Exothermic & Endothermic (Edexcel IGCSE Chemistry (Modular))

Exothermic and 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 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