Exothermic & Endothermic (Edexcel IGCSE Chemistry: Double 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

Exothermic reactions

• An exothermic reaction releases heat energy
  • This means that the temperature increases
• Examples of exothermic reactions include neutralisation and combustion

Exothermic reaction diagram

In exothermic reactions, the temperature of the surroundings increases and the heat content of the system falls

Endothermic reactions

• An endothermic reaction takes heat energy in
  • This means that the temperature decreases

Endothermic reaction diagram

In endothermic reactions, the temperature of the surroundings falls and the heat content of the system increases

• 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