Energy level diagrams

This page covers one Form 5 Thermochemistry content standard: energy level diagrams. Having classified reactions as exothermic or endothermic in the previous standard, you now learn to represent that heat change as a diagram. Energy level diagrams appear again and again in Paper 2, both as questions that ask you to draw one and as diagrams you must interpret, so a reliable method here earns marks across the whole chapter.

What an energy level diagram shows

An energy level diagram is a simple chart with energy (heat content) on the vertical axis. It shows two horizontal lines: one for the energy level of the reactants and one for the energy level of the products. The difference in height between the two lines is ΔH, the heat change of the reaction. The horizontal axis represents the progress of the reaction; it carries no scale and simply orders reactants on the left, products on the right.

The single most important feature is which line is higher:

• If the products are lower than the reactants, energy has been released, the reaction is exothermic.
• If the products are higher than the reactants, energy has been absorbed, the reaction is endothermic.

Drawing an exothermic diagram

For an exothermic reaction:

1. Draw and label the vertical axis “Energy”.
2. Draw the reactants line higher up.
3. Draw the products line lower down.
4. Write the formulae of the reactants on the upper line and the products on the lower line.
5. Draw a downward arrow from the reactants level to the products level and label it ΔH (negative).

Because the products have less heat content than the reactants, the “lost” energy has been released to the surroundings as heat, which is why the temperature of the mixture rises.

Drawing an endothermic diagram

For an endothermic reaction:

1. Draw and label the “Energy” axis.
2. Draw the reactants line lower down.
3. Draw the products line higher up.
4. Label both lines with the correct formulae.
5. Draw an upward arrow from reactants to products and label it ΔH (positive).

Here the products have more heat content than the reactants; that extra energy was absorbed from the surroundings, which is why the temperature falls.

Reading a diagram in reverse

Examiners often give you the diagram and ask you to classify the reaction and state the sign of ΔH. Reverse the logic: look at the products. Products low → exothermic → ΔH negative. Products high → endothermic → ΔH positive. The ΔH arrow confirms it, an arrow pointing down means heat is released, an arrow pointing up means heat is absorbed.

Worked example

Question. The combustion of methane is exothermic. Sketch an energy level diagram for the reaction CH₄ + 2O₂ → CO₂ + 2H₂O, and mark ΔH.

Step 1, Set up the axis. Draw a vertical axis and label it “Energy”. The horizontal direction shows the progress of the reaction.

Step 2, Decide the relative levels. The reaction is exothermic, so the products (CO₂ + 2H₂O) must be drawn lower than the reactants (CH₄ + 2O₂).

Step 3, Draw and label the two levels. Draw an upper horizontal line for CH₄ + 2O₂ and a lower horizontal line for CO₂ + 2H₂O, writing each formula against its line.

Step 4, Add the ΔH arrow. Draw a vertical arrow pointing downwards from the reactants line to the products line. Label it ΔH (negative), because heat is released.

Answer. The diagram shows CH₄ + 2O₂ on the higher level and CO₂ + 2H₂O on the lower level, with a downward ΔH arrow between them labelled negative, the standard picture of an exothermic reaction.

Practice question

Question. The thermal decomposition of copper(II) carbonate, CuCO₃ → CuO + CO₂, is endothermic. Describe how its energy level diagram should be drawn, including the position of reactants and products and the direction of the ΔH arrow.

Answer. Because the reaction is endothermic, the reactant (CuCO₃) is drawn on the lower energy level and the products (CuO + CO₂) on the higher level. The ΔH arrow points upwards from the lower reactant line to the higher product line and is labelled ΔH (positive), showing that heat has been absorbed from the surroundings.

Exam tip

A full-mark energy level diagram needs five things: a labelled energy axis, a reactants line, a products line at the correct relative height, the formulae written against each line, and a labelled ΔH arrow pointing the right way. The two errors that cost marks most often are (1) putting the products on the wrong side, remember exothermic products are LOW, endothermic products are HIGH, and (2) forgetting to label the arrow or drawing it the wrong way. A quick self-check: the ΔH arrow always points from reactants to products; if that arrow goes down, the reaction is exothermic, and if it goes up, it is endothermic. Keep the two lines clearly separated so the marker can see the height difference at a glance.

Linking the diagram to bond energy

You can also explain the height difference using bonds. In an exothermic reaction, more energy is released when bonds form in the products than is absorbed when bonds break in the reactants, so the products end up at a lower energy level. In an endothermic reaction the opposite is true: more energy is absorbed breaking reactant bonds than is released forming product bonds, so the products end up higher. Being able to say this in one sentence turns a “draw the diagram” question into a full explanation, which is exactly what the longer Paper 2 items reward.

Common mistakes to avoid

Do not draw the two energy levels at the same height, there must be a clear vertical gap equal to ΔH. Do not leave the arrow unlabelled or omit the negative or positive sign. Do not confuse this diagram with a rate-of-reaction energy profile that shows an activation-energy “hump”; at Form 5 thermochemistry level you are usually asked only for the reactant and product levels and the ΔH arrow, unless the question specifically asks for activation energy. Finally, always write the actual formulae, not just the words “reactants” and “products”, when the equation is given.

Where this fits

This is content standard 11.2 of the Thermochemistry chapter, following directly from exothermic and endothermic reactions and feeding into the heats of neutralisation, displacement, precipitation and combustion, each of which you may be asked to represent as a diagram. Reinforce the drawing rules with the chapter revision notes and practise interpreting diagrams with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers check every part of your diagram against the marking scheme, because these are among the most reliable marks in SPM Chemistry.

Quick recap

• Energy axis vertical; reactants and products drawn as two horizontal lines.
• Exothermic: products lower, ΔH arrow down, ΔH negative.
• Endothermic: products higher, ΔH arrow up, ΔH positive.
• Label the axis, both lines, the formulae and the ΔH arrow for full marks.