Chapter 18, Problem 8RAT

A substance consisting of which molecule shown below should have a higher boiling point?

$\text{S=C=O}$ $\text{O=C=O}$

(a) The molecule on the left, SCO, because it comes later in the periodic table.

(b) The molecule on the left, SCO, because it has less symmetry.

(c) The molecule on the right, OCO, because it has more symmetry.

(d) The molecule on the right, OCO, because it has more mass.

Solution

To determine: The correct option for the statement “A substance consisting of which molecule shown below should have a higher boiling point?”

Answer:

The correct option for the statement “A substance consisting of which molecule shown below should have a higher boiling point?” is option (b).

Explanation:

The dipole. is the segregation of charges. It happens because of difference in the values of electronegativities.

In the $\text{O}=\text{C}=\text{O}$ molecule, 2 $\text{C}=\text{O}$ bonds exist. The carbon atom contains less electronegativity compared to oxygen. So, the carbon will carry partial positive charge whereas oxygen will carry partial negative charge. The ${\text{CO}}_2$ molecule is basically symmetrical. The dipoles of two $\text{C=O}$ bonds will be identical and opposite. So, the identical and opposite values cancel each other and the overall dipole-dipole moment will be zero.

In $\text{S}=\text{C}=\text{O}$ molecule, $\text{S}=\text{C}$ one and one $\text{C=O}$ bonds are present. This molecule is not symmetrical. The dipole moment for $\text{S}=\text{C}$ bond is weaker compared to $\text{C=O}$ bond. So, the overall dipole will be generated, and the sulphur will carry partial positive charge whereas oxygen will carry partial negative charge.

The $\text{S}=\text{C}=\text{O}$ molecule will possess dipole-dipole interactions because of its less symmetry, but the ${\text{CO}}_2$ molecule will not possess dipole-dipole interactions because of its symmetry.

Thus, the $\text{S}=\text{C}=\text{O}$ molecule on the left will have higher boiling point because of extra dipole-dipole interactions and less symmetry.

Conclusion:

Therefore, the correct option for the statement “A substance consisting of which molecule shown below should have a higher boiling point?” is option (b).