The standard molar enthalpies of formation of cyclohexane (l) and benzene (l) at `25^(@)C` are `-156` and `+49 kJ mol^(-1)`, respectively. The standard enthaly of hydrogenation of cyclohenxene (l) at `25^(@)C` is `-119 kJ mol^(-1)` Use this data to estimate the magnitude of the resonance enegry of benzene.

Explanation

Cyclohexene `(l)+H_(2)(g)to"Cyclohexane"(l),DeltaH=-119kJ`
Enthalpy of formation of cyclohexane(l)=-156kJ `mol^(-1)`
So, enthalpy of formation of cyclohexane`=-156-(-119)kJ`
`=-37kJ` `mol^(-1)`
Since, `DeltaH_("cyclohexane")` is `-156" kJ "mol^(-1)`, we can say that for energy double bond the energy decreases by an amount +119kJ `mol^(-1)` and therefore for the introduction of three double bonds (present in benzene ring) the energy required
`=3xx119kJ" "mol^(-1)=357" kJ "mol^(-1)`
Hence, theoretical `DeltaH_(f)` for benzene`=(357-156)kJ" "mol^(-1)` ,brgt Resonance energy=Theoretical `DeltaH_(f)-`Observed `DeltaH_(f)`
`=(201~49)" kJ "mol^(-1)=152kJ" "mol^(-1)`