What Are Intermolecular Forces?
Intermolecular forces are attractive forces between separate molecules — weaker than chemical bonds but crucial to physical properties like boiling point and solubility. The main types are London dispersion forces, dipole-dipole interactions and hydrogen bonding.
Intermolecular forces are weak attractions between molecules caused by electrostatic interactions. They are much weaker than covalent or ionic bonds but determine bulk physical properties.
- •London dispersion: induced dipole–induced dipole (all molecules)
- •Dipole-dipole: permanent dipole interactions (polar molecules)
- •Hydrogen bonding: O-H/N-H/F-H + lone pair (special case of dipole-dipole)
Step-by-step worked examples
Why is the boiling point of Br@sub{2} (79@deg C) higher than F@sub{2} (−188@deg C), even though both are diatomic?
Br@sub{2} has more electrons than F@sub{2} (molar mass Br@sub{2} = 160 g/mol vs. F@sub{2} = 38 g/mol)
More electrons = larger, more polarisable electron cloud
London dispersion forces are stronger in Br@sub{2}
Stronger intermolecular forces require more energy to overcome
Result: Br@sub{2} has a much higher boiling pointWhy is HF a liquid (boiling point 19.5@deg C) while HCl is a gas (boiling point −85@deg C)?
HF is highly polar (H-F bond very electronegative) More importantly, HF molecules form hydrogen bonds with each other HCl is polar but cannot form strong hydrogen bonds (Cl is less electronegative) Hydrogen bonds are much stronger than dipole-dipole forces Result: HF has a much higher boiling point
Compare the water solubility of hexane (C@sub{6}H@sub{14}) and methanol (CH@sub{3}OH).
Hexane is nonpolar; water is polar and forms hydrogen bonds Hexane–water intermolecular forces are very weak (London dispersion only) 'Like dissolves like': hexane does not dissolve well Methanol is polar and can form hydrogen bonds with water Methanol dissolves readily in water Result: methanol >> hexane in water solubility
Flashcards
Quick quiz
Q1.Which type of intermolecular force occurs in all molecules, regardless of polarity?
Q2.Why is the boiling point of HF (19.5@deg C) much higher than HCl (−85@deg C)?
Q3.Which of these molecules can exhibit hydrogen bonding?
Q4.London dispersion forces are stronger in…
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Common mistakes
Confusing intermolecular forces with covalent/ionic bonds. — Correct: Intermolecular forces are much weaker; they act between separate molecules.
Thinking only polar molecules have intermolecular forces. — Correct: All molecules have London dispersion; polar molecules also have dipole-dipole.
Assuming hydrogen bonding requires a H-H bond. — Correct: Hydrogen bonding involves H bonded to N, O or F, and a lone pair on another molecule.
Ignoring intermolecular forces' role in physical properties. — Correct: Boiling point, melting point, solubility and vapour pressure all depend on them.
FAQ
What are intermolecular forces?
Weak electrostatic attractions between separate molecules. Much weaker than chemical bonds.
What are the types of intermolecular forces?
London dispersion forces, dipole-dipole interactions and hydrogen bonding.
Why do intermolecular forces matter?
They determine melting point, boiling point, solubility, surface tension and other bulk properties.
Can nonpolar molecules have intermolecular forces?
Yes — London dispersion forces occur in all molecules.