Endothermic & Exothermic Reactions (Cambridge (CIE) IGCSE Chemistry)

Exothermic & endothermic 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 

• This process involves the transfer of thermal energy into and out of reaction mixtures

• The terms used to describe this are:

  • System: the reacting chemicals 

  • Surroundings: anything other than the chemicals reacting

• The energy within the system comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy

Exothermic reactions

• In exothermic reactions, thermal energy is transferred from the chemical energy store of the chemical system to the surroundings

  • The energy of the system decreases, which means that the energy change is negative

  • The temperature of the surroundings increases because thermal energy is given out / released

• The overall transfer is from the system to the surroundings

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

• Typical examples of exothermic reactions include:

  • Combustion

  • Oxidation

  • Neutralisation 

• 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

Endothermic reactions

• In endothermic reactions, thermal energy is transferred from the surroundings system to the system surroundings

  • The energy of the system increases, which means that the energy change is positive

  • The temperature of the surroundings decreases because thermal energy is taken in / absorbed

• The overall transfer is from the surroundings to the system

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

• Endothermic reactions are less common than exothermic reactions

• Typical examples of endothermic reactions include:

  • Electrolysis

  • Thermal decomposition

  • The first stages of photosynthesis 

• Cold packs for sports injuries are based on endothermic reactions, designed to take heat away from a recently injured area to prevent swelling

Reaction pathway diagrams

• Reaction pathway diagrams are graphical representations of the relative energies of the reactants and products in chemical reactions

• On a reaction pathway diagram:

  • Progress of the reaction is shown on the x-axis

  • Energy is shown on the y-axis

  • The difference in height between the energy of reactants and products is the overall energy change of a reaction

Reaction pathway diagram of an exothermic reaction and an endothermic reaction

• In exothermic reactions:

  • Energy is given out to the surroundings

  • The energy of the products will therefore be lower than the energy of the reactants

  • The overall energy change is negative

  • This is represented on the reaction profile with a downwards-arrow as the energy of the products is lower than the reactants

• In endothermic reactions:

  • Energy is taken in from the surroundings

  • The energy of the products will be higher than the energy of the reactants

  • The overall energy change is positive

  • This is represented on the reaction profile with an upwards-arrow as the energy of the products is higher than the reactants