Changes in Heat Energy

Specification Point 7.9:
  • Recall that changes in heat energy accompany the following changes:
    a) salts dissolving in water
    b) neutralisation reactions
    c) displacement reactions
    d) precipitation reactions
    and that, when these reactions take place in solution, temperature changes can be measured to reflect the heat changes
  • Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons.
  • This is done by chemical bonding (we have already seen ionic and covalent bonding) where old bonds are broken and new bonds are formed.
  • This process involves the transfer of energy into and out of reaction mixtures.
  • The energy comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy.
  • Breaking bonds requires energy which is taken in from the surroundings.
  • Bond forming releases energy which is given out to the surroundings.
  • In the majority of reactions the energy is in the form of heat energy, although sometimes other types of energy are produced such as light or sound.
  • The changes in heat can be observed and measured with a thermometer.
  • The following are some examples of heat changes in reactions.
  • Salts dissolving in water: 
    • These can either take energy in or give it out.
  • Neutralisation reactions:
    • These always give energy out.
  • Displacement reactions:
    • These can either take energy in or give it out.
  • Precipitation reactions:
    • These always give energy out.

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.