Bond Energies

KEY POINTS:

• In molecules there are chemical bonds – and for this purpose of this page those bonds are all covalent.
• Each bond possesses a certain amount of internal energy (bond energy – measured in kJ/mol).
• In a reaction the bonds in the reactants need to be broken – this requires energy to be put into the reaction. This is an endothermic process, with the total energy required to break all the bonds equivalent to the activation energy for the reaction.
• Bonds are then made as the products form. This is an exothermic process as the atoms are moving from a high energy state to a lower one as new molecules form.
• To calculate the overall enthalpy change for the reactions the sum total to break all the bonds in the reactants is calculated with the total energy released when new bonds are formed then taken away.
• Whether or not a reaction is endo- or exothermic overall depends on if it takes more energy to break then bonds than is released when new bonds are made.
• Catalysts act by weakening the bonds in the reactants, therefore lowering the overall activation energy and allowing more particles to successfully react.