Values of measured bond energies may vary greatly depending on the molecule studied. Consider the following reactions:
NCl$_3$ (g) $\rightarrow$ NCl$_2$ (g) + Cl(g) $\Delta$H = 375 kJ/mol
ONCl(g) $\rightarrow$ NO(g) + Cl(g) $\Delta$H = 158 kJ/mol
Rationalize the difference in the values of $\Delta$H for these reactions, even though each reaction appears to involve only the breaking of one N$-$Cl bond. (Hint: Consider the bond order of the NO bond in ONCl and in NO.)
O$-$N$-$Cl: The bond order of the NO bond in ONCl is
NO: From molecular orbital theory, the bond order of this NO bond is
In the reaction
ONCl(g) $\rightarrow$ NO(g) + Cl(g),
some energy is in forming the stronger NO bond, lowering the value of $\Delta$H.
