Energy Transfer in Reactions (AQA GCSE Chemistry: Combined Science)

• 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

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

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

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