Thermodynamics and Thermochemistry 2 Question 20
20. The standard molar enthalpies of formation of cyclohexane $(l)$ and benzene $(l)$ at $25^{\circ} \mathrm{C}$ are -156 and $+49 \mathrm{~kJ} \mathrm{~mol}^{-1}$ respectively. The standard enthalpy of hydrogenation of cyclohexene $(l)$ at $25^{\circ} \mathrm{C}$ is $-119 \mathrm{~kJ} \mathrm{~mol}^{-1}$.
Use these data to estimate the magnitude of the resonance energy of benzene.
(1996, 2M)
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Solution:
$-357 \mathrm{~kJ}=\Delta H_{f}^{\circ}($ cyclohexane $)-\Delta H_{f}^{\circ}\left(\mathrm{C}{6} \mathrm{H}{6}\right)$
$\Rightarrow \Delta H_{f}^{\circ}\left(\mathrm{C}{6} \mathrm{H}{6}\right)_{\text {Theoretical }}=-156+357=201 \mathrm{~kJ}$
$\Rightarrow$ Resonance energy $=\Delta H_{f}^{\circ}$ (exp.) $-\Delta H_{f}^{\circ}$ (Theoretical)
$$ =49-201=-152 \mathrm{~kJ} / \mathrm{mol} $$
