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Delta(f)H^(Theta) of Cyclohexene (l) and...

`Delta_(f)H^(Theta)` of Cyclohexene `(l)` and benzene at `25^(@)C` is `-156` and `+46 kJmol^(-1)`, respectively. `Delta_("hydrogenation")H^(Theta)` of cyclohexene `(l) at 25^(@)C is -119 kJ mol^(-1)`.
Reasonance energy of benzene is found to be `-38x kJ mol^(-1)`. Find the value of `x`.

Text Solution

Verified by Experts


This shows that the generation of one `(C =C)` bond in cyclohexene requires `119kJ mol^(-1)` of enthalpy. To calculate `RE` resonance energy.

`DeltaH_(1)^(Theta) = DeltaH_(2)^(Theta) = DeltaH_(3)^(Theta) = 119 kJ mol^(-1), DeltaH_(4)^(Theta) = RE`
`DeltaH_(5)^(Theta) = Delta_(f)H^(Theta) ("benzene") - Delta_(f)H^(Theta) ("cyclohexene")`
`= 46 -(-156) = 205 kJ mol^(-1)`.
`:.` From Hess' law:
`DeltaH_(4)^(Theta) = DeltaH_(5)^(Theta) - (DeltaH_(1)^(Theta) +DeltaH_(2)^(Theta) +DeltaH_(3)^(Theta))`
`= 205 - 3 xx 119 =- 152 kJ mol^(-1)`
`:. -38x =- 152`
`x = 4`
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The enthalpy of hydrogenation of cyclohexene is -119.5kJ mol^(-1) . If resonance energy of benzene is -150.4kJ mol^(-1) , its enthalpy of hydrogenation would be :

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 enthaly of hydrogenation of cyclohenxene (l) at 25^(@)C is -119 kJ mol^(-1) Use this data to estimate the magnitude of the resonance enegry of benzene.

Knowledge Check

  • Enthalpy of hydrogenation of cyclohexene is -119-5 "kJ mol"^(-1) . If resonance energy of benzene is - 150.4 kJ mol^(-1) , its enthalpy of hydrogenation would be

    A
    `-358.5 "kJ mol"^(-1)`
    B
    `-508.9 "kJ mol"^(-1)`
    C
    `-208.1 "kJ mol"^(-1)`
    D
    `-269.9 "kJ mol"^(-1)`

  • The enthalpy of hydrogenation of cyclohexene is -119.5 "kJ mol"^(-1) . If resonance energy of benzene is -150.4 "kJ mol"^(-1) its enthalpy of hydrogenation would be

    A
    `-358.5 "kJ mol"^(-1)`
    B
    `-508.9 "kJ mol"^(-1)`
    C
    `-208.1 "kJ mol"^(-1)`
    D
    `-269.9 "kJ mol"^(-1)`

  • The enthalpy of hydrogenation of cyclohexene is -119.5 kJ mol^(-1) . If resonance energy of benzene is -150.4kJ mol^(-1) . Its enthalpy of hydrogention would be

    A
    `-269.9 kJ mol^(-1)`
    B
    `-358.5 kJ mol^(-1)`
    C
    `-508.9 kJ mol^(-1)`
    D
    `-208.1 kJ mol^(-1)`