Reactions and temperature changes

Energy is conserved in chemical reactions, so the total amount of energy in the universe at the end of a reaction is the same as it was before the reaction.

When a chemical reaction happens, energy is transferred to or from the surroundings. When energy is transferred to the surroundings, this is called an exothermic reaction, and the temperature of the surroundings increases. Examples of exothermic reactions include:

• combustion reactions
• many oxidation reactions
• most neutralisation reactions

Everyday uses of exothermic reactions include self-heating cans and hand warmers.

When energy is taken in from the surroundings, this is called an endothermic reaction and the temperature of the surroundings decreases. Examples of endothermic reactions include:

• thermal decomposition reactions
• the reaction of citric acid and sodium hydrogencarbonate

Everyday uses of endothermic reactions include instant ice packs which can be used to treat sports injuries.

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. In exothermic reactions, energy exits.