Energy must always be supplied to break bonds

• 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

Bond energy calculations-need to be practised

• 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