Endothermic reactions are chemical reactions that take in energy from the surroundings, usually in the form of heat. As energy is absorbed, the surroundings become cooler. These reactions are important in chemistry because they help explain how energy changes during chemical processes. Endothermic reactions are closely linked to topics such as bond energies, activation energy and reaction profiles in GCSE Chemistry.
Summary
- Endothermic reactions transfer energy from the surroundings.
- The surroundings become colder during endothermic reactions.
- Temperature decreases can be measured using a thermometer.
- Thermal decomposition and photosynthesis are endothermic reactions.
- Endothermic reactions require more energy to break bonds than is released when new bonds form.
What are Endothermic Reactions?
- Endothermic reactions are reactions that transfer energy from the surroundings, in the form of heat. As energy is taken in, the temperature of the surroundings decreases.
- In endothermic reactions, more energy is required to break bonds than is released when new bonds form
- The word endothermic can be split into two parts, the first endo indicates on the inside (think of endoskeletons in biology) and thermic – relating to heat (thermal clothing for example)
Endothermic vs Exothermic Reactions
| Endothermic | Exothermic |
| Takes in energy from surroundings | Transfers energy to surroundings |
| Surroundings get colder | Surroundings get hotter |
| Temperature decreases | Temperature increases |
| Examples: thermal decomposition, photosynthesis | Examples: combustion, respiration |
Exothermic and endothermic reactions are commonly compared in GCSE Chemistry exams, especially when interpreting reaction profiles and temperature changes. Learn more about Exothermic reactions in our Exothermic Reactions GCSE guide.
Temperature Changes in Endothermic Reactions
During an endothermic reaction, energy is transferred from the surroundings so the temperature drops. This temperature decrease can be measured using a thermometer, measuring temperature change is an important GCSE required practical skill.
- The reaction mixture becomes colder.
- The surroundings lose energy.
- The temperature decreases.
Examples of Endothermic Reactions
Many common chemical reactions are endothermic. GCSE students are expected to know several important examples.
- Photosynthesis in plants.
- Thermal decomposition reactions.
- Some reactions between citric acid and sodium hydrogencarbonate.
- Sports injury cold packs.
Some GCSE practical investigations also demonstrate endothermic reactions by measuring temperature decreases.
Everyday Uses of Endothermic Reactions
- Instant cold packs for sports injuries absorb heat energy.
- Photosynthesis stores energy from sunlight in plants.
- Some cooking processes require energy to break down substances.
Bond Breaking and Bond Making
Chemical reactions involve both bond breaking and bond making.
- Breaking bonds requires energy.
- Making bonds releases energy.
In endothermic reactions, the energy released when new bonds form is lower than the energy needed to break the original bonds. This causes an overall transfer of energy from the surroundings. For more detail on bond breaking and bond forming see our bond energies guide.
Endothermic Reaction Profiles
Reaction profile diagrams for endothermic reactions show the products at a higher energy level than the reactants because energy has been transferred from the surroundings. As with all reactions, endothermic reactions require activation energy before the reaction can occur. Endothermic reaction profiles are commonly compared with exothermic reaction profiles in GCSE exam questions. For more details on reaction profiles see our reaction profiles guide.
Common GCSE Exam Mistakes
- Mixing up endothermic and exothermic reactions.
- Thinking endothermic reactions release energy.
- Forgetting that the surroundings get colder.
- Drawing reaction profiles with products at a lower energy level.
Exam Tips
- Remember that endothermic reactions gain energy from the surroundings.
- Use the phrase ‘temperature decreases’ in written answers.
- Learn common examples such as thermal decomposition and photosynthesis.
- Practise drawing endothermic reaction profiles.
How Endothermic Reactions Link To Other GCSE Chemistry Topics
- Energy changes
- Exothermic reactions
- Reaction profiles
- Activation energy
- Bond energies
Final thoughts
Endothermic reactions are a key part of GCSE Chemistry because they explain how chemical reactions gain energy from the surroundings. Understanding endothermic 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.