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When a chemical reaction occurs, energy is transferred to or from the surroundings. There is usually a temperature change. For example, when a bonfire burns it transfers heat energy to the surroundings. Objects near a bonfire become warmer. The temperature rise can be measured with a thermometer.

Exothermic reactions

These are reactions that transfer energy to the surroundings (ie the energy exits from the reaction, hence the name exothermic). The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. A thermometer is used to detect the temperature increase.

Some examples of exothermic reactions are:

• combustion (burning)
• neutralisation reactions between acids and alkalis
• the reaction between water and calcium oxide

The slideshow shows an exothermic reaction between sodium hydroxide and hydrochloric acid.

Endothermic reactions

These are reactions that take in energy from the surroundings (ie energy enters the reaction, which will help you to remember the name endothermic). The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become colder. A thermometer is used to detect the temperature decrease.

Some examples of endothermic reactions are:

• electrolysis
• the reaction between ethanoic acid and sodium carbonate
• the thermal decomposition of calcium carbonate in a blast furnace

The slideshow shows an endothermic reaction between sodium carbonate and ethanoic acid.