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
−535.5kJ/mol
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B
−238kJ/mol
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C
−357kJ/mol
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D
−119kJ/mol
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Solution
The correct option is D−119kJ/mol Point to remember: 1. Resonance energy =ΔHexperimental−ΔHcalculated
To find heat of hydrogenation of C6H10+H2→C6H12ΔH=? We need to find the energy required to break one C=C and the energy released when 2 C−H bonds are formed. In benzene, 3 C=C bonds break and 6 C−H bonds are formed. So Ebreak(C=C)+form2(C=H)=calculatedenergyofC6H63 Ebreak(C=C)+form2(C=H)=−205+(−152)3=−119kJ