Energy Level Diagram – IGCSE Chemistry Definition

Energy level diagrams (also called reaction profiles or energy profile diagrams) are graphical representations of the energy changes during a reaction. Drawing and interpreting them is tested on the 0620 syllabus across Papers 1, 2, and 4. You must be able to draw diagrams for both exothermic and endothermic reactions and show the effect of a catalyst.

The 0620 definition

An energy level diagram shows the relative energy levels of reactants and products in a chemical reaction, plotted against the progress of the reaction.

Drawing the diagram

Both axes must be labelled:

• y-axis: Energy
• x-axis: Progress of reaction (or reaction pathway)

Exothermic reaction

• Reactants are drawn at a higher energy level
• Products are drawn at a lower energy level
• A downward arrow between them shows the energy released (negative enthalpy change)
• A hump between reactants and products represents the activation energy

Endothermic reaction

• Reactants are drawn at a lower energy level
• Products are drawn at a higher energy level
• An upward arrow between them shows the energy absorbed (positive enthalpy change)
• A hump above the reactants represents the activation energy

Key features to label

1. Reactants (on the left, at their energy level)
2. Products (on the right, at their energy level)
3. Activation energy (Ea) — the height of the hump above the reactants
4. Energy change — the difference in height between reactants and products (labelled with an arrow showing direction)

Effect of a catalyst

A catalyst lowers the activation energy by providing an alternative reaction pathway. On the diagram:

• The reactant and product energy levels stay the same
• The hump is lower (smaller activation energy)
• The overall energy change is unchanged

Worked exam question

Draw an energy level diagram for an exothermic reaction. On your diagram, label: (i) reactants, (ii) products, (iii) activation energy, (iv) energy change. (4)

Mark scheme

(i) Reactants labelled at a higher energy level on the left [1]

(ii) Products labelled at a lower energy level on the right [1]

(iii) Activation energy labelled as the height from reactants to the top of the hump/curve [1]

(iv) Energy change labelled as the vertical distance from reactants down to products (with a downward arrow) [1]

Common exam mistakes

• Forgetting to draw the activation energy hump. Even exothermic reactions need energy to start — the hump must be present above the reactant level.
• Drawing the catalyst line at a different product energy level. A catalyst lowers the activation energy only — it does not change the energy of the products.
• Labelling the axes incorrectly. The y-axis must be “Energy” and the x-axis must be “Progress of reaction” (not “Time”).