Q10.67CP - Chemistry: Molecular Nature Of Matter And Change

Question

When ${\mathbf{SO}}_{\mathbf{3}}$ gains two electrons, ${\mathbf{SO}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}$ forms. 

1. Which depiction showed below best illustrates the change in molecular shape around S?
2. Does molecular polarity change during this reaction?

Step-by-Step Solution

Answer

(a) The depiction “C” best illustrates the change in molecular shape around S. 

(b) Yes, the molecular polarity changes during this reaction.

Step 1: The depiction which best illustrates the change in molecular shape around S.

The molecular shapes of the molecules from the Lewis structure are determined by employing Valence-Shell Electron-Pair Repulsion (VSEPR) theory. The molecular shapes are assigned specific ${\mathrm{AX}}_{\mathrm{m}}{\mathrm{E}}_{\mathrm{n}}$ designations.

Where m and n are integers

A - central atom

X - Surrounding atom 

E - Nonbonding valence-electron group (usually a lone pair). 

As ${\mathrm{SO}}_3$ having trigonal planar shape gains two electrons it forms ${\mathrm{SO}}_3^{2-}$. This has a trigonal bipyramidal shape. The depiction “C” alone shows the change in molecular shape around S, while the rest of the other depiction does not match to the change in molecular shape around S. The trigonal planar shape of ${\mathrm{SO}}_3$ with ${\mathrm{AX}}_3$ designation changes to the trigonal bipyramidal shape of ${\mathrm{SO}}_3^{2-}$ with ${\mathrm{AX}}_3{\mathrm{E}}_1$ designation on gaining two electrons.

Step 2: Change in the Molecular Polarity

The molecule ${\mathrm{SO}}_3$ having a trigonal planar shape gains two electrons to form ${\mathrm{SO}}_3^{2-}$. The molecular polarity changes during this reaction. The molecule ${\mathrm{SO}}_3$ is nonpolar, while ${\mathrm{SO}}_3^{2-}$ is polar. 

In ${\mathrm{SO}}_3$ the shape of the molecule is a trigonal planar. As  has more electronegativity value than  with an electronegativity difference of 0.9, making the S-O bond more polar as each bond dipole points towards O. Even though the S-O bond is more polar as each bond dipole points towards O, the bond polarities are counterbalanced and the molecule shows no significant dipole moment making the molecule ${\mathrm{SO}}_3$ nonpolar. 

In ${\mathrm{SO}}_3^{2-}$ the S-O bond is more polar as each bond dipole points towards O. The lone pair in ${\mathrm{SO}}_3^{2-}$ contributing for the net dipole moment. As the bond polarities are not counterbalanced, the molecule shows a significant dipole moment making the molecule polar.

Thus, depiction “C” correctly shows the change in molecular shape around S and the molecular polarity changes during this reaction.