Exothermic

Key Stage 3

An exothermic process is one that gives out energy. This usually causes surroundings to increase in temperature.

Most chemical reactions are exothermic with means they release energy to the environment and this is observed by an increase in temperature.


The energy stored in the reactants is released in the chemical reaction making the material increase in temperature. The products now have less energy than the reactants.

Freezing, Condensing and Depositing are exothermic changes because they release energy when they happen. The material has less after they have happened. However, there is usually no increase in temperature because the material is usually cooled to change state.

An exothermic process is one that gives out energy. This usually causes surroundings to increase in temperature.

Exothermic reactions usually require energy to begin. This is called the activation energy.

In an exothermic process the potential energy stored in the products is less than the potential energy stored in the reactants.


To start the reaction an activation energy is needed. This is usually achieved by initially heating the reactants. Once the reaction starts the energy stored in the reactants is released in the chemical reaction making the material increase in temperature. The products now have less energy than the reactants.

In a chemical reaction energy is needed to break the chemical bonds holding the atoms together.

This happens because there is less energy stored in the chemical bonds of the products than the reactants.


In the reaction between Methane and Oxygen the chemical bonds in the reactants must be broken first before the bonds in the products are formed.

There are 4 C-H bonds and 2 O=O bonds.

4 x 413 + 2 x 498 = 2648kJ

Therefore 2648kJ/mol are needed to break the bonds in the reactants.

There are 2 C=O bonds and 4 O-H bonds.

2 x 799 + 4 x 464 = 3454kJ

Therefore 3454kJ/mol is released when the bonds in the products form.

Once the reaction is complete 806kJ will be released.