Exothermic and endothermic reactions

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. 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:

• burning
• neutralisation reactions between acids and alkalis.
• the reaction between water and calcium oxide

Endothermic reactions

These are reactions that take in energy from the surroundings. 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 exothermic reaction between sodium hydroxide and hydrochloric acid, and an endothermic reaction between sodium carbonate and ethanoic acid.

1. Sodium hydroxide solution is poured into a beaker of hydrochloric acid which contains a thermometer showing room temperature

2. The beaker now contains sodium chloride and water, and the thermometer is showing a rise in temperature, so the neutralisation reaction is exothermic

3. Sodium carbonate powder is tipped into a beaker of ethanoic acid which contains a thermometer showing room temperature

4. The beaker now contains sodium ethanoate, water and carbon dioxide, and the thermometer is showing a fall in temperature, so this was an endothermic reaction