3.9 Intermolecular Forces (Forces Between Molecules)

This topic introduces intermolecular forces — the forces of attraction between molecules. It explains their importance in determining physical properties and introduces the three main types.

Definition

Intermolecular forces are the forces of attraction between covalent molecules.

Types of Intermolecular Forces

• Present between polar molecules.
• Example: HCl molecules.

• Present between nonpolar molecules.
• Example: CH₄, Cl₂.

• A special case of permanent dipole-dipole forces.
• Present when H is bonded to F, N, or O.
• Example: H₂O, NH₃, HF.

Strength Order

Hydrogen bonding > Permanent dipole-dipole > London forces

Importance

- Determines melting point, boiling point, density, heat of vaporization, and heat of sublimation.

Key Point

• Melting and boiling points of covalent molecules depend on intermolecular forces, not on covalent bonds.
• Covalent bonds are much stronger (hundreds of kJ mol⁻¹) than intermolecular forces.

Main Idea

Intermolecular forces are the forces that act between molecules (not within a molecule). They are much weaker than covalent bonds but play a crucial role in determining the physical properties of substances, such as melting point, boiling point, and density.

Core Concept

The physical properties of covalent compounds depend on the strength of intermolecular forces, not on the strength of covalent bonds. Stronger intermolecular forces mean higher melting points, boiling points, heats of vaporization, and heats of sublimation.

Detailed Explanation

What are Intermolecular Forces?

• Intermolecular forces are those forces which are present between covalent molecules.
• They are also called van der Waals forces (in a broad sense).
• They are much weaker than ionic, covalent, and metallic bonds.
• They determine many physical properties of substances.

Why are Intermolecular Forces Important?

• Melting point
• Boiling point
• Density
• Heat of vaporization
• Heat of sublimation

can be explained on the basis of intermolecular forces.

Types of Intermolecular Forces

There are three types of intermolecular forces:

• Permanent Dipole-Dipole Forces
• Instantaneous Dipole-Induced Dipole Forces
• Hydrogen Bonding

Comparison of Strengths

The strength of intermolecular forces follows this order:

Hydrogen bonding > Permanent dipole-dipole > Instantaneous dipole-induced dipole forces

Intermolecular vs. Intramolecular Forces

• Intermolecular forces: Forces between molecules.
• Intramolecular forces: Forces within a molecule (e.g., covalent bonds).

Key Point: Melting and boiling points of covalent molecules depend upon the intermolecular forces, not on the covalent bonds in the molecule. Covalent bonds are very strong (values in hundreds of kJ mol⁻¹). The forces between molecules are much weaker than covalent bonds.

Important Points

• 1. Intermolecular forces = forces between molecules.
• 2. They are weaker than covalent bonds.
• 3. Three types: Dipole-dipole, London forces, Hydrogen bonding.
• 4. Strength: H-bonding > Dipole-dipole > London forces.
• 5. Determine MP, BP, density, heat of vaporization, and heat of sublimation.
• 6. MP/BP depend on intermolecular forces, not covalent bonds.

Remember

• - Intermolecular forces: Between molecules
• - Intramolecular forces: Within a molecule
• - Types: Dipole-dipole, London, H-bonding
• - Strength: H-bonding > Dipole-dipole > London
• - Effect: Determines MP, BP, density

Common Mistakes

• 1. Confusing intermolecular and intramolecular forces: Inter = between; Intra = within.
• 2. Thinking covalent bonds break during melting/boiling: Only intermolecular forces break; covalent bonds remain intact.
• 3. Forgetting that London forces exist in all molecules: They are especially important for nonpolar molecules.
• 4. Thinking hydrogen bonding is a covalent bond: It is a strong intermolecular force, not a bond.

Quick Revision

• Intermolecular forces = forces between molecules.
• They are weaker than covalent bonds.
• Three types: Dipole-dipole, London forces, Hydrogen bonding.
• Strength: H-bonding > Dipole-dipole > London forces.
• Determine MP, BP, density, heat of vaporization, and heat of sublimation.
• MP/BP depend on intermolecular forces, not covalent bonds.

60-Second Revision

• Intermolecular forces: Between molecules
• Intramolecular forces: Within a molecule
• Types: Dipole-dipole, London, H-bonding
• Strength: H-bonding > Dipole-dipole > London
• Effect: Determines MP, BP, density

QUALITY VALIDATION

• Topic: 3.9 Intermolecular Forces (Forces Between Molecules)
• Chapter: 3 (Chemical Bonding)

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