The standard molar enthalpies of formation of cyclohexane (l) and benzene (l) at 25∘C are -156 and +49 kJ/mol. The standard enthalpy of hydrogenation of cyclohexene (l) at 25∘C is -119 kJ/mol. Estimate the magnitude of resonance energy.
152 kJ/mol
C6H10(l)+H2(g)→C6H12(l) ΔH=−119kJ/mol
6C(s)+6H2(g)→C6H12(l) ΔfH=−156kJ/mol
6C(s)+3H2(g)→C6H6(l) ΔfH=+49kJ/mol
Two things are needed here,
C6H6(l)+3H2(g)→C6H12(l)
and benzene losing all its double donds and forming cyclohexane.
Subtracting the first from the second will give us the resonance energy.
We flip the formation of benzene equation, add it to the formation of cyclohexane equation:
C6H6(l)→6C(s)+3H2(g) ΔH=−49kJ/mol
6C(s)+6H2(g)→C6H12(l) ΔfH=−156kJ/mol––––––––––––––––––––––––––––––––––––––––––––––––––––––––
C6H6(l)+3H2(g)→C6H12(l) ΔH=−205kJ/mol
Hypothetical heat of hydrogenation of benzene
Resonance energy = |–357 – (–205)|
= |–152| kJ/mol
= 152 kJ/mol