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Question

If the enthalpy of hydrogenation of C6H6(l) into C6H12(l) is - 205 kJ and the resonance energy of C6H6(l) is -152 kJ/mol then the enthalpy of hydrogenation of cyclohexene is: (Assume ΔHvap of C6H6(l), C6H8(l), C6H12(l) all are equal).

A
535.5 kJ/mol
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B
238 kJ/mol
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C
357 kJ/mol
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D
119 kJ/mol
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Solution

The correct option is D 119 kJ/mol
Point to remember:
1. Resonance energy =ΔHexperimentalΔHcalculated

To find heat of hydrogenation of
C6H10+H2C6H12 ΔH=?
We need to find the energy required to break one C=C and the energy released when 2 CH bonds are formed. In benzene, 3 C=C bonds break and 6 CH bonds are formed.
So Ebreak(C=C)+form 2(C=H)=calculated energy of C6H63
Ebreak(C=C)+form 2(C=H)=205+(152)3=119kJ

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