Question

The enthalpy of hydrogenation of cyclohexane os -119.5 Kj/mol. If resonance energy of bemzene is -150.4 kj/mol, its enthalpy of hydrogenation would be?

Ans: -208.1 kj/mol.

How?

Answer 1

Dear student!

The hydrogenation of Cyclohexene forms cyclohexane as:

C₆ H₁₀ + H₂ -> C₆ H₁₂ ΔH = –119.5 kJ /mol

As the breaking of 1 C=C bond and the formation of 2 C-H bonds has a net release of 119.5 kJ/ mol, so in case of benzene, 3 C=C bond is hydrogenated by, 3 x 119.5 kJ/ mol = 358.5 kJ/ mol energy.

Now, since benzene has resonance energy so enthalpy change of hydrogenation is = 358.5 - 150.4 =208.1kJ/mol.