GCSE Chemistry

GCSE Chemistry exothermic and endothermic reactions, sorted for the exam

Last updated September 2026

Exothermic reactions release energy to the surroundings, so the temperature goes up. Endothermic reactions take energy in, so the temperature goes down. That one rule answers most exam questions on its own — this guide covers the examples, the energy diagrams, and the bond energy calculations that trip people up, then points you at a game that covers this alongside the rest of the syllabus.

Exothermic or endothermic — how to tell

On an energy level diagram, exothermic reactions show products sitting below the reactants — the difference is the energy given out. Endothermic reactions show products sitting above the reactants — the difference is the energy taken in. Both diagrams still show a hump above the reactants first: that's the activation energy, the minimum needed to get the reaction started at all, and it applies either way.

• Exothermic examples — combustion, neutralisation (acid plus alkali), most oxidation reactions, and the thermite reaction.
• Endothermic examples — thermal decomposition (like heating calcium carbonate to make calcium oxide and carbon dioxide), and dissolving certain salts such as ammonium nitrate, which is why it's used in instant cold packs.

Bond energies — where the marks actually are

Breaking bonds always needs energy put in — that step is endothermic on its own. Making new bonds always releases energy — that step is exothermic on its own. Every reaction does both, so the overall energy change depends on which one wins: release more energy making bonds than you spent breaking them, and the reaction is exothermic overall; spend more than you get back, and it's endothermic. This is also the required practical to know cold: measure the temperature change of a reaction — neutralisation or dissolving a salt — in an insulated cup rather than an open beaker, because an insulated cup is what stops heat escaping to the room and throwing the result off. It's exactly the kind of rule that needs spaced revision rather than a single read-through the night before.

Questions students actually ask

Why does the temperature rise in an exothermic reaction if energy is being "released"? Because that energy doesn't disappear — it transfers straight into the surrounding particles (and the thermometer), making them move faster, which is what a temperature rise actually measures.

Is dissolving always endothermic? No. It depends on the substance. Dissolving ammonium nitrate is endothermic, which is why it's used in cold packs, but dissolving other salts, like calcium chloride, is exothermic instead.

How do I read an energy level diagram in an exam? Find which line is higher. Products below reactants means exothermic, energy released. Products above reactants means endothermic, energy absorbed. The gap between the two lines is the overall energy change; the hump above the reactants is the activation energy.