Intermolecular Forces Attractions between molecules Overview l Molecules
Intermolecular Forces Attractions between molecules
Overview l Molecules may be weakly attracted to each other. l There are various categories of intermolecular attractions. l Intermolecular attractions affect macroscopic properties of compounds.
Intermolecular attractions Also called “van der Waals forces” or “weak forces” l Generally weak l Depend on several factors l l l Molecule size Molecule polarity Number of bonding electrons Affects molecular properties l Boiling point, evaporation time, melting point, viscosity
Ion-dipole attractions Strongest of the weak forces l Ions attract polar molecules l Important role in dissolving ions in water l Enough of these can break apart a crystal lattice l
Dipole dipole attractions Dipoles arrange themselves to maximize attractions & minimize repulsions l Strength depends on the nature of the dipoles involved l Example: using a magnet to induce a dipole in a nail l
Hydrogen bonding Subset of dipole-dipole interactions l Important in protein, DNA structure l Hydrogen bonding in water Hydrogen bonding in DNA
Dipole-Induced Dipole Attractions Presence of a permanent dipole can “induce” a teporary dipole in another molecule l Temporary effect l Explains why O 2, CO 2 can dissolve in water l Visuals on next slide l
Dipole-induced dipole interactions + d- d+ Spherical nonpolar atom + Cation approaches from a distance Ion causes temporary uneven distribution of electrons
Induced dipole-induced dipole attractions l Also “London dispersion forces” l Weakest of the intermolecular attractions l Randomness in electron motion can result in uneven electron distribution for a moment l Transient effect l This can temporarily induce a dipole in another molecule
Induced dipole-induced dipole attractions
Induced dipole-induced dipole attractions l Most significant for larger atoms l Compare l I 2 iodine and fluorine molecules larger atoms solid at room temperature l F 2 smaller atoms gas at room temperature
Effects of intermolecular attractions l Lots of intermolecular attractions difficult to separate molecules High boiling point l May be solid or liquid at room temperature l l Few intermolecular attractions easy to separate molecules Low boiling point l May be gases or liquids with low boiling points (“volatile”) l
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