The standard molar enthalpies of formation of cyclohexane $(l)$ and benzene $(l)$ at $25^{o} C$ are $- 156$ and $+ 49 k J {m o l}^{- 1}$ respectively. The standard enthalpy of hydrogenation of cyclohexene $(l)$ at $25^{o} C$ is $- 119 k J {m o l}^{- 1}$ . Use these data to estimate the magnitude of resonance energy of benzene.

Open in App

Solution

Cyclohexene $(l) + H_{2} (g) \to$ Cyclohexene $(l); Δ H = - 119 k J$
Enthalpy of formation of cyclohexane $(l) = - 156 k J {m o l}^{- 1}$
So, enthalpy of formation of cyclohexane $= - 156 - (119) k J$
$= - 37 k J {m o l}^{- 1}$
Since, $Δ H_{c y c l o h e x a n e}$ is $- 156 k J {m o l}^{- 1}$ , we can say that for every double bond the energy decreases by an amount $+ 119 k J {m o l}^{- 1}$ and therefore for the introducton of three double bonds (present in benzene ring) the energy required
$3 \times 119 k J {m o l}^{- 1} = 357 k J {m o l}^{- 1}$
Hence, theoritical $Δ H_{f}$ for benzene $= (357 - 156) k J {m o l}^{- 1}$
$= 201 k J {m o l}^{- 1}$
Resonance energy $=$ Theoretical $Δ H_{f} -$ Observed $Δ H_{f}$
$(201 - 49) k J {m o l}^{- 1} = 152 k J {m o l}^{- 1}$

Suggest Corrections

Similar questions

Q. The standard molar enthalpies of formation of cyclohexane

(l)

and benzene

(l)

25^{o} C

are

- 156

and

+ 49

k J m o l^{- 1}

respectively. The standard enthalpy of hydogenation of cyclohexene

(l)

25^{o} C

- 119 k J m o l^{- 1}

.
Use these data to estimate the magnitude of resonance energy of benzene in

k J / m o l

The standard molar enthalpies of formation of cyclohexane (l) and benzene (l) at $25^{\circ} C$ are -156 and +49 kJ/mol. The standard enthalpy of hydrogenation of cyclohexene (l) at $25^{\circ} C$ is -119 kJ/mol. Estimate the magnitude of resonance energy.