3:07 (Triple only) use bond energies to calculate the enthalpy change during a chemical reaction

Each type of chemical bond has a particular bond energy. The bond energy can vary slightly depending what compound the bond is in, therefore average bond energies are used to calculate the change in heat (enthalpy change, ΔH) of a reaction.

Example: dehydration of ethanol

Note: bond energy tables will always be given in the exam, e.g:

BondAverage bond energy in kJ/mol

So the enthalpy change in this example can be calculated as follows:

Breaking bondsMaking bonds
BondsEnergy (kJ/mol)BondsEnergy (kJ/mol)
H-C x 5(412 x 5) = 2060C-H x 4(412 x 4) = 1648
C-O360O-H x 2(463 x 2) = 926
Energy needed to break all the bonds3231Energy released to make all the new bonds3186

Enthalpy change, ΔH = Energy needed to break all the bonds - Energy released to make all the new bonds

ΔH = 3231 – 3186 = +45 kJ/mol (ΔH is positive so the reaction is endothermic)