Question

Rationalize the following differences in physical properties in terms of intermolecular forces. Compare the first three substances with each other, compare the last three with each other, and then compare all six. Can you account for any anomalies?

Short answer

In benzene, the only attractions between the neighboring molecules are the van der Waals forces. It also has London dispersion forces. In benzene molecules, there is no permanent dipole present.

Naphthalene only has London dispersion forces in between its molecules because it is a large flat molecule; the London dispersion forces are strong, making it have a higher boiling point than benzene and carbon tetrachloride.

Carbon tetrachloride is nonpolar, and so the only kind of force that can exist is induced dipole. In all three, only naphthalene has a stronger intermolecular force than benzene and carbon tetrachloride. Thus it contains higher boiling, melting points, and enthalpy of vaporization.

Acetone is known to have a dipole moment in between its molecules, so dipole-dipole forces will be present.

In acetic acid ( CH₃COOH ), out of four, three types of intermolecular forces, hydrogen bonding, dipole-dipole interactions, and dispersion forces, are present.

Benzoic acid has dipole-dipole force, hydrogen bonding, and London dispersion force.

In all three, only benzoic acid has stronger intermolecular force in comparison to acetone and acetic acid; that is why it contains higher boiling, melting, and enthalpy of vaporization

Among all the six compounds, benzoic acid has the strongest intermolecular forces. Hence, benzoic acid has a higher melting point, boiling point, and enthalpy of vaporization. Acetone has the weakest intermolecular forces, and hence it has the lowest melting point, boiling point, and enthalpy of vaporization.

Step-by-step solution

Intermolecular force

Intramolecular forces are well-known forces that work between the molecules as they hold atoms of the molecule together within a molecule.Intermolecular forces exist between molecules of any compound, such as in water molecules.

Types of intermolecular forces

There are mainly four types of intermolecular forces present.

1. London depression force

2. Dipole-Dipole force

3. Hydrogen bonding

4. Ion-Dipole force

Explanation

In benzene, the only attractions between the neighboring molecules are the van der Waals forces. It also has London dispersion forces. In benzene molecules, there is no permanent dipole present.

Naphthalene only has London dispersion forces in between its molecules because it is a large flat molecule; the London dispersion forces are large, making it have a higher boiling point than benzene and carbon tetrachloride.

Carbon tetrachloride is nonpolar, and so the only kind of force that can exist is induced dipole. In all three, only naphthalene has a stronger intermolecular force than benzene and carbon tetrachloride; that is why it contains higher boiling, melting points, and enthalpy of vaporization.

Acetone has a dipole moment between its molecules, so dipole-dipole forces will be present.

In acetic acid (), out of four, three types of intermolecular forces, hydrogen bonding, dipole-dipole interactions, and dispersion forces, are present.

Benzoic acid has dipole-dipole force, hydrogen bonding, and London dispersion force.

In all three, only benzoic acid has stronger intermolecular force in comparison to acetone and acetic acid; that is why it contains higher boiling, melting, and enthalpy of vaporization

Among all the six compounds, benzoic acid has the strongest intermolecular forces. Hence, benzoic acid has a higher melting point, boiling point, and enthalpy of vaporization. Acetone has the weakest intermolecular forces, and hence it has the lowest melting point, boiling point, and enthalpy of vaporization.