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Chemistry Question on Thermodynamics

The enthalpy of hydrogenation of cyclohexene is −119.5 kJ mol−1-119.5\, kJ\, mol ^{-1}. If resonance energy of benzene is −150.4 kJ mol−1-150.4\, kJ\, mol ^{-1}, its enthalpy of hydrogenation would be

A

−208.1 kJ mol−1-208.1\, kJ\, mol ^{-1}

B

−269.9 kJ mol−1-269.9\, kJ\, mol ^{-1}

C

−358.5 kJ mol−1-358.5\, kJ\, mol ^{-1}

D

−508.9 kJ mol−1-508.9\, kJ\, mol ^{-1}

Answer

−208.1 kJ mol−1-208.1\, kJ\, mol ^{-1}

Explanation

Solution

The enthalpy of hydrogenation of cyclohexene (having one double bond) is −119.5 kJ mol−1-119.5\, kJ\, mol ^{-1}. Hence, theoretically enthalpy of hydrogenation of Kekule's structure of benzene should be 3×(−119.5)kJ mol−13 \times(-119.5) kJ\, mol ^{-1}. Since the resonance energy of benzene is −150.4 kJ mol−1-150.4\, kJ\, mol ^{-1}, the actual enthalpy of hydrogenation of benzene will be, =[3×(−119.5)]−(−150.4)=[3 \times(-119.5)]-(-150.4) =−208.1 kJ mol−1=-208.1\, kJ\, mol ^{-1}.