Intermolecular forces (IMFs) are a foundational idea in IB Chemistry Topic 4 (Bonding). They explain why substances have different boiling points, solubilities, viscosities, and physical states. Although students often focus on covalent bonds within molecules, intermolecular forces are just as important because they determine how molecules interact with each other. Understanding IMFs helps you predict physical properties and answer questions across many IB topics.
What Are Intermolecular Forces?
Intermolecular forces are the attractive forces between molecules.
They are weaker than covalent, ionic, and metallic bonds, but strong enough to influence the behavior of substances.
These forces determine:
- Melting and boiling points
- Solubility
- Surface tension
- Viscosity
- Volatility
- State of matter at room temperature
IMFs are essential for explaining why water boils at 100°C while methane boils at –164°C.
Types of Intermolecular Forces
IB Chemistry focuses on three main types:
- London dispersion forces (LDFs)
- Dipole–dipole forces
- Hydrogen bonding
All molecules have at least one type of IMF.
1. London Dispersion Forces (LDFs)
London dispersion forces are temporary attractions caused by momentary dipoles in electron clouds.
Key points:
- Present in all molecules, including non-polar ones
- Weakest type of IMF
- Caused by temporary fluctuations in electron distribution
Larger molecules with more electrons have stronger LDFs.
Factors that increase LDF strength:
- 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.
