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The standard molar enthalpies of formati...

The standard molar enthalpies of formation of cyclohexane (l) and benzene at `25^@C` are `-156` and `+ 49 kJ mol^(–1)` respectively. The standard enthalpy of hydrogenation of cyclohexene at is `kJ mol^(–1)`. Use these data to estimate the magnitude of the resonance energy of benzene.

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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
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