Overview
This lecture covers intermolecular forces, including their types, relative strengths, effects on boiling point and solubility, and how to identify them in various chemical compounds.
Types of Intermolecular Forces
- Ion-ion interactions occur between two charged ions and are the strongest intermolecular force.
- Ion-dipole interactions happen between an ion and a polar molecule (dipole), like salt dissolving in water.
- Dipole-dipole interactions occur between two polar molecules with partial positive and negative charges attracting.
- Hydrogen bonds are a special, strong type of dipole-dipole interaction, only present when H is bonded to N, O, or F.
- London dispersion forces (LDF), or van der Waals forces, exist in all molecules, but are the only force in non-polar molecules, arising from temporary induced dipoles.
Factors Affecting Force Strength & Properties
- Ion-ion strength increases with higher charges and decreases with larger ion size.
- Lattice energy is proportional to the product of charges and inversely proportional to the distance between ions.
- Dipole-dipole and hydrogen bonding require polarity; non-polar molecules only have LDF.
- Larger, heavier molecules have stronger LDF due to greater polarizability and higher boiling points.
- Straight-chain alkanes have higher boiling points than branched isomers due to larger surface area for interactions.
- Polar molecules dissolve better in water; longer non-polar chains reduce solubility.
Examples & Comparative Analysis
- MgO exhibits ion-ion interactions.
- KCl and water show ion-dipole interactions.
- Methane and CO₂ are non-polar, so only LDF present.
- SO₂ is polar, so it has dipole-dipole interactions.
- HF shows hydrogen bonding (strongest among H-X halides).
- Methanol vs. methane: methanol (polar, H-bonding) has higher boiling point.
- Propanol has a higher boiling point and lower vapor pressure than methanol due to size.
- Methanol dissolves better in water than propanol or octanol because of smaller non-polar region.
- Straight-chain pentane has a higher boiling point than neopentane due to surface area.
- H₂O > H₂Se > H₂S in boiling points due to H-bonds and size.
- HF > HI > HBr > HCl in boiling point due to H-bonding and size.
Key Terms & Definitions
- Intermolecular force — Attraction between different molecules.
- Ion-ion interaction — Electrostatic attraction between two ions.
- Ion-dipole interaction — Force between an ion and a polar molecule.
- Dipole-dipole interaction — Attraction between two polar molecules.
- Hydrogen bond — Strong dipole-dipole force when H is attached to N, O, or F.
- London dispersion force (LDF) — Weak force from temporary dipoles in all molecules.
- Lattice energy — Energy holding ions in an ionic solid together.
Action Items / Next Steps
- Review and practice identifying the dominant intermolecular force in given compounds.
- Memorize the order of strength for intermolecular forces: ion-ion > ion-dipole > hydrogen bond > dipole-dipole > LDF.
- Work on boiling point and solubility ranking exercises for related compounds.