Question

The standard molar enthalpies of formation of cycohexane (l) and benzene (l) at $25^\circ C$ are -156 and +49 KJ $mol^{- 1}$ respectively. The standard enthalpy of hydrogenation of cyclo hexene (l) at $25^\circ C$ is -116 KJ $mol^{- 1}$ . Use these data to estimate the magnitude of the resonance energy of benzene.

• A. $\text{152} \, \text{kJ} \, \text{mol}^{- 1}$
• B. $1.52 \, \text{kJ} \, \text{mol}^{- 1}$
• C. $15.2 \, \text{kJ} \, \text{mol}^{- 1}$
• D. None of these

Answer 1

Answer: (A)

Enthalpy of hydrogenation of these double bond in benzene
$= 3 \times - 119 = - 357 \, \text{kJ} \, \text{mol}^{- 1}$
$\text{ΔH(reaction)} \, \text{=} \, \text{ΣH(products)} - \text{ΔΣH(reactants)}$
$= - 156 - \left(49 + 0\right) = - 205 \, \text{kJ} \, \left(\text{mol}\right)^{- 1}$
Resonance energy = Calculated heat of hydrogenation - Actual heat of hydrogenation
$= 357 - 205 = 152 \, \text{kJ} \, \text{mol}^{- 1} .$