How to draw energy level diagrams

An energy level diagram turns a heat of reaction into a picture, and SPM loves asking you to draw one and label it. The good news is that there are only two shapes to learn, one for exothermic, one for endothermic, and once you know which way the product level goes and which way the ΔH arrow points, you have the marks. This guide builds both from the axes up, using the thermochemistry syllabus.

What the axes mean

Every energy level diagram has the same axes:

• The vertical axis is energy (increasing upwards). Sometimes it is labelled “energy” or “enthalpy”.
• The horizontal axis is the progress of the reaction, the path from reactants to products. It carries no numbers; it just shows the direction of the reaction from left to right.

You draw the reactants as a horizontal line on the left and the products as a horizontal line on the right, at different heights.

The exothermic shape

In an exothermic reaction, heat is released to the surroundings and the temperature rises, so the products store less energy than the reactants. That means:

• Reactants level is high, products level is low.
• The ΔH arrow points downwards, from the reactants level to the products level.
• ΔH is negative, because energy has been lost to the surroundings.

To draw it: axes first, then a higher reactant line, a lower product line, and a downward arrow between them labelled with ΔH and its negative value.

The endothermic shape

In an endothermic reaction, heat is absorbed from the surroundings and the temperature falls, so the products store more energy than the reactants. That means:

• Reactants level is low, products level is high.
• The ΔH arrow points upwards, from the reactants level to the products level.
• ΔH is positive, because energy has been gained from the surroundings.

The two diagrams are mirror images in the vertical sense: exothermic drops, endothermic climbs.

Step-by-step: draw an exothermic diagram

Take the combustion of ethanol, which is strongly exothermic, see heat of combustion.

1. Draw the vertical energy axis and the horizontal progress-of-reaction axis.
2. Draw a horizontal line high up, labelled with the reactants: C₂H₅OH + 3O₂.
3. Draw a horizontal line lower down, labelled with the products: 2CO₂ + 3H₂O.
4. Draw a vertical arrow from the reactant line down to the product line.
5. Label that arrow ΔH = negative (heat released). If a value is given, write it, for example ΔH = −1376 kJ mol⁻¹.

For an exothermic diagram of neutralisation, the same shape applies with acid + alkali on top and salt + water below, see heat of neutralisation.

Adding the activation energy hump

For a fuller energy profile, add the activation energy, the minimum energy colliding particles need to react. Between the reactant and product levels, draw a hump (a peak) that rises above the reactant level and then comes down to the product level. The height from the reactant line up to the top of the hump is the activation energy, labelled Eₐ. This is where thermochemistry meets collision theory.

A catalyst does not change ΔH, but it lowers the hump: draw a second, lower peak for the catalysed path. The reactant and product levels stay exactly where they are, only the height of the hump changes, which is the visual way of saying a catalyst provides an alternative path with lower activation energy.

The full labelling checklist

A complete diagram shows:

• Both axes labelled (energy vertical, progress of reaction horizontal).
• Reactants line and products line, each labelled with the actual substances.
• Products lower for exothermic, higher for endothermic.
• A ΔH arrow in the correct direction, labelled with its sign (and value if given).
• The activation energy hump with Eₐ, if the question asks for an energy profile.

Common mistakes to avoid

• Putting the products at the wrong height. Exothermic products are lower; endothermic products are higher. Decide from the temperature change first.
• Pointing the ΔH arrow the wrong way. Down for exothermic (energy out), up for endothermic (energy in).
• Forgetting the sign of ΔH. Negative for exothermic, positive for endothermic.
• Raising the product level with a catalyst. A catalyst only lowers the hump; the reactant and product levels, and ΔH, never change.

Practise both shapes side by side

Sketch an exothermic and an endothermic diagram next to each other a few times, labelling every part, then add the activation energy hump and a catalysed path to each. Once the two shapes are automatic, these become reliable marks in Paper 2. If the direction of the ΔH arrow or the effect of a catalyst keeps confusing you, that is quick to untangle with a teacher, our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial. See how it works if you would like your diagrams checked against real Paper 2 answers.