Question

The standard molar enthalpies of formation of cyclohexane (l) and benzene (l) at 25°C are - 156 and + 49 kJ mol^-1 , respectively. The standard enthalpy of hydrogenation of cyclohexene (l) at 25°C is – 119 kJ mol^-1 . The magnitude of the resonance energy is :

a) - 357 kJ mol^-1

b) - 152 kJ mol^-1 

c) - 205 kJ mol^-1

d) + 152 kJ mol_-1

Answer 1

Correct option is b) - 152 kJ mol^-1

Enthalpy change for the hydrogenation of benzene should be three times the enthalpy of hydrogenation of cyclohexene

This reaction also gives the energy of formation of cyclohexane.

The difference in the enthalpy of formation calculated by these alternate paths is due to resonance stabilization of benzene.

Resonance energy of benzene = - 308 – (-156) -1 = -152 kJ mol^-1