Bond Energies

During a chemical reaction, old bonds are broken and new bonds are formed
Energy must be supplied to break existing bonds, so bond breaking is an endothermic process
Energy is released when new bonds are formed, so bond formation is an exothermic process
In exothermic reactions the energy released by forming bonds is greater than the energy to break them
In endothermic reactions the energy used to break bonds is greater than the energy released by forming them

Every chemical bond has a particular bond energy associated with it
This bond energy varies slightly depending on the compound the bond occurs in
You can use these known bond energies to calculate the overall energy changes for a reaction
The overall energy change is the sum of the energies needed to break bonds in the reactants minus the energy released when the new bonds are formed in the products
You can’t compare the overall energy changes of reactions unless you know the numerical differences in the bond energies