Exothermic and endothermic reactions: energy changes explained
Tell exothermic from endothermic reactions, read reaction profiles and calculate energy change from bond energies, with worked examples.
GCSEHigh SchoolCurriculum fit: GCSE Chemistry and Combined Science (energy changes; bond energy calculations are mainly Higher). US: high school physical science (energy in chemical processes).
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What is it?
In an exothermic reaction, energy is released to the surroundings, so the temperature rises. In an endothermic reaction, energy is taken in from the surroundings, so the temperature falls.
Chemical reactions involve breaking old bonds, which takes in energy, and making new bonds, which releases energy. The overall energy change depends on which is larger.
Key formulas and ideas
- Exothermic: temperature rises (for example combustion, neutralisation).
- Endothermic: temperature falls (for example thermal decomposition).
- Energy change = energy to break bonds − energy released making bonds.
- A negative energy change means exothermic; a positive one means endothermic.
Worked examples
Example 1: Identify
Question: The temperature of a reaction mixture rises from 20 °C to 35 °C. What type of reaction is this?
- The temperature has increased.
- Energy has been released to the surroundings.
Answer: Exothermic
Example 2: Bond energies
Question: Calculate the energy change for H₂ + Cl₂ → 2HCl. Bond energies: H–H 436, Cl–Cl 243, H–Cl 432 kJ/mol.
- Energy to break bonds: 436 + 243 = 679 kJ.
- Energy released making bonds: 2 × 432 = 864 kJ.
- Energy change = 679 − 864 = −185 kJ/mol.
Answer: −185 kJ/mol (exothermic)
Example 3: Reaction profile
Question: On the profile for an exothermic reaction, where are the products relative to the reactants?
- Energy is released, so the products have less energy.
- The products line sits below the reactants line.
Answer: Products are lower in energy than reactants
Common mistakes
- Saying bond breaking releases energy.
- Subtracting in the wrong order, giving the wrong sign.
- Forgetting to multiply by the number of bonds in the balanced equation.
- Calling a reaction exothermic just because it needs heating to start.
Practice questions
Frequently asked questions
Why do exothermic reactions often need heat to start?
Activation energy must be supplied to start breaking bonds, even though more energy is released overall.
Give an everyday example of an endothermic reaction.
Cold packs and the reaction of citric acid with sodium hydrogencarbonate, which cool the surroundings.
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GCSEHigh SchoolSources and review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.