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 |
A) - 132 kJ
B) - 72 kJ
C) + 80 kJ
D) + 162 kJ
Correct Answer: B
Solution :
\[n(C{{H}_{2}}=C{{H}_{2}})\xrightarrow{\ }\,{{(-C{{H}_{2}}-C{{H}_{2}}-)}_{n}},\]\[\Delta H=?\] Thus, ?n? double bonds are dissociated to form a molecule with 2n single bonds. \[\Delta H=\left( \frac{n}{n} \right)({{\Sigma }_{C=C}})-\left( \frac{2n}{n} \right)({{\Sigma }_{C=C}})\]\[=+590-2\times 331=-72\,kJ\,mo{{l}^{-1}}\]You need to login to perform this action.
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