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UPSEE Solved Model Paper 3

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Direction : The bond dissociation energy of a diatomic molecule is also called bond energy. Bond energy is also called, the heat of formation of the bond from the gaseous atoms constituting the bond with reverse sign.
Example : H (g) + Cl (g) → H - Cl (g) , ΔHf\Delta H_f = −431 kJ mol−1-431\,\mathrm{kJ}\,\mathrm{mol}^{-1} or bond energy of H - Cl = −(ΔHf)-(\Delta H_f) = - (431) = + 431 kJ mol−1\mathrm{mol}^{-1}. When a compound shows resonance there occurs a fair agreement between the calculated values of heat of formation obtained from bond enthalpies and any other method. However deviation occur insane of compounds having alternate double bonds.
Example : C6H6(g)\underset{(g)}{\mathrm{C}_6\mathrm{H}_6} → 6C(g)+6H(g)\underset{(g)}{6\mathrm{C}} + \underset{(g)}{6\mathrm{H}}
Resonance energy = experimental heat of formation - calculated heat of formation
If (i) ΔHf∘\Delta H_f^\circ (benzene) = −358.5 kJ mol−1-358.5\,\mathrm{kJ}\,\mathrm{mol}^{-1}
(ii) Heat of atomization of graphite = 716.8 kJ mol−1716.8\,\mathrm{kJ}\,\mathrm{mol}^{-1}
(iii) Bond energy of C - H, C - C, C = C and H - H bonds are 490, 340, 620 and 436.9 kJ mol−1\mathrm{mol}^{-1} respectively. The resonance energy (in kJ mol−1\mathrm{mol}^{-1}) of C6H6\mathrm{C}_6\mathrm{H}_6 using Kekule formula is
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