Bond Making & BreakingOxford AQA IGCSE Chemistry: Revision notes

Chemical reactions involve breaking bonds in the reactants and forming new bonds in the products.

• Breaking bonds requires energy to be supplied (an input of energy) — this is an endothermic process
• Forming bonds releases energy to the surroundings — this is an exothermic process

The overall energy change of a reaction depends on the balance between these two processes.

In an exothermic reaction, more energy is released when new bonds form in the products than is needed to break the bonds in the reactants.

• Energy released (bond forming) is greater than energy supplied (bond breaking)
• The extra energy is transferred to the surroundings, so the temperature of the surroundings increases
• Examples: combustion, many oxidation reactions, neutralisation

In an endothermic reaction, more energy is needed to break the bonds in the reactants than is released when new bonds form in the products.

• Energy supplied (bond breaking) is greater than energy released (bond forming)
• The reaction takes in more energy than it releases, so the surroundings cool down (temperature decreases)
• Example: thermal decomposition reactions

The overall energy change of a reaction can be calculated using bond dissociation energies (the energy needed to break one mole of a particular bond):

Energy change = (total energy to break bonds in reactants) minus (total energy released forming bonds in products)

• If the result is positive, more energy was needed to break bonds than was released, so the reaction is endothermic
• If the result is negative, more energy was released forming bonds than was needed to break them, so the reaction is exothermic