Activation Energy Diagram – IGCSE Chemistry Definition

An activation energy diagram, often called an energy level diagram or energy profile diagram, is a graphical representation of the energy changes that occur during a chemical reaction. The y-axis shows energy and the x-axis shows the progress of the reaction (sometimes labelled “reaction coordinate”). The diagram displays the energy of the reactants, the energy of the products, the activation energy barrier, and the overall energy change.

Key features

The diagram begins with a horizontal line or label showing the energy of the reactants. A curve rises to a peak representing the transition state, where bonds are being broken and new bonds are beginning to form. The height of this peak above the reactant level is the activation energy (Ea). After the peak, the curve falls to the energy level of the products.

For an exothermic reaction, the product level is lower than the reactant level. The difference represents the energy released to the surroundings. For an endothermic reaction, the product level is higher, and the difference represents the energy absorbed from the surroundings.

Labels examiners require

Mark schemes typically require: the reactant and product levels labelled with their names or formulae, an arrow showing the activation energy (Ea) from the reactant level to the top of the peak, and an arrow showing the overall energy change between reactant and product levels.

Showing the effect of a catalyst

A second, lower curve can be drawn to show the catalysed pathway. It starts and finishes at the same energy levels (same reactants, same products) but has a lower peak, indicating a lower activation energy.

Exam context

Drawing and interpreting energy diagrams is one of the most commonly examined topics in chemical energetics. Paper 4 questions may ask candidates to draw the diagram from scratch, add labels to a pre-drawn diagram, or interpret a given diagram to identify whether the reaction is exothermic or endothermic.

Worked exam question

The diagram shows the energy profile for a reaction. The reactants are at 200 kJ and the products are at 120 kJ. The peak is at 280 kJ. (a) Calculate the activation energy. (b) Calculate the overall energy change. (c) State whether the reaction is exothermic or endothermic. (3 marks)

(a) Ea = 280 - 200 = 80 kJ [1]. (b) Energy change = 200 - 120 = -80 kJ (or 120 - 200 = -80 kJ) [1]. (c) Exothermic, because the products have less energy than the reactants / the energy change is negative [1].

Common mistakes

Candidates often measure the activation energy from the product level instead of the reactant level. Activation energy is always measured from the reactant level upwards to the peak. Another mistake is drawing the peak below the reactant level, which is physically impossible --- some energy barrier must always be overcome, even for exothermic reactions.