• # 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 Estimate the average S?F bond energy in $S{{F}_{6}}$. The standard heat of formation values of$S{{F}_{6}}(g),\,\,S(g)$ and $F(g)$ are -1100, 275 and $80\,\,kJ\,mo{{l}^{-1}}$ respectively. A)  309.17 kJ                    B)  206 kJ              C)  109.2 kJ                      D)  275.8 kJ

Given $S(s)+3{{F}_{2}}(g)\xrightarrow{\,}S{{F}_{6}};$ $\Delta H=-1100\,kJ$                                    ?(i) $\frac{1}{2}{{F}_{2}}(g)\xrightarrow{\,}\,F(g);$    $\Delta H=80\,\,kJ$    ?(ii) $S(s)\xrightarrow{\,}\,S(g);\,\,\Delta H=+275\,kJ$                ?(iii) Subtracting Eq. (iii) from Eq. (ii) with 6 gives $S(g)+3{{F}_{2}}(g)\,\beta \,\xrightarrow{\,}\,6{{F}_{6}};$$\Delta H=-1100-275$ $=-1375\,\,kJ$            ?(iv) Subtracting Eq. (ii) with 6 gives $3{{F}_{2}}(g)\xrightarrow{\,}\,6F(g);\,\,\Delta H=6\times 80\,kJ$ $=480\,kJ$     ?(v) Subtracting Eq. (v) from Eq. (iv). $S(g)+6F(g)\xrightarrow{\,}\,S{{F}_{6}};$ $\Delta H=-1375-480=-1855\,kJ$ or         $S{{F}_{6}}\xrightarrow{\,}\,S(g)+6F(g);$ $\Delta H=+1855\,kJ$ Average $BE=\frac{1855}{6}=309.17\,kJ$