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:

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:

The slideshow shows an exothermic reaction between sodium hydroxide and hydrochloric acid, and an endothermic reaction between sodium carbonate and ethanoic acid.

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

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