GCSE Chemistry Energy Changes Notes
Chemical reactions transfer energy to or from the surroundings. Bond breaking is endothermic; bond making is exothermic — the difference is the overall energy change.
Key concepts
- Exothermic
- Reaction transfers energy to the surroundings. Temperature of surroundings rises. Example: combustion, neutralisation.
- Endothermic
- Reaction takes energy from the surroundings. Temperature falls. Example: thermal decomposition, dissolving ammonium nitrate.
- Activation energy
- Minimum energy needed for reactant particles to react on collision.
- Reaction profile
- Diagram showing reactant, activation and product energy levels.
- Breaking bonds requires energy (endothermic); making bonds releases energy (exothermic).
- Overall ΔH = energy to break bonds − energy released making bonds.
- In an exothermic reaction, products are lower in energy than reactants.
Key formulas
Overall energy change
ΔH = Σ(bond energies broken) − Σ(bond energies made)
Worked example
Calculate the energy change for H₂ + Cl₂ → 2HCl. (Bond energies (kJ/mol): H–H 436, Cl–Cl 243, H–Cl 432.)
- 1Energy to break: 436 + 243 = 679 kJ.
- 2Energy released making: 2 × 432 = 864 kJ.
- 3ΔH = 679 − 864 = −185 kJ/mol.
- 4Negative → exothermic.
Answer: ΔH = −185 kJ/mol (exothermic)
Common mistakes
✗ Getting the sign of ΔH wrong.
✓ Exothermic is negative; endothermic is positive.
✗ Confusing 'reaction gets hot' with 'endothermic'.
✓ If surroundings get hot, energy is released — the reaction is exothermic.
✗ Forgetting to count every bond in the molecule.
✓ Draw the displayed formula and count each bond individually.
Quick check
Try these, then reveal the answers.
Is combustion exothermic or endothermic?Reveal answer
Exothermic.
What is activation energy?Reveal answer
The minimum energy needed for a reaction to occur on collision.
If ΔH = +50 kJ/mol, is the reaction exo or endo?Reveal answer
Endothermic.
Exam tips
- Draw reaction profiles with reactants and products clearly labelled and Ea marked from reactants to the peak.
- State the sign of ΔH with the number.
- Give a real example (e.g. self-heating cans = exothermic, sports cold packs = endothermic).
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Frequently asked questions
Why is bond breaking endothermic?+
Energy must be supplied to overcome the attractive forces holding atoms together in a bond.
How does a catalyst affect the reaction profile?+
It provides an alternative route with a lower activation energy but does not change ΔH.
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