Question

The standard molar enthalpies of hydrogenation of benzene ($l$) at $25{}^{o}C$ is $-208{\text{kJ mol}}^{-1}$. The standard enthalpy of hydrogenation of cyclohexene$\left(l\right)$ at $25{}^{o}C$ is $-119{\text{kJ mol}}^{-1}$. Use these data to estimate the magnitude of the resonance energy of benzene.
(correct answer +1, wrong answer -0.25)

• A. $+208{\text{kJ mol}}^{-1}$
• B. $180{\text{kJ mol}}^{-1}$
• C. $151{\text{kJ mol}}^{-1}$
• D. $+100{\text{kJ mol}}^{-1}$

Answer 1

The correct option is C $151{\text{kJ mol}}^{-1}$

Enthalpy of hydrogenation of cylcohexene $=-119{\text{kJ mol}}^{-1}$
Expected hydrogenation of benzene $=3\times -119=-357{\text{kJ mol}}^{-1}$
Experimental hydrogenation of benzene $=-208{\text{kJ mol}}^{-1}$
Resonance energy of benzene $=-208-(-357)=151{\text{kJ mol}}^{-1}$