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

Standard enthalpy of hydrogenation of cyclohexene `= -119 kJ mol^(-1)`. This is the observed enthalpy of hydrogenation ofone double bond ( present in cyclohexene) . If benezene is considered as cyclohexatriene, the observed enthalpy of the reaction.
wouldbe `= 3 xx(-119)=- 357 kJ mol^(-1)`
Calculated enthalpy of the above reaction is
`Delta_(r)H =DeltaH_(f)^(@) (C_(6)H_(14)) - [ DeltaH_(f)^(@) (C_(6)H_(6))+3DeltaH_(f)^(@)(H_(2))]= - 156 - ( 49+0) = - 2205 kJmol^(-1)`
`:. ` Resonance energy = Observed `Delta_(r)H^(@) -` Calculated `Delta_(r)H^(@)`
`= ( - 357 ) - ( - 205) kJ mol^(-1) = -152 kJ mol^(-1)`