6.3 - Intermolecular forces

Question 1

What is the difference between covalent bonding and intermolecular forces?

Answer 1

Covalent bonding is strong and holds the atoms in a molecule together, while intermolecular forces are weak interactions between dipoles of different molecules.

Question 2

What are the three main types of intermolecular forces?

Answer 2

1. Induced dipole-dipole interactions (London forces)
2. Permanent dipole-dipole interactions
3. Hydrogen bonding

Question 3

What determines the physical properties like melting and boiling points?

Answer 3

Intermolecular forces largely determine physical properties such as melting and boiling points.

Question 4

How do the strengths of different bonds compare?

Answer 4

The bond strengths (measured by bond enthalpy in kJ/mol) are:

• London forces: 1-10
• Permanent dipole-dipole interactions: 3-25
• Hydrogen bonds: 10-40
• Single covalent bonds: 150-500

Question 5

What are London forces?

Answer 5

London forces are weak intermolecular forces that exist between all molecules, polar or non-polar.

Question 6

How do London forces arise?

Answer 6

London forces arise due to the following process:

1. Movement of electrons creates a changing dipole in a molecule.
2. An instantaneous dipole forms, which induces a dipole in a neighboring molecule.
3. The induced dipole causes further dipoles in nearby molecules, creating attractive forces.

Question 7

Are London forces permanent?

Answer 7

No, London forces are temporary and constantly form and disappear as electrons move.

Question 8

What factors affect the strength of London forces?

Answer 8

The strength of London forces increases with:

1. More electrons in the molecule.
2. Larger instantaneous and induced dipoles.
3. Stronger attractive forces, leading to higher boiling points.

Question 8

Why is the term “Van der Waals’ forces” ambiguous?

Answer 8

The term “Van der Waals’ forces” is used to describe both permanent and induced dipole-dipole interactions. “London forces” is the recommended term for induced dipole-dipole interactions.

Question 9

What are permanent dipole-dipole interactions?

Answer 9

These are forces that act between the permanent dipoles in different polar molecules.

Question 10

How do permanent dipole-dipole interactions compare with London forces?

Answer 10

Permanent dipole-dipole interactions are generally stronger than London forces and contribute to higher boiling points in polar molecules.

Question 11

How does hydrogen chloride compare with fluorine in terms of boiling points?

Answer 11

1. Fluorine (F₂) molecules are non-polar and only have London forces.
2. Hydrogen chloride (HCl) molecules are polar and have both London forces and permanent dipole-dipole interactions.
3. As a result, HCl has a higher boiling point than F₂.

Question 12

Why do simple molecular substances have low melting and boiling points?

Answer 12

The weak intermolecular forces in the lattice can be broken by small amounts of energy, even at low temperatures.

Question 12

What are simple molecular substances?

Answer 12

Simple molecular substances are made up of small molecules with a definite number of atoms and molecular formula (e.g., H₂, H₂O, CO₂).

Question 12

What happens when a simple molecular lattice melts?

Answer 12

1. Only the weak intermolecular forces are broken.
2. The strong covalent bonds within the molecules remain intact.

Question 12

What is a simple molecular lattice?

Answer 12

In the solid state, simple molecules form a regular structure where molecules are held together by weak intermolecular forces, while the atoms within each molecule are bonded by strong covalent bonds.

Question 13

How does the solubility of non-polar simple molecular substances depend on the solvent?

Answer 13

1. In non-polar solvents: Non-polar substances are soluble because intermolecular forces form between the solute and solvent molecules, breaking the lattice.
2. In polar solvents: Non-polar substances are insoluble because there is little interaction, and the strong intermolecular forces in the polar solvent cannot be broken.

Question 13

How does the solubility of polar simple molecular substances depend on the solvent?

Answer 13

1. In polar solvents: Polar substances may dissolve due to attractions between the polar solute and solvent molecules (e.g., sugar in water).
2. In non-polar solvents: Solubility depends on the balance between polar and non-polar regions of the molecule (e.g., ethanol can dissolve in both types of solvents).

Question 13

Why don’t simple molecular substances conduct electricity?

Answer 13

Simple molecular substances lack mobile charged particles, so there is nothing to carry a charge or complete an electrical circuit.