3.9.1 Types of Intermolecular Forces

This topic explains the three types of intermolecular forces: permanent dipole-dipole forces, instantaneous dipole-induced dipole forces (London forces), and hydrogen bonding.

Definition

Intermolecular Forces: The forces of attraction present between covalent molecules.

Main Idea

• Permanent Dipole-Dipole Forces
• Instantaneous Dipole-Induced Dipole Forces (London Forces)
• Hydrogen Bonding

Each type differs in strength and in the type of molecules it acts between.

Core Concept

• Polar molecules: Permanent dipole-dipole forces (and London forces).
• Nonpolar molecules: Instantaneous dipole-induced dipole forces (London forces).
• Molecules with H bonded to F, N, or O: Hydrogen bonding.

The strength order is:

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

Detailed Explanation

1. Permanent Dipole-Dipole Forces

• These forces are present between polar molecules.
• Polar molecules are those where, due to electronegativity difference, one atom bears a partial positive charge (δ+) and the other, which is more electronegative, bears a partial negative charge (δ−).
• Some molecules with polar bonds have an overall dipole (e.g., Hδ+—Clδ−).
• However, not all do — e.g., in CCl₄, the individual dipoles cancel each other out, so CCl₄ is a nonpolar molecule.
• There are attractions between these permanent dipoles in neighbouring molecules, e.g., between H—Cl molecules.

H^δ+ - Cl^δ- ⋯ H^δ+ - Cl^δ-

- Stronger the dipole-dipole forces, higher will be the value of melting point, boiling point, heat of vaporization, and heat of sublimation of the particular compound.

2. Instantaneous Dipole-Induced Dipole Forces (London Forces)

• Those forces which are present between nonpolar molecules are called instantaneous dipole-induced dipole forces.
• Even in molecules with no polar bonds, there are temporary dipoles.
• It is because of uneven electron distribution due to the constant movement of electrons.
• This induces a temporary dipole in a neighbouring molecule, producing a temporary dipole-induced dipole force of attraction.

• At any instant, the electron cloud of a nonpolar molecule may be unevenly distributed.
• This creates an instantaneous dipole (temporary δ+ and δ−).
• This instantaneous dipole induces a dipole in a neighbouring molecule.
• The two dipoles attract each other.

• Molecular size
• Atomic size
• Molecular mass
• Number of electrons in the molecule

The bigger the molecule (i.e., more electrons, larger size, or higher relative molecular mass), the greater the van der Waals forces.

Example: Boiling Points of Alkanes

The boiling point of propane (C₃H₈) is greater than that of ethane (C₂H₆). This is because the molecular mass of C₃H₈ is 44, which is greater than the Mr of C₂H₆, which is 30.

3. Hydrogen Bonding

• This is a special case of permanent dipole-dipole forces.
• It is present between those molecules where hydrogen is covalently bonded with higher electronegative elements such as F, N, and O.
• There is a strong attraction from the lone pair on the N, O, or F of one molecule to the exposed hydrogen nucleus of another molecule.
• This is simply a strong intermolecular force — it is NOT a bond.
• Those covalent compounds which form hydrogen bonding with water molecules are water soluble.
• For example, ammonia is soluble in water.

Example: Hydrogen bonding between ammonia and water molecules.

Strength of Intermolecular Forces

H-bonding > permanent dipole-dipole > instantaneous dipole-induced dipole forces

Comparison of Boiling Points

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.

Intermediate Level

Types of Intermolecular Forces

Permanent Dipole-Dipole Forces

• Between polar molecules.
• δ+ of one molecule attracts δ− of another.
• Stronger dipole → higher MP, BP, heat of vaporization.

London Forces

• Between nonpolar molecules.
• Temporary dipoles due to electron movement.
• Strength increases with:
• Molecular size
• Molecular mass
• Number of electrons

Example: Boiling point of alkanes increases with molecular mass.

Hydrogen Bonding

• Special case of dipole-dipole forces.
• H bonded to F, N, or O.
• Strong attraction between lone pair and exposed H nucleus.
• NOT a bond — it is an intermolecular force.
• Compounds that H-bond with water are water-soluble.

Strength Order

H-bonding > Dipole-dipole > London forces

Comparison Table

Important Points

• 1. Three types of intermolecular forces: Dipole-dipole, London, H-bonding.
• 2. Dipole-dipole: Between polar molecules.
• 3. London forces: Between nonpolar molecules; strength depends on size, mass, electrons.
• 4. Hydrogen bonding: Special dipole-dipole; H bonded to F, N, or O.
• 5. Strength: H-bonding > Dipole-dipole > London forces.
• 6. MP/BP depend on intermolecular forces, not covalent bonds.
• 7. Compounds that H-bond with water are water-soluble.

Remember

• - Types: Dipole-dipole, London, H-bonding
• - Dipole-dipole: Polar molecules
• - London: Nonpolar molecules
• - H-bonding: H—F, H—N, H—O
• - Strength: H-bonding > Dipole-dipole > London
• - MP/BP: Depend on intermolecular forces

Common Mistakes

• 1. Confusing dipole-dipole with London forces: Dipole-dipole = polar; London = nonpolar.
• 2. Thinking hydrogen bonding is a covalent bond: It is an intermolecular force.
• 3. Forgetting that London forces exist in all molecules: They are dominant in nonpolar molecules.
• 4. Thinking CCl₄ is polar: It has polar bonds but is nonpolar overall.
• 5. Ignoring the effect of molecular size on London forces: Larger molecules have stronger London forces.

Quick Revision

• Three types of intermolecular forces: Dipole-dipole, London, H-bonding.
• Dipole-dipole: Between polar molecules.
• London forces: Between nonpolar molecules; strength depends on size, mass, electrons.
• Hydrogen bonding: Special dipole-dipole; H bonded to F, N, or O.
• Strength: H-bonding > Dipole-dipole > London forces.
• MP/BP depend on intermolecular forces, not covalent bonds.
• Compounds that H-bond with water are water-soluble.

60-Second Revision

• Types: Dipole-dipole, London, H-bonding
• Dipole-dipole: Polar molecules
• London: Nonpolar molecules
• H-bonding: H—F, H—N, H—O
• Strength: H-bonding > Dipole-dipole > London
• MP/BP: Depend on intermolecular forces

QUALITY VALIDATION

• Topic: 3.9.1 Types of Intermolecular Forces
• Chapter: 3 (Chemical Bonding)

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