10.51_P - Chemistry: Molecular Nature Of Matter And Change

Question

In the gas phase, phosphorous pentachloride exists as separate molecules. In the solid phase, however, the compound is composed of alternating ${{\mathbf{PCl}}_4}^{+}$  and ${{\mathbf{PCl}}_6}^{-}$ ions. What change(s) in molecular shape occur(s) as  ${\mathbf{PCl}}_{\mathbf{5}}$ solidifies? How does the Cl-P-Cl angle change?

Step-by-Step Solution

Answer

In the solid phase, the molecular shape of phosphorous pentachloride becomes changes to tetrahedral with the bond angle $\mathbf{109}\mathbf{.}{\mathbf{5}}^o$  in ${{\mathbf{PCl}}_4}^{+}$   and octahedral with bond angles ${{\mathbf{PCl}}_6}^{-}$  and in ${{\mathbf{PCl}}_6}^{-}$  ions

gas phase

Phosphorous pentachloride has a molecular geometry of trigonal bipyramidal with ideal bond angles ${\mathbf{90}}^o$  and  ${\mathbf{120}}^o$ in gas phase.

solid phase

As ${\mathbf{PCl}}_5$  solidifies, the molecular shape and bond angle change. In solid phase, ${\mathbf{PCl}}_5$ is composed of alternating  ${{\mathbf{PCl}}_4}^{+}$  and  ${{\mathbf{PCl}}_6}^{-}$ ions. 

 In  ${{\mathbf{PCl}}_4}^{+}$, phosphorous has four valence electrons which forms four sigma bonds with chlorine atoms. Hence, the hybridization of  ${{\mathbf{PCl}}_4}^{+}$ ion is ${\mathbf{sp}}^3$   and the molecular shape becomes tetrahedral with ideal bond angle of   $\mathbf{109}\mathbf{.}{\mathbf{5}}^o$

 

In  ${{\mathbf{PCl}}_6}^{-}$ ,phosphorous has six valence electrons which forms six bonds with chlorine atoms. Hence, the hybridization ${{\mathbf{PCl}}_6}^{-}$ of ${\mathbf{sp}}^3{\mathbf{d}}^2$  ions   and the molecular shape becomes octahedral with ideal bond angle of  ${\mathbf{90}}^o$.