When a chemical reaction occurs, old bonds are broken and new bonds are formed. Breaking bonds always requires energy, and forming a bond always releases energy. The amount of energy required to break one mole of a particular bond to form neutral atoms is called the bond dissociation energy. In contrast, the amount of energy released when neutral atoms form one mole of a bond is called bond energy. The difference between the bond dissociation energy and the bond energy determines whether the reaction absorbs or releases energy.
The enthalpy change in a chemical reaction is the sum of energies absorbed and released in bond breaking and bond forming:
ΔH° = Sum of bond dissociation energies of reactants - Sum of bond energies of products
Bond energy of H-H, I-I, and H-I bonds are 436 kJ/mol, 151 kJ/mol, and 299 kJ/mol respectively.
Problem Solving Strategy:
ΔH° = Sum of bond dissociation energies of reactants - Sum of bond energies of products
ΔH° = [B.E of H-H + B.E. of I-I] - [2 × B.E of H-I]
ΔH° = [436 + 151] - [2 × 299]
ΔH° = 587 - 598
ΔH° = -11 kJ/mol
Reaction Progress with Time
Note that the enthalpy of reaction calculated using bond energy data is often different from values determined experimentally.
Example: Calculate the enthalpy of the following reaction from the given bond energy data. Bond energy of H-H, F-F, and H-F bonds are 436 kJ/mol, 155 kJ/mol, and 567 kJ/mol respectively.