Enduring Understanding 2.B: Intermolecular Forces of Attraction
- The properties of matter depend on the intermolecular forces between the particles the matter is composed of.
- London Dispersion Forces are attractive forces that exist between all atoms and molecules.
- Temporary dipoles can be induced in particles by uneven distribution of electrons. These temporary dipoles attract each other.
- These forces are strongest in large, polarizable molecules.
- Example 1: Iodine (I2) is a nonpolar molecule, but it is large (MW: 253.8 g/mol) and has a very polarizable electron cloud. This results in it having large London Dispersion forces between particles, and therefore being a solid at ambient conditions.
- Example 2: London forces between large CO2 atoms in the gas phase result in significant non-ideal behavior of CO2, whereas the much smaller, less polarizable helium (He) shows less deviation from ideal behavior.
- Dipole Forces result from attraction between the positive and negative ends of molecules with permanent dipoles.
- Dipoles are stronger than London Forces alone, so polar molecules tend to have stronger intermolecular forces than nonpolar molecules of a similar size and polarity.
- Hydrogen Bonds are a special type of dipole forces, in which a hydrogen atom is covalently bound to a very electronegative atom (N, O, F), resulting in a large dipole. This results in even small molecules having strong intermolecular bonding.
- Example: Water (H2O), has strong hydrogen bonds between molecules and therefore boils at 100°C. Hydrogen sulfide (H2S) and hydrogen selenide (H2Se) are larger and might be expected to have larger London forces, but they do not form strong hydrogen bonds and therefore have much lower boiling points, -60°C and -41°C, respectively.
- Ionic interactions are Coulombic interactions between positively and negatively charged ions. They are usually very strong, which is why ionic materials (such as table salt, NaCl) tend to be solids.
- Ions can also form strong interactions with dipoles of solvents in solution. This is why ionic solids tend to dissolve in polar solvents like water.
- Properties such as boiling point, vapor pressure, solubility in polar or nonpolar solvents, all depend on the types of intermolecular forces in a substance.
- Sample Problem: On the bases of intermolecular forces, rank the following elements/compounds by increasing boiling point: LiF, H2S, H2O, Ne.
- Answer: Ne < H2S < H2O < LiF
- Neon (Ne) is a noble gas, nonpolar and with only modest London Dispersion forces between atoms. It will be a gas at (and well below) room temperature, boiling at -246°C.
- Hydrogen sulfide (H2S) is a polar molecule. It will have polar interactions as well as London forces between molecules, and boils at -60°C.
- Water (H2O) has strong hydrogen bonding between molecules, and therefore will boil at a higher temperature than H2S: 100°C.
- Lithium fluoride is an ionic solid, with strong ionic interactions between particles. It boils at 1,676°C.
- The secondary structure of biological macromolecules (e.g. folding of proteins, base pairing in DNA) depend on many of the forces listed above, such as H-bonding (base pairs in DNA) and hydrophobic interactions (London dispersion forces).
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