Question

Using one of the Lewis structures for benzene${\mathbf{\text{C}}}_{\mathbf{\text{6}}}{\mathbf{\text{H}}}_{\mathbf{\text{6}}}$estimate ${\mathbf{\Delta Hf}}^o$for(${\mathbf{\text{C}}}_{\mathbf{\text{6}}}{\mathbf{\text{H}}}_{\mathbf{\text{6}}}$g) using bond energies and given that the standard enthalpy of formation of C(g) is$\mathbf{\text{717kJ/mol}}$. The experimental ${\mathbf{\Delta Hf}}^o$value for${\mathbf{\text{C}}}_{\mathbf{\text{6}}}{\mathbf{\text{H}}}_{\mathbf{\text{6}}}$(g) is 83 kJ/mol. Explain the discrepancy between the experimental value and the calculated ${\mathbf{\Delta Hf}}^o$value for${\mathbf{\text{C}}}_{\mathbf{\text{6}}}{\mathbf{\text{H}}}_{\mathbf{\text{6}}}$(g).

Short answer

The resonance there is the discrepancy between the experimental value and the calculated ${\mathbf{\text{ΔH}}}_{\mathbf{\text{f}}}$value for benzene. Resonance makes benzene more stable than expected.

Step-by-step solution

Step 1:Discrepancy of Benzene

We need to explain the discrepancy between the experimental value of ${\text{ΔH}}_{\text{f}}$and the calculated${\text{ΔH}}_{\text{f}}$ for benzene.

Step 2: Lewis structures for benzene

Benzene has a planar ring structure. Benzene has two resonance structures, and all C-C bonds in benzene are equivalent. The delocalization of the electrons makes benzene ring behave quite differently from a typical unsaturated hydrocarbon.

Therefore, because of the resonance there is the discrepancy between the experimental value and the calculated ${\text{ΔH}}_{\text{f}}$value for benzene. Resonance makes benzene more stable than expected.