UPSEE Solved Model Paper 3

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 :

C6H6

(g)

→

6C

(g)

+

6H

(g)

Resonance energy = experimental heat of formation - calculated heat of formation
Estimate the average SF bond energy in SF6. The standard heat of formation values of SF6(g),S(g) and F (g) are -1100, 275 and 80 kJ mol−1 respectively.