Intermolecular Forces

1. Intermolecular Forces Attractions between molecules

2. Overview • Molecules may be weakly attracted to each other. • There are various categories of intermolecular attractions. • Intermolecular attractions affect macroscopic properties of compounds.

3. Intermolecular attractions • Also called “van der Waals forces” or “weak forces” • Generally weak • Depend on several factors • Molecule size • Molecule polarity • Number of bonding electrons • Affects molecular properties • Boiling point, evaporation time, melting point, viscosity

4. Ion-dipole attractions • Strongest of the weak forces • Ions attract polar molecules • Important role in dissolving ions in water • Enough of these can break apart a crystal lattice

5. Dipole dipole attractions • Dipoles arrange themselves to maximize attractions & minimize repulsions • Strength depends on the nature of the dipoles involved • Example: using a magnet to induce a dipole in a nail

6. Hydrogen bonding • Subset of dipole-dipole interactions • Important in protein, DNA structure Hydrogen bonding in water Hydrogen bonding in DNA

7. Dipole-Induced Dipole Attractions • Presence of a permanent dipole can “induce” a teporary dipole in another molecule • Temporary effect • Explains why O2, CO2 can dissolve in water • Visuals on next slide

8. + d - d+ + Dipole-induced dipole interactions Spherical nonpolar atom Ion causes temporary uneven distribution of electrons Cation approaches from a distance

9. Induced dipole-induced dipole attractions • Also “London dispersion forces” • Weakest of the intermolecular attractions • Randomness in electron motion can result in uneven electron distribution for a moment • Transient effect • This can temporarily induce a dipole in another molecule

10. Induced dipole-induced dipole attractions

11. Induced dipole-induced dipole attractions • Most significant for larger atoms • Compare iodine and fluorine molecules • I2 larger atoms solid at room temperature • F2 smaller atoms gas at room temperature

12. Effects of intermolecular attractions • Lots of intermolecular attractions  difficult to separate molecules • High boiling point • May be solid or liquid at room temperature • Few intermolecular attractions  easy to separate molecules • Low boiling point • May be gases or liquids with low boiling points (“volatile”)