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UPSEE Solved Model Paper 3

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Question : 69

Total: 150

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 = −431kJmol−1 or bond energy of H - Cl = −(ΔHf) = - (431) = + 431 kJ 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 :

Resonance energy = experimental heat of formation - calculated heat of formation
If (i) ΔHf° (benzene) = −358.5kJmol−1
(ii) Heat of atomization of graphite = 716.8kJmol−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 respectively. The resonance energy (in kJ mol−1) of C6H6 using Kekule formula is

- 150
- 50
- 250
+ 150

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