Exothermic reactions are an important GCSE Chemistry topic because they explain how some reactions transfer energy to the surroundings. These reactions commonly appear in exam questions and are important for understanding fuels, respiration and everyday chemical reactions. Exothermic reactions are closely linked to topics such as reaction profiles, activation energy and bond energies in GCSE Chemistry.
Summary
- Exothermic reactions transfer energy to the surroundings.
- The surroundings become hotter during exothermic reactions.
- Temperature increases can be measured using a thermometer.
- Combustion, respiration and neutralisation are exothermic reactions.
- Exothermic reactions release more energy when bonds form than is needed to break bonds.
What Are Exothermic Reactions?
Exothermic reactions are reactions that transfer energy to the surroundings, in the form of heat. As energy is released, the temperature of the surroundings increases.
In exothermic reactions, more energy is released when new bonds form than is needed to break the original bonds in the reactants.
The word exothermic can be split into two parts, the first exo indicates on the outside (think of exoskeletons in biology) and thermic – relating to heat (thermal clothing for example)
Exothermic vs Endothermic Reactions
| Exothermic | Endothermic |
| Transfers energy to surroundings | Takes in energy from surroundings |
| Surroundings get hotter | Surroundings get colder |
| Temperature increases | Temperature decreases |
| Examples: combustion, respiration | Examples: thermal decomposition, photosynthesis |
Learn more about Endothermic reactions in our Endothermic Reactions GCSE guide.
Temperature Changes in Exothermic Reactions
During an exothermic reaction, energy is transferred to the surroundings so the temperature rises. This temperature increase can be measured using a thermometer, measuring temperature change is an important GCSE required practical skill.
- The reaction mixture becomes warmer.
- The surroundings gain energy.
- The temperature increases.
Examples of Exothermic Reactions
Many common chemical reactions are exothermic. GCSE students are expected to know several important examples.
- Combustion reactions
- Respiration
- Neutralisation reactions
- Hand warmers
Bond Breaking and Bond Making
Chemical reactions involve both bond breaking and bond making.
- Breaking bonds requires energy.
- Making bonds releases energy.
In exothermic reactions, the energy released when new bonds form is greater than the energy needed to break the original bonds. This causes an overall transfer of energy to the surroundings. For more detail on bond breaking and bond forming see our bond energies guide (coming soon).
Exothermic Reaction Profiles
Reaction profile diagrams for exothermic reactions show the products at a lower energy level than the reactants because energy has been transferred to the surroundings. Even exothermic reactions require activation energy before the reaction can begin. Exothermic reaction profiles are commonly compared with endothermic reaction profiles in GCSE exam questions. For more details on reaction profiles see our reaction profiles guide.
Common GCSE Exam Mistakes
- Mixing up exothermic and endothermic reactions.
- Thinking exothermic reactions absorb energy.
- Forgetting that the surroundings get hotter.
- Drawing reaction profiles with products at a higher energy level.
Exam Tips
- Remember that exothermic reactions transfer energy to the surroundings.
- Use the phrase ‘temperature increases’ in written answers.
- Learn common examples such as combustion and respiration.
- Practise drawing exothermic reaction profiles.
How Exothermic Reactions Link To Other GCSE Chemistry Topics
- Energy changes
- Endothermic reactions
- Reaction profiles
- Activation energy
- Bond energies
Final Thoughts
Exothermic reactions are a key part of GCSE Chemistry because they explain how chemical reactions transfer energy to the surroundings. Understanding exothermic reactions helps students explain temperature changes, reaction profiles and bond energy ideas throughout the GCSE Chemistry course. For a detailed overview of all topics, see our full GCSE Chemistry revision guide.
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