Bond Energy (AQA A Level Chemistry)

• During a reaction, enthalpy changes take place because bonds are being broken and formed
• Energy (in the form of heat) is needed to overcome attractive forces between atoms
• Bond breaking is therefore endothermic
• Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed
• Bond forming is therefore exothermic

To break bonds energy is required from the surroundings and to make new bonds energy is released from the reaction to the surroundings

• The sum of the energy in and energy out determines whether the reaction is overall exothermic or endothermic
• If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic
• If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic
• In reality, only some bonds in the reactants are broken and then new ones are formed