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Intermolecular Forces

Intermolecular Forces. Attractions between molecules. Overview. Molecules may be weakly attracted to each other. There are various categories of intermolecular attractions. Intermolecular attractions affect macroscopic properties of compounds. Intermolecular attractions.

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Intermolecular Forces

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  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”)

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