Click image to open full size
Chemistry
Intermolecular Forces
Intermolecular Forces
Related Tools
Related Labs
Related Worksheets
Intermolecular forces are the attractions between separate molecules, atoms, or ions in a substance. They are weaker than chemical bonds inside a molecule, but they strongly affect boiling point, melting point, viscosity, surface tension, and solubility. Understanding these forces helps explain why water is a liquid at room temperature, why oil and water do not mix well, and why some substances evaporate quickly while others do not.
Key Facts
- London dispersion forces occur in all atoms and molecules because electrons can form temporary dipoles.
- Dipole-dipole forces occur between polar molecules with permanent partial positive and partial negative ends.
- Hydrogen bonding occurs when H is bonded to N, O, or F and is attracted to a lone pair on N, O, or F nearby.
- Ion-dipole forces occur between an ion and a polar molecule, such as Na+ attracted to the O end of water.
- For substances with similar molar mass, stronger intermolecular forces usually mean higher boiling point.
- Coulomb's law helps describe attraction strength: F = kq1q2/r^2, so larger charges and shorter distances increase attraction.
Vocabulary
- Intermolecular force
- An attraction between separate particles, such as molecules, atoms, or ions.
- London dispersion force
- A weak attraction caused by temporary shifts in electron density that create temporary dipoles.
- Dipole
- A separation of electric charge in a molecule, with one region partially positive and another partially negative.
- Hydrogen bond
- A strong type of dipole-dipole attraction involving hydrogen bonded to nitrogen, oxygen, or fluorine.
- Ion-dipole force
- An attraction between an ion and the oppositely charged end of a polar molecule.
Common Mistakes to Avoid
- Calling intermolecular forces chemical bonds is wrong because they act between particles, while covalent, ionic, and metallic bonds hold atoms together within a substance.
- Assuming bigger molecules always have higher boiling points is wrong because molar mass matters, but polarity and hydrogen bonding can have an even stronger effect.
- Saying every molecule with hydrogen can hydrogen bond is wrong because hydrogen bonding requires H directly bonded to N, O, or F.
- Ignoring molecular shape is wrong because bond polarity alone does not guarantee a polar molecule if the bond dipoles cancel.
Practice Questions
- 1 Rank these substances from lowest to highest expected boiling point and explain your order: CH4, H2O, H2S. Use the main intermolecular force in each substance.
- 2 A liquid has a normal boiling point of 78 °C and another similar-sized liquid has a normal boiling point of 35 °C. Which liquid likely has stronger intermolecular forces, and by how many degrees Celsius are the boiling points different?
- 3 A student says CO2 must be polar because each C=O bond is polar. Explain why this conclusion is incorrect using molecular shape and dipole cancellation.