DPST1031 Chap.6 Intermolecular Forces and Liquid Properties
Intermolecular Forces and Liquid Properties
This chapter connects the structure of a molecule to the way a bulk substance behaves. It begins with bond polarity from electronegativity difference and uses VSEPR shapes to decide whether bond dipoles cancel. It then sets out the three intermolecular forces the syllabus names, dispersion, dipole-dipole and hydrogen bonding, with the structural requirement for each and where ionic attraction sits above them.
Those forces are used to rank boiling points, melting points, viscosity, surface tension and vapour pressure, and to predict whether two liquids are miscible, which is the reasoning the Mastery questions on this topic expect.
What this chapter covers
- 01
Electronegativity and bond polarity
- 02
Molecular polarity from bond dipoles
- 03
Polarizability and dispersion forces
- 04
Dipole-dipole forces
- 05
Hydrogen bonding requirements
- 06
Ranking boiling and melting points
- 07
Viscosity, surface tension and vapour pressure
- 08
Miscibility of liquids
Worked example · free
Why ethanol boils far above methoxymethane
- +1Equal molar mass, 46 g mol⁻¹, means dispersion forces are similar for the two isomers.
- +1Methoxymethane is bent at oxygen and polar, so it adds dipole-dipole forces but has no O−H bond.
- +1Ethanol has H bonded to oxygen and lone pairs on oxygen, so its molecules hydrogen bond to each other.
- +1Hydrogen bonding is much stronger, so ethanol needs far more energy to separate its molecules.
Key terms
- Bond dipole
- The separation of partial charges along a polar bond, drawn towards the more electronegative atom.
- Dispersion force
- An attraction between temporary dipoles that acts between all molecules.
- Polarizability
- The ease with which an electron cloud is distorted to form a temporary dipole.
- Dipole-dipole force
- An attraction between molecules that carry permanent net dipoles.
- Surface tension
- The energy needed to increase the surface area of a liquid, high when attractions are strong.
- Viscosity
- A liquid's resistance to flow, which rises with stronger intermolecular forces.
- Miscibility
- The ability of two liquids to mix completely in any proportion.
Intermolecular Forces and Liquid Properties FAQ
Is a polar molecule always the one with the higher boiling point?
No. For molecules of similar size, polarity raises the boiling point, but a large non-polar molecule can have dispersion forces strong enough to beat a small polar one. Octane boils above water's boiling point, so always compare sizes as well as polarity.
When can a molecule form hydrogen bonds?
It needs hydrogen covalently bonded to nitrogen, oxygen or fluorine, and a lone pair on N, O or F on a neighbouring molecule. Methanol and ammonia qualify; methoxymethane has oxygen but no O−H, so it cannot hydrogen bond with itself, although it can accept a hydrogen bond from water.
Why does a stronger intermolecular force lower vapour pressure?
Vapour pressure comes from molecules escaping the liquid surface. Stronger attractions hold molecules in the liquid more tightly, so at a given temperature fewer have enough energy to escape, and the equilibrium vapour pressure is lower.
How do I decide whether two liquids are miscible?
Compare the forces each liquid uses. If the molecules can form attractions with each other that are about as strong as those within each pure liquid, they mix. Water and ethanol mix because both hydrogen bond; water and hexane separate because hexane cannot.
Why does ionic bonding sit above intermolecular forces?
In an ionic solid every ion is held by full electrostatic charges to many oppositely charged neighbours in a lattice. Those attractions are far stronger than the partial-charge attractions between molecules, so salts such as NaCl melt at several hundred degrees, while molecular solids of similar mass melt far lower.
Exam move
For every substance you meet, write a three-part note: polar or non-polar with the shape reason, the forces present, and the strongest of them. Practise with sets that hold one variable constant, such as same molar mass or same functional group, so that each comparison isolates a single cause. Then extend the same reasoning to viscosity, surface tension, vapour pressure and miscibility.
Finish with mixed ranking sets that combine ionic, metallic and molecular substances, because mastery questions often ask you to pick the extreme member of a mixed group and justify it.
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