In chemistry, an exothermic reaction is one that releases heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:

Reactants → Products + Energy

When using a calorimeter, the change in heat of the calorimeter is equal to the opposite of the change in heat of the system. This means that when the solution in which the reaction is taking place gains heat, the reaction is exothermic.

In an exothermic reaction the total energy absorbed in bond breaking is less than the total energy released in bond making.

The absolute amount of energy in a chemical system is extremely difficult to measure or calculate. The enthalpy change, ΔH, of a chemical reaction is much easier to measure and calculate. A bomb calorimeter is very suitable for measuring the energy change, ΔH, of a combustion reaction. Measured and calculated ΔH values are related to bond energies by:

ΔH = energy used in bond breaking reactions - energy released in bond making products.

For an exothermic reaction, this gives a negative value for ΔH as a larger value is subtracted from a smaller value. For example, when hydrogen burns:

2H₂(g) + O₂(g) → 2 H₂O(g)

ΔH = −83.6 kJ/mol of O₂

Examples of exothermic reactions

• Combustion
• Neutralisation
• Adding water to concentrated acid
• Adding water to anhydrous copper(II) sulphate

External link

• GCSE Science: Reactions