The BasicsEdit

An endothermic reaction takes energy from the environment.

This means it makes its container's temperature fall.

At GCSE there are few endothermic reactions except Thermal Decomposition but even cooking is endothermic since it requires you to heat food.

Photosynthesis also counts even though it doesn't take in heat - the definition simply says that it takes in energy, which photosynthesis does in the form of light.


Note: The Activation Energy corresponds to the bond-breaking, the Enthalpy (H) change is the difference between bond-breaking and bond-forming

Exam HintsEdit

Remember that all reactions start with bond-breaking - this requires energy and so is endothermic.

However, new bonds will form, releasing energy. And bond formation is always exothermic.

So a reaction can only be endothermic if more energy is required to break bonds than is then released when the new bonds are formed.

If you have been told that reactions occur so that the reactants can release energy then the existence of endothermic reactions that occur without being heated to provide the energy needed may be a little confusing. However, at A2 you will learn that Entropy  has a bearing and that the spontaneity of a reaction is governed by a combination of Enthalpy and Entropy that is called Gibbs Free Energy.
Endothermic reaction very, VERY cool

Endothermic reaction very, VERY cool.

The standard demo of an endothermic reaction.