• question_answer 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)\xrightarrow{\,}\,H-Cl(g),$$\Delta {{H}_{f}}=-431\,\,kJ\,\,mo{{l}^{-1}}$or bond energy of $H-Cl=-(\Delta {{H}_{f}})$$=-(431)=+431\,kJ\,mo{{l}^{-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 incase of compounds having alternate double bonds. Example                $\underset{(g)}{\mathop{{{C}_{6}}{{H}_{6}}}}\,\xrightarrow{\,}\underset{(g)}{\mathop{\,6C}}\,+\underset{(g)}{\mathop{6H}}\,$ Resonance energy = experimental heat of formation - calculated heat of formation The polymerization of ethylene to linear polyethylene is represented by the reaction$nC{{H}_{2}}=C{{H}_{2}}\xrightarrow{\,}\,{{(-C{{H}_{2}}-C{{H}_{2}}-)}_{n}}$ when 'n' has a large integral value. Given that average enthalpies of bond dissociation for $C=C$ and $C-C$ at 298K are + 590 and$+331\,\,kJ\,\,mo{{l}^{-1}}$ respectively. Then the enthalpy of polymerization/mol of ethylene at298 K is A)  - 132 kJ                      B)  - 72 kJ C)  + 80 kJ                      D)  + 162 kJ

$n(C{{H}_{2}}=C{{H}_{2}})\xrightarrow{\ }\,{{(-C{{H}_{2}}-C{{H}_{2}}-)}_{n}},$$\Delta H=?$ Thus, ?n? double bonds are dissociated to form a molecule with 2n single bonds. $\Delta H=\left( \frac{n}{n} \right)({{\Sigma }_{C=C}})-\left( \frac{2n}{n} \right)({{\Sigma }_{C=C}})$$=+590-2\times 331=-72\,kJ\,mo{{l}^{-1}}$
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