Grade 10
Intermolecular forces (van der Waals, hydrogen bonds)
London dispersion, dipole–dipole attraction and hydrogen bonding influence boiling, melting and solubility. These attractions act between particles, not as covalent bonds within a molecule.
IntuitionIntuition: particles and change
Water beads on some surfaces and boils much hotter than similarly small molecules. Attractions between molecules help explain such everyday observations.
SchoolSchool: models and rules
Definition: Key idea
Intermolecular forces are attractions between separate molecules or particles. They are generally weaker than covalent bonds, but many attractions acting together strongly affect bulk properties.
London dispersion forces arise from fluctuating electron distributions and occur in all atoms and molecules. They become stronger with greater polarizability and, often, larger contact surface.
| Force | Between | Trend |
|---|---|---|
| London dispersion | all atoms and molecules | grows with size and polarizability |
| Dipole–dipole | permanent dipoles | stronger than dispersion at similar mass |
| Hydrogen bond | H bonded to N/O/F and a lone pair on N/O/F | the strongest of these three |
Example: Worked example
Why is water’s boiling point high for a small molecule?
Solution
Water molecules form many hydrogen-bond attractions. Boiling separates molecules from this network, without breaking the covalent O–H bonds within each molecule.
Example: Second example
NH₃ (M = 17) boils at −33 °C, PH₃ (M = 34) at −88 °C. Why?
Solution
NH₃ has hydrogen bonding between molecules, while PH₃ relies on weaker dispersion forces — despite PH₃ being heavier, its intermolecular attractions are weaker.
A permanent dipole can attract another dipole. Hydrogen bonding is a particularly strong, directional interaction when H bonded to N, O or F interacts with a lone pair on N, O or F.
Comparing methane (CH₄), ammonia (NH₃), water (H₂O) and hydrogen fluoride (HF) shows the effect clearly: the three hydrogen-bonding molecules have far higher boiling points than their similar-mass neighbours.
UndergraduateDeeper view
At a deeper level, classify observations by the particles involved and by the quantities that remain invariant. Models describe charge, electron density or particle counting; choose the simplest model that accounts for the measured evidence.
Example: Calculation / application example
Why does butane (C₄H₁₀, bp −0.5 °C) have a higher boiling point than propane (C₃H₈, bp −42 °C)?
Solution
Both are non-polar and interact only via London dispersion forces. Butane has more electrons and a longer chain (larger contact area), giving stronger dispersion attractions.