Intermolecular Forces: Flashcards
Revise dispersion forces, dipole–dipole forces, hydrogen bonding and their effects on boiling point and solubility.
Intermolecular Forces: Flashcards
- QuestionIntermolecular force vs covalent bond?Answer
Intermolecular forces act between molecules and are much weaker; covalent bonds hold atoms together within a molecule.
- QuestionWhat are London dispersion forces?Answer
Attractions between temporary dipoles caused by moving electrons. Present in all molecules; stronger for bigger, longer molecules.
- QuestionWhen do dipole–dipole forces occur?Answer
Between polar molecules: the δ+ end of one attracts the δ− end of another.
- QuestionWhat is needed for hydrogen bonding?Answer
An H atom bonded to N, O or F, and a lone pair on an N, O or F atom of another molecule.
- QuestionWhat is overcome when a molecular liquid boils?Answer
Only the intermolecular forces; covalent bonds inside the molecules stay intact.
- QuestionWhy does water (100 °C) boil so much higher than H₂S (−60 °C)?Answer
Water forms hydrogen bonds (O–H); H₂S does not, and has only weaker dipole–dipole and dispersion forces.
- QuestionWhy does pentane boil higher than 2,2-dimethylpropane?Answer
Unbranched pentane has more surface contact, so stronger dispersion forces (36 °C against 10 °C).
- QuestionWhy does ice float?Answer
Hydrogen bonds hold the molecules in an open network in ice, so ice is less dense than liquid water.
- QuestionHow do stronger intermolecular forces affect vapour pressure and viscosity?Answer
Vapour pressure is lower; viscosity and surface tension are higher.
- QuestionWhat does "like dissolves like" mean?Answer
Solutes dissolve best in solvents with similar forces: polar in polar (water), non-polar in non-polar (hexane).
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