Exothermic and endothermic reactions

During a chemical reaction, energy can be transferred between the reacting chemicals and their surroundings. The surroundings include the solution and container around the reacting substances. Energy is conserved: it is not created or destroyed, but transferred between the chemicals and the surroundings.

Exothermic reactions

An exothermic reaction transfers energy from the reacting chemicals to the surroundings. The surroundings gain energy and become warmer, so their temperature rises.

Because energy has been transferred out, the products store less energy than the reactants. The decrease in energy stored by the chemicals equals the energy transferred to the surroundings.

Combustion, many oxidation reactions and neutralisation are examples of exothermic reactions.

Endothermic reactions

An endothermic reaction takes in energy from the surroundings. The surroundings lose energy and become cooler, so their temperature falls.

Because energy has been transferred in, the products store more energy than the reactants. The increase in energy stored by the chemicals equals the energy taken in from the surroundings.

Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate are examples of endothermic reactions.

Identifying the direction of energy transfer

A temperature change in the surroundings provides evidence of energy transfer. A temperature rise indicates that the reaction has transferred energy to the surroundings and is exothermic. A temperature fall indicates that the reaction has taken in energy from the surroundings and is endothermic.

Applications of exothermic and endothermic reactions

Exothermic and endothermic processes can be used to change the temperature of their surroundings. The choice depends on whether the surroundings need to be warmed or cooled.

Exothermic processes for warming

An exothermic reaction transfers energy to its surroundings, making them warmer. This makes it useful in a self-heating can, where the energy released warms the contents, or in a hand warmer, where it warms the user's hands.

Endothermic processes for cooling

An endothermic process absorbs energy from its surroundings, making them cooler. Some sports injury packs use an endothermic process to absorb energy from the injured area, cooling it.

Judging suitability for heating or cooling

To judge an application, link its intended use to the direction of energy transfer and the resulting temperature change:

• For warming, the process must release enough energy to raise the temperature of the surroundings sufficiently.
• For cooling, the process must absorb enough energy to lower the temperature of the surroundings sufficiently.

A process is not necessarily suitable just because it is exothermic or endothermic: its heating or cooling effect must be sufficient for the intended use. Base the judgement on the information given about that effect; enthalpy calculations are not needed.