Reactions and temperature changes

When a chemical reaction happens, energy is transferred to or from the surroundings. For example, changes in heat energy happen in the following situations:

• saltsdissolving in water
• neutralisation reactions
• displacement reactions
• precipitation reactions

When these reactions happen in solution, there are temperature changes that can be observed. These can be measured using a thermometer.

Reactions can be endothermic or exothermic. The table shows some differences.

The slideshow describes an exothermic reaction between dilute 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

In endothermic reactions, energy enters from the surroundings. In exothermic reactions, energy exits to the surroundings.