Question

The enthalpy of hydrogenation of cyclohexene is $-119.5\text{kJ/ mol}$. If resonance energy of benzene is $-150.4\text{kJ/mol}$ then what is its enthalpy of hydrogenation?

• A. $-259.9\text{kJ/mol}$
• B. $-358.5\text{kJ/mol}$
• C. $-508.9\text{kJ/mol}$
• D. $-208.1\text{kJ/mol}$

Answer 1

The correct option is D $-208.1\text{kJ/mol}$

As, Enthalpy of hydrogenation of cyclohexene is $-119.5\text{kJ/mol}$, i.e, Hydrogenation of one $C=C$ is $-119.5\text{kJ/mol}$.
$\therefore \Delta H_{cal}\text{for benzene is}3\times -119.5=-357\text{kJ/mol}$
So,
$\Delta H_{obs}=\Delta H_{cal}+\text{Energy needed to disturb resonance}$
$=(3\times -119.5)+150.4=-208.1\text{kJ/mol}$
Thus, enthalpy of hydrogenation of benzene is $-208.1\text{kJ/mol}$.