Exothermic & Endothermic ReactionsOxford AQA IGCSE Chemistry: Revision notes
Section 1
What happens to energy during a chemical reaction?
During a chemical reaction, energy is always transferred to or from the surroundings. This transfer can be detected as a change in temperature of the surroundings (the air, water, or solution around the reaction).
- If the surroundings get hotter, energy has been given out by the reaction
- If the surroundings get colder, energy has been taken in by the reaction
Section 2
What is an exothermic reaction?
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases.
Examples of exothermic reactions:
- Combustion (burning fuels)
- Many oxidation reactions
- Neutralisation (acid + alkali)
When methane burns in a Bunsen burner, the surrounding air and any nearby objects heat up — a clear exothermic reaction.
Section 3
What is an endothermic reaction?
An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.
Examples of endothermic reactions:
- Thermal decomposition reactions (e.g. heating a metal carbonate to break it down)
Students sometimes confuse 'energy is needed to start the reaction' with endothermic — even an exothermic reaction like combustion needs an initial spark. Classify a reaction by what happens to the surroundings' temperature overall, not by whether it needs starting.
Section 4
How is the energy change represented?
Chemists use the symbol ΔH (delta H) to represent the overall energy change of a reaction, following this convention:
| Reaction type | Sign of ΔH |
|---|---|
| Exothermic | Negative (−) |
| Endothermic | Positive (+) |
A negative ΔH means the products have less energy than the reactants (energy was released). A positive ΔH means the products have more energy than the reactants (energy was absorbed).
In an exam, always link the sign of ΔH to the direction of energy transfer — don't just quote the sign without explaining what it means.
Must Know
- All chemical reactions transfer energy to or from the surroundings
- Exothermic reactions transfer energy to the surroundings — temperature increases; ΔH is negative
- Endothermic reactions take in energy from the surroundings — temperature decreases; ΔH is positive
- Combustion, oxidation and neutralisation are exothermic
- Thermal decomposition is endothermic
- ΔH describes the overall energy change of the reaction
That's the notes covered.
Carry on to the next subtopic.