6.7 Bond Enthalpies

Calculate the enthalpy change of a reaction based on the average bond energies of bonds broken and formed in the reaction.

• During a chemical reaction, bonds are broken and/or formed, and these events change the potential energy of the system.
• The average energy required to break all of the bonds in the reactant molecules can be estimated by adding up the average bond energies of all the bonds in the reactant molecules. Likewise, the average energy released in forming the bonds in the product molecules can be estimated. If the energy released is greater than the energy required, the reaction is exothermic. If the energy required is greater than the energy released, the reaction is endothermic.