction to happen. A big hump means a slow reaction at room temperature; a small one means an easy reaction. Label it from the reactant level up to the top of the curve.

Showing a catalyst

A catalyst provides an easier route, so on the diagram it gives a lower hump — a smaller activation energy — while the reactant and product levels stay exactly where they were. That's the key exam point: a catalyst changes the activation energy, not ΔH.