Fiducia TogetherEnergy changes in reactions
Exothermic and endothermic, and the energy needed to start.
Please note: This page explains a word or a process in education. It is general information about the system, the law and the guidance in England — not advice about your child, your school or your exam. If a decision is going against you, get advice from one of the free services we link to before you agree to anything or miss a deadline. If you are revising, check your own exam board's specification: boards word things differently and yours is the one that marks you.
What it is
An exothermic reaction transfers energy to the surroundings and the temperature rises. An endothermic reaction takes energy in and the temperature falls.
Where it applies
GCSE chemistry and combined science, with a required practical.
How it works
Bond breaking is endothermic and bond making is exothermic. Whether a reaction overall releases or absorbs energy depends on which is larger.
Activation energy is the minimum needed to start, shown on a reaction profile as the hump.
How to use it in your work
Draw reaction profiles with the axes labelled and the activation energy marked from the reactants to the peak, not from the baseline.
For bond energy calculations, use energy in minus energy out and be careful with signs.
A catalyst lowers activation energy; it does not change the overall energy change. Both halves are marks.
Where to get help
Your teacher and your board's specification and mark scheme.
Revise this with a song
These are ours. We write revision songs because a lot of people remember a tune when they cannot remember a page, and because revising out loud in the kitchen beats staring at a highlighter.
- The Chemistry Song The GCSE chemistry specification, set to music.
Where to read more
Last reviewed 2026-08-28 by Fiducia Together · Next review due 2027-08-28
Please note: This page explains a word or a process in education. It is general information about the system, the law and the guidance in England — not advice about your child, your school or your exam. If a decision is going against you, get advice from one of the free services we link to before you agree to anything or miss a deadline. If you are revising, check your own exam board's specification: boards word things differently and yours is the one that marks you.