Question

Values of measured bond energies may vary greatly depending on the molecule studied. Consider the following reactions:

$$\begin{gathered} \mathit{N}\mathit{C}{\mathit{l}}_{\mathbf{3}\mathbf{\left(}\mathbf{g}\mathbf{\right)}}\mathbf{\to }\mathit{N}\mathit{C}{\mathit{l}}_{\mathbf{2}\mathbf{\left(}\mathbf{g}\mathbf{\right)}}\mathbf{+}\mathit{C}{\mathit{l}}_{\mathbf{\left(}\mathbf{g}\mathbf{\right)}}\mathit{\Delta }\mathit{H}\mathbf{=}\mathbf{375}\mathit{k}\mathit{J}\mathbf{/}\mathit{m}\mathit{o}\mathit{l} \\ \mathit{O}\mathit{N}\mathit{C}{\mathit{l}}_{\mathbf{\left(}\mathbf{g}\mathbf{\right)}}\mathbf{\to }\mathit{N}{\mathit{O}}_{\mathbf{\left(}\mathbf{g}\mathbf{\right)}}\mathbf{+}\mathit{C}{\mathit{l}}_{\mathbf{\left(}\mathbf{g}\mathbf{\right)}}\mathit{\Delta }\mathit{H}\mathbf{=}\mathbf{158}\mathit{k}\mathit{J}\mathbf{/}\mathit{m}\mathit{o}\mathit{l} \end{gathered}$$
Rationalize the difference in the values of$\Delta H$for these reactions, even though each reaction appears to involve the breaking of only one N-Cl bond. (Hint: Consider the bond order of the NO bond in ONCl and in NO.)

Short answer

$\Delta H$represents the change in the enthalpy which tells us the difference between the energy required for the formation and dissociation of a bond.

Step-by-step solution

Molecular orbital configuration of NO and ONCl

$$\begin{gathered} NO\left(11\right)=(\sigma 1s{)}^2({\sigma }^{*}1s{)}^2(\sigma 2s{)}^2({\sigma }^{*}2s{)}^2(\sigma 2p_z{)}^2(\pi 2p_x{)}^1 \\ ONCI\left(18\right)=(\sigma 1s{)}^2({\sigma }^{*}1s{)}^2(\sigma 2s{)}^2({\sigma }^{*}2s{)}^2(\sigma 2p_z{)}^2(\pi 2p_x{)}^2(\pi 2p_y{)}^2({\pi }^{*}2p_x{)}^2({\pi }^{*}2p_y{)}^2 \end{gathered}$$

Bond order of NCl3 and ONCl

$$\begin{gathered} BondorderofNO=\frac{No.ofbondingelectrons-no.ofnonbondingelectrons}{2} \\ =\frac{7-4}{2} \\ =1.5 \end{gathered}$$

$$\begin{gathered} BondorderofONCl=\frac{No.ofbondingelectrons-no.ofnonbondingelectrons}{2} \\ =\frac{10-8}{2} \\ =1 \end{gathered}$$

Step 3: Difference in ∆H

As we can see that the bond order of ONCl is less than NO, so it will require less than energy to dissociate the bond as compared to the formation of the bond. Moreover, among NCl₃and ONCl, the value of enthalpy change is more for NCl₃because the nitrogen is bonded with same electronegative atom that is chlorine so each bond will have same strength. Whereas in ONCl, oxygen is the most electronegative element so it will attract electron more as compared to the chlorine atom and thus it weakens the bond strength of N-Cl. That’s why the enthalpy is more for NCl₃ .