Lesson 7 of 8

Exothermic and endothermic

Goal: Tell exothermic from endothermic reactions.

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An exothermic reaction gives out energy and warms its surroundings. An endothermic reaction takes in energy and cools them.

Two gloved hands holding a hand warmer in the snow
Exothermic. The reaction inside a hand warmer gives out energy, so your hands get warmer.
A cold pack held on a footballer’s knee
Endothermic. When the chemicals inside an instant cold pack mix, they take in energy, so the pack turns icy cold.
  • Use a thermometer. If the temperature goes up, the reaction is exothermic. If it goes down, it is endothermic.
  • Exothermic: burning a fuel, and an acid neutralising an alkali.
  • Endothermic: sherbet fizzing on your tongue, and thermal decomposition, which only keeps going while you heat it.
  • Bonds explain it. Breaking the bonds between atoms takes energy in. Making new bonds gives energy out. A reaction is exothermic when more is given out than taken in.

A way to remember: in an exothermic reaction the energy exits.

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Key words in this lesson

Fuel A substance that stores energy and is burned to release it

Thermal decomposition When heat breaks one substance down into simpler ones

Exothermic Gives out energy, so the surroundings get hotter

Endothermic Takes in energy, so the surroundings get colder

For teachers: AQA 3.6.3 · Pearson Edexcel 22.1 · Cambridge 8Cc.02