Exothermic and Endothermic Reactions

GCSE Chemistry · Energy Changes

Energy changes in reactions

Every chemical reaction involves an energy change with the surroundings. Reactions are either exothermic (give out energy) or endothermic (take in energy). Overall, energy is conserved — it's transferred, not created or destroyed.

Exothermic reactions

Exothermic reactions transfer energy TO the surroundings, usually as heat, so the temperature rises.

Examples: combustion (burning), neutralisation, oxidation (e.g. rusting), most hand warmers.
Everyday uses: self-heating cans, hand warmers.

Endothermic reactions

Endothermic reactions take IN energy FROM the surroundings, so the temperature falls.

Examples: thermal decomposition (e.g. heating a carbonate), the reaction of citric acid with sodium hydrogen carbonate, photosynthesis.
Everyday uses: sports injury cold packs.

Reaction profiles

A reaction profile is a diagram showing the energy of reactants and products:

Exothermic: products are lower in energy than reactants (energy released).
Endothermic: products are higher in energy than reactants (energy absorbed).
The activation energy is the minimum energy needed to start the reaction — the "hump" reactants must get over. A catalyst lowers the activation energy (provides an easier route) without being used up.

Bond energies

Energy is involved in breaking and making bonds:

Breaking bonds is endothermic — energy must be put in.
Making bonds is exothermic — energy is given out.

Overall energy change = energy to break bonds − energy released making bonds.

If more energy is released (making) than taken in (breaking) → exothermic (negative overall).
If more energy is needed (breaking) than released (making) → endothermic.

Worked example

Breaking bonds needs 1500 kJ; making bonds releases 1800 kJ. Is the reaction exo- or endothermic?

Overall = 1500 − 1800 = −300 kJ. More energy is released than taken in → exothermic. ✓

Common mistakes

Getting bond energy the wrong way round — breaking = endothermic (in), making = exothermic (out).
Saying a catalyst changes the overall energy change — it only lowers the activation energy.
Mixing up the temperature change: exothermic rises, endothermic falls.

Exam tips

Learn examples of each type — a common recall question.
On reaction profiles, correctly place products lower (exo) or higher (endo) and label the activation energy.
For bond-energy sums: overall = (bonds broken) − (bonds made).

Key facts to remember

Exothermic = releases energy, temperature rises (combustion, neutralisation, oxidation).
Endothermic = absorbs energy, temperature falls (thermal decomposition, photosynthesis).
Breaking bonds = endothermic; making bonds = exothermic; a catalyst lowers activation energy only.