AQA GCSE Chemistry

Revision Notes

5.1.1 Energy Transfer in Reactions

Heat Exchange in Reactions

  • 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 terms used to describe this are the system (what happens in the chemical reaction) and the surroundings
  • The energy comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy
  • 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

Exam Tip

Physical processes can also involve heat exchange. Examples include freezing or melting which involve a change in state.

Exothermic Reactions

  • In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases
  • Combustion, oxidation, and neutralisation reactions are typical exothermic reactions
  • Hand warmers used in the wintertime are based on the release of heat from an exothermic reaction
  • Self-heating cans of food and drinks such as coffee and hot chocolate also use exothermic reactions in the bases of the containers

Exothermic- reaction, IGCSE & GCSE Chemistry revision notes

Diagram showing the transfer of heat energy outwards from an exothermic reaction

Endothermic Reactions

  • In endothermic reactions energy is taken in from the surroundings so the temperature of the surroundings decreases
  • These types of reactions are much less common than the exothermic reactions
  • Electrolysis, thermal decomposition reactions and the first stages of photosynthesis are typical endothermic reactions
  • Sports injury treatment often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling

Endothermic- reaction, IGCSE & GCSE Chemistry revision notes

Diagram showing the transfer of heat energy from the surroundings into an endothermic reaction

Exothermic Vs. Endothermic Reactions

  • The changes in heat content can be determined and measured with a thermometer
  • Note that the overall amount of energy does not change as energy is conserved in reactions
  • This means that it cannot be created or destroyed but it can be transferred
  • So, if energy is transferred to the surroundings during a chemical reaction, then the products formed must have less energy than the reactants by the same amount as that transferred
  • The following are some examples of heat changes in reactions
    • Neutralisation reactions:
      • These always give energy out
    • Displacement reactions:
      • These can either take energy in or give it out
    • Combustion reactions:
      • These always give energy out
  • In summary:
    • In exothermic reactions, the temperature of the surroundings increases and the heat content of the system falls
    • In endothermic reactions, the temperature of the surroundings falls and the heat content of the system increases

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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