Intermolecular Forces Lecture

Question-and-answer flashcards covering definitions, types, factors, comparisons, and effects of intermolecular forces for exam review.

What are intermolecular forces?

Weak forces of attraction between molecules that influence physical properties such as melting point, boiling point, and solubility. They are different from covalent, ionic, or metallic bonds (which are intramolecular).

What are the three main types of intermolecular forces?

1. Van der Waals (induced dipole–dipole or London dispersion) forces
2. Permanent dipole–dipole interactions
3. Hydrogen bonding

What are Van der Waals (London dispersion) forces?

Temporary attractions caused by instantaneous dipoles arising from electron movement. They are present between all molecules and are the only intermolecular force in non-polar molecules (e.g., O₂, Cl₂). Their strength increases with molecular size and electron count.

What factors affect the strength of Van der Waals forces?

• More electrons → stronger induced dipoles
• Larger surface area/longer chains → more contact between molecules → stronger forces
• Therefore, long-chain alkanes have stronger Van der Waals forces than branched alkanes.

What are permanent dipole–dipole interactions?

Attractive forces between polar molecules where the δ⁺ end of one molecule is electrostatically attracted to the δ⁻ end of another.

How are permanent dipole–dipole forces different from Van der Waals forces?

Permanent dipole–dipole forces are generally stronger and occur only between polar molecules, whereas Van der Waals forces occur between all molecules and are usually weaker.

What is hydrogen bonding?

A strong type of permanent dipole attraction that occurs when a hydrogen atom covalently bonded to N, O, or F is attracted to a lone pair on another N, O, or F atom.

What are the requirements for hydrogen bonding?

1. Hydrogen must be covalently bonded to N, O, or F.
2. A lone pair must be present on an adjacent N, O, or F atom.

Give examples of molecules that exhibit hydrogen bonding.

Water (H₂O), ammonia (NH₃), hydrogen fluoride (HF), and ethanol (C₂H₅OH).

What are the effects of hydrogen bonding on physical properties?

• Raises boiling and melting points
• Increases solubility in water (e.g., alcohols)
• Causes ice to be less dense than liquid water due to its open lattice structure.

How do intermolecular forces affect boiling points?

Stronger intermolecular forces require more energy to overcome, leading to higher boiling points. Relative strength: hydrogen bonding > permanent dipole–dipole > Van der Waals forces.

Rank the intermolecular forces from weakest to strongest.

1. Van der Waals (induced dipole–dipole) – weakest
2. Permanent dipole–dipole
3. Hydrogen bonding – strongest