The standard molar enthalpies of formation of cyclohexane (I) and benzene(I) at `25^(@)C` are `-156` and `+49 KJ mol^(-1)` respectivelyl. The standard enthalpy of hydrogentation of cyclohexene (I) at `25^(@)` is `-119 KJ mol^(-1)`. Use these data to estimate the magnitude of the resonance energy of benzene.

Explanation

This implies that the generation of one double bond in cyclohexane requires 119 kJ/mol of enthalpy. To calculate the resonance energy we may proceed as follows:

`DeltaH_(1)^(@) = DeltaH_(2)^(@) = DeltaH_(3)^(@) = 119` kJ/mol.
`DeltaH_(4)^(@)` = resonance energy
`DeltaH_(5)^(@) = DeltaH_(r)^(@)("Benzene") - DeltaH_(f)^(@)("Cyclohexane")`
`=49-(-156)`
According to Hess.s law
`DeltaH_(4)^(@) = DeltaH_(5)^(@) - (DeltaH_(1)^(@) + DeltaH_(2)^(@) + DeltaH_(3)^(@))`
=205 - 3 `xx 119`
`=-152` kJ/mole