Reaction profiles: exothermic and endothermic

An exothermic reaction profile is a graph of energy against the progress of a reaction where the products finish lower than the reactants, because energy is given out to the surroundings. An endothermic profile is the opposite: the products finish higher, because energy is taken in. Both curves rise to a peak first, and the height from the reactants to the peak is the activation energy.

Exothermic and endothermic reactions

Those examples are the ones AQA lists in its GCSE Chemistry specification.

Energy level diagram

The simplest picture is an energy level diagram: one line for the energy stored in the reactants and one for the products, with an arrow for the energy change between them.

Exothermic reaction profile

A reaction profile adds what happens on the way. Particles need a minimum amount of energy to react, so the energy has to rise first before the new bonds form and energy is released. That gives the curve its hump.

Here x is the progress of the reaction, from start to finish, and y is energy in kJ per mole. The reactants start at 40, the curve climbs to 90, then drops to the products at 10. Reading off the graph:

• Activation energy = peak − reactants = 90 − 40 = 50 kJ/mol.
• Overall energy change = products − reactants = 10 − 40 = −30 kJ/mol. Negative, so 30 kJ/mol is given out: exothermic.

Endothermic reaction profile

Again x is progress and y is energy. Now the products end higher than the reactants. The activation energy is 90 − 10 = 80 kJ/mol, and the overall energy change is 40 − 10 = +30 kJ/mol: 30 kJ/mol is taken in.

Practise reading values from a profile, then press Mark to check your answer in the course.

How to draw a reaction profile

1. Draw and label the axes: energy up the side, progress of reaction along the bottom.
2. Draw a short horizontal line for the reactants on the left and label it.
3. Draw the products line on the right: lower for exothermic, higher for endothermic. Label it.
4. Join them with a smooth curve that rises to a single peak above both.
5. Add a vertical arrow from the reactants up to the peak, labelled activation energy.
6. Add a vertical arrow from the reactants line to the products line, labelled energy change (or energy given out, or taken in).

AQA expects GCSE students to draw simple reaction profiles for exothermic and endothermic reactions, and to use them to identify which kind a reaction is. At A-level, the overall energy change is written ΔH.

What a catalyst does to the profile

A catalyst gives the reaction a different route with a lower activation energy, so the hump is lower: here 60 − 40 = 20 kJ/mol instead of 50. More particles have enough energy to react, so the reaction is faster. The reactants and products are the same, so the overall energy change is still −30 kJ/mol. A catalyst never changes the energy change.

Energy changes from bond energies

Bond breaking is endothermic and bond making is exothermic. The overall energy change is the difference between the two totals. In AQA GCSE Chemistry this calculation is Higher tier, with the bond energies supplied.

Example: nitrogen and hydrogen make ammonia, N₂ + 3H₂ → 2NH₃. Given bond energies of N≡N 945, H-H 436 and N-H 391 kJ/mol:

• Bonds broken: one N≡N and three H-H, 945 + 3 × 436 = 2,253 kJ/mol.
• Bonds made: two NH₃ molecules have six N-H bonds, 6 × 391 = 2,346 kJ/mol.
• Energy change = 2,253 − 2,346 = −93 kJ/mol. The new bonds release more energy than breaking the old ones took in, which makes the reaction exothermic.

The enthalpy calculator works through bond energy and enthalpy change calculations step by step, and the Reaction Kinetics course has an interactive energy profile you can change.

Common mistakes

• Drawing the products on the wrong side. Exothermic means lower products; endothermic means higher.
• Measuring activation energy from the products. It goes from the reactants to the peak.
• Leaving out the hump. Every reaction profile rises before it falls, even exothermic ones.
• Saying a catalyst changes the energy change. It only lowers the activation energy.
• Arrows without arrowheads or labels. Mark schemes usually want both, pointing the right way.

To practise energy changes under time pressure, try today's GCSE Chemistry Higher or GCSE Combined Science Higher short timed paper. Each paper mixes topics.

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Chemistry · CoursesReaction kinetics course: rates, collisions and catalystsAn interactive reaction kinetics course in 25 short sections: measuring rates, collision theory, energy profiles, catalysts and rate equations.