Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!
Print

Science

Energy changes and reversible reactions

Page:

  1. 1
  2. 2
  3. 3
  1. Next

Exothermic [exothermic: Heat energy is released in an exothermic reaction. We know this because the surroundings get warm. ] reactions transfer energy to the surroundings. Endothermic [endothermic: In an endothermic reaction, energy is taken in from the surroundings. The surroundings then have less energy than they started with, so the temperature falls. ] reactions take in energy from the surroundings.

Reversible reactions are where the productsproduct: A product is a substance formed in a chemical reaction. can react to remake the original reactantsreactants: substances present at the start of a chemical reaction. If the forward reaction is exothermic, the reverse reaction is endothermic.

Exothermic and endothermic reactions

When a chemical reaction occurs, energy is transferred to, or from, the surroundings - and there is often 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. The temperature increase can be detected using a thermometer. 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 get colder. The temperature decrease can also be detected using a thermometer. Some examples of endothermic reactions are:

  • electrolysis [electrolysis: Electrolysis is the decomposition (separation or break-down) of a compound using an electric current. ]
  • the reaction between ethanoic acid and sodium carbonate
  • the thermal decomposition [thermal decomposition : A reaction in which substances are broken down by heat. ] of calcium carbonate in a blast furnace

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

Page:

  1. 1
  2. 2
  3. 3
  1. Next

Back to Chemical reactions index

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.