The enthalpy of hydrogenation of cyclohexene is -119.5 ~kJ ~mol^ -1 . If resoance energy of benzene is -150 kJ mol^ -1 , its enthalpy of hydrogenation would be:

Correct answer: C. -208.1 ~kJ ~mol^ -1 Heat of hydrogenation of cyclohaxene =-119.5 ~kJ / mol Heat of hydrogenation of benzene =3 ×-119.5=-358.5 ~kJ / mol Resonance energy = Observed Δ H- Calculated Δ H =-150.4=-358.5-x x=-208.1 ~kJ Related Theory The energy calculated for a resonance hybrid is lower than the energies of any of the alternative structures; the molecule is then said to be stabilized by resonance.

NEET 2006

Exam: NEET
Year: 2006
Subject: Chemistry
Chapter: Thermodynamics (C)

NEET 2006 · Chemistry · Thermodynamics (C)

The enthalpy of hydrogenation of cyclohexene is -119.5 ~kJ ~mol^ -1...

The enthalpy of hydrogenation of cyclohexene is

-119.5 \mathrm{~kJ} \mathrm{~mol}^{-1}

. If resoance energy of benzene is -150

\mathrm{kJ} \mathrm{mol}^{-1}

, its enthalpy of hydrogenation would be:

Options

A $-358.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$
B $-508.9 \mathrm{~kJ} \mathrm{~mol}^{-1}$
C $-208.1 \mathrm{~kJ} \mathrm{~mol}^{-1}$
D $-269.9 \mathrm{~kJ} \mathrm{~mol}^{-1}$

Correct answer

C. $-208.1 \mathrm{~kJ} \mathrm{~mol}^{-1}$

Step-by-step solution

\therefore

Heat of hydrogenation of cyclohaxene

=-119.5 \mathrm{~kJ} / \mathrm{mol}

\therefore

Heat of hydrogenation of benzene

=3 \times-119.5=-358.5 \mathrm{~kJ} / \mathrm{mol}

Resonance energy = Observed

\Delta \mathrm{H}-

Calculated

\Delta \mathrm{H}

=-150.4=-358.5-x

x=-208.1 \mathrm{~kJ}

Related Theory The energy calculated for a resonance hybrid is lower than the energies of any of the alternative structures; the molecule is then said to be stabilized by resonance.

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