Question

How I can calculate the resonance energy from the enthalpy of formation and energy bonds for an aromatic compound.

Answer 1

Determination of resonance energy

Resonance energy is the extra energy released from the molecule for extra stability ( due to resonance )

If a compound exhibits resonance, there is a considerable difference between the enthalpies of formation as calculated from bond energies and those determined experimentally. As an example we may consider the dissociation of benzene.

      C6H6 (g) ———> 6C(g) + 6H(g)

Assuming that benzene ring consists of three single and three double bonds (Kekule’s structure) the calculated dissociation energy comes out to be 5384.1 KJ from bond energies data.

  ΔHd = 3ΔHC–C + 3ΔHC=C + 6ΔHC–H

The experimental value is known to be 5535.1 KJ/mol. Evidently, the energy required for the dissociation of benzene is 151 KJ more that the calculated value 5384.1 KJ. The difference of 151 KJ gives the resonance energy of benzene.