Reactions

When chemical reactions take place, energy is transferred as bonds are broken and made.

• Reactions that transfer energy to the surroundings are called exothermic reactions
• Reactions that take in energy from the surroundings are called endothermic reactions

In exothermic reactions the energy transferred often heats up the surroundings and so the TEMPERATURE INCREASES. Exothermic reactions include combustion, oxidation reactions like respirations and neutralisation reactions involving acids and bases.

In endothermic reactions energy is taken in from the surroundings. Some cause a decrease in temperature whilst others just require a supply of energy. When some solid compounds are mixed with water, the temperature decreases because endothermic changes happen as they dissolve. Thermal decomposition reactions need to be heated continuosly in order for the reaction to keep going.

• Exothermic reactions can be used to heat things and examples of this include handwarmers and self-heating cans. In some handwarmers and cans the reactants are used up and the reaction is irreversible. However in some products a reversible reaction is used such as the crystallisation of a salt. Once used, the pack can be heated in boiling water in order to re-dissolve the salt and therefore these can be re-used many times.

• Endothermic changes can be used to cool things down. Some chemical cold packs contain ammonium nitrate and water that are kept separated. When mixed, the ammonium nitrate dissolves and takes in energy from the surroundings causing that area to lose heat. The reaction is reversible but no in the pack and so this type of pack can only be used once. The pack can be used on sports injuries or to cool drinks before consumption.