Energy Transfer in Reactions (AQA GCSE Chemistry)

Heat exchange in 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 is known as the law of conservation of energy

• 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

Exothermic reactions

What is an exothermic reaction?

• An exothermic reaction releases heat energy

  • This means that the temperature increases

• Examples of exothermic reactions include neutralisation and combustion

Exothermic reaction diagram

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

Endothermic reactions

What is an endothermic reaction?

• An endothermic reaction takes heat energy in

  • This means that the temperature decreases

• Electrolysis, thermal decomposition reactions, the first stages of photosynthesis and the reaction between citric acid and sodium hydrogencarbonate 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 diagram

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

Exothermic & endothermic reactions

Measuring heat changes

• 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

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