Thermodynamics And Thermochemistry Result 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 $-120 \mathrm{~kJ} \mathrm{~mol}^{-1}$.
Use these data to estimate the magnitude of the resonance energy of benzene.
(1996, 2 M)
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Answer:
Correct Answer: 20. $(-152 $ $kJ/mol)$
Solution:
$-357 \mathrm{~kJ}=\Delta H_f^{\circ}\left(\mathrm{C}_6 \mathrm{H}_6\right)-\Delta H_f^{\circ} \text { (cyclohexane) }$
$\Rightarrow \Delta H_f^{\circ}\left(\mathrm{C}_6 \mathrm{H}6\right){\text {Theoretical }}=+201 \mathrm{~kJ} $
$\Rightarrow \text { Resonance energy }=\Delta H_f^{\circ} \text { (exp. })-\Delta H_f^{\circ} \text { (Theoretical) } $
$=49-201=-152 \mathrm{~kJ} / \mathrm{mol}$
BETA
