Intermolecular Forces Explained

• Higher molar mass
• Larger electron clouds
• Longer and less compact molecular shapes

This is why iodine (I₂) is a solid, bromine (Br₂) is a liquid, and chlorine (Cl₂) is a gas.

2. Dipole–Dipole Forces

Dipole–dipole forces occur between polar molecules due to permanent partial charges.

Key features:

• Stronger than dispersion forces
• Only present in molecules with permanent dipoles
• Positive end of one molecule attracts the negative end of another

Example:
HCl molecules attract each other through dipole–dipole interactions.

Dipole–dipole forces raise boiling points compared to nonpolar molecules of similar mass.

3. Hydrogen Bonding

Hydrogen bonding is the strongest type of intermolecular force, occurring when hydrogen is bonded to N, O, or F.

Requirements:

• H attached to N, O, or F
• Lone pair on the neighboring N, O, or F atom

Hydrogen bonding causes:

• High boiling points (e.g., water)
• High surface tension
• Unique density behavior of ice
• Strong solubility of alcohols in water

Water’s unusually high boiling point is due to hydrogen bonding networks between molecules.

Why Intermolecular Forces Matter

IMFs explain many physical properties:

1. Boiling and melting points

Stronger IMFs → more energy needed to separate molecules → higher boiling/melting points.

2. Solubility

Polarity and IMFs determine solubility:

• “Like dissolves like.”
• Polar molecules dissolve in polar solvents.
• Nonpolar molecules dissolve in nonpolar solvents.

3. Volatility

Weak IMFs → molecules escape as gas easily → high volatility.

4. Viscosity

Stronger IMFs → thicker, more resistant flow (e.g., glycerol).

5. Surface tension

Hydrogen bonding gives water abnormally high surface tension.

Intermolecular Forces vs Intramolecular Bonds

A common IB confusion:

Intramolecular forces:

• Bonds within a molecule
• Include covalent, ionic, metallic
• Much stronger

Intermolecular forces:

• Attractions between molecules
• Determine physical properties

Breaking IMFs changes state; breaking covalent bonds changes chemical identity.

IMFs in IB Exam Questions

You must be able to:

• Identify the strongest IMF present
• Compare boiling points using IMFs
• Predict solubility based on polarity
• Explain trends down groups or with molar mass
• Distinguish IMF effects from covalent bonding effects

These questions appear in Papers 1, 2, and 3.

Common IB Misunderstandings

“Nonpolar molecules have no forces.”

Incorrect—London dispersion forces always exist.

“Hydrogen bonding occurs whenever hydrogen is present.”

False—hydrogen must be bonded to N, O, or F.

“Dipole–dipole is stronger than hydrogen bonding.”

No—hydrogen bonding is the strongest IMF.

“Breaking IMFs is the same as breaking covalent bonds.”

Not true. Physical changes involve IMFs; chemical changes involve covalent bonds.

FAQs

Why does water boil so high?

Hydrogen bonding creates strong attractions that require lots of energy to break.

Are IMFs responsible for solubility?

Yes—matching intermolecular forces is essential for dissolving substances.

Do heavier molecules always have stronger IMFs?

Generally yes, because they have larger electron clouds.

Conclusion

Intermolecular forces are the attractions between molecules that influence boiling points, solubility, and many other physical properties. The three main types—London dispersion forces, dipole–dipole interactions, and hydrogen bonding—appear throughout IB Chemistry. Mastering IMFs helps you explain trends, predict behavior, and understand the real-world properties of substances.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.