Chemistry: Liquids and Solids

Study Notes

Kinetic energies of gas molecules decrease with lower temperatures.
Intermolecular attractive forces cause gases to condense into liquids.
Liquids have molecules closer together than gases, with greater attractive forces and restricted molecular motion.
Further cooling decreases kinetic energy, forming solids.
Crystalline solids have molecules in a fixed crystal lattice, restricted to vibrations around fixed points.
Intermolecular attractive forces are minimized in gases due to high kinetic energy.
Attractive forces easily overcome the low kinetic energy of molecules or ions in crystals, forming ordered crystalline structures.
Understanding the liquid state is less complete than the other two states (gas and solid).

Covalent bonds hold atoms together in molecules.
Intermolecular attractive forces hold molecules together in liquids and solids.
Two types of intermolecular forces are dipole-dipole forces and London dispersion forces (discussed in a subsequent section).
Dipole-dipole forces exist between polar molecules, aligning in electric fields. Polar molecules have positive and negative poles.
Polar molecules line up in crystals reflecting dipole-dipole forces.
Electronegativity differences predict polarity and position of positive/negative poles of diatomic and other molecules.
Predicting polarity in larger molecules requires considering molecular geometry, bond polarity, and nonbonding electron pairs.
London dispersion forces are postulated to exist between nonpolar molecules. These forces result from temporary dipoles created by the movement of electrons, leading to attractions between molecules.
Instantaneous fluctuations in electron distribution cause temporary dipoles in a molecule, and these temporary dipoles induce corresponding dipoles in neighboring molecules, producing attractive forces.
These forces are stronger in larger, more complex molecules with larger electron clouds.
London forces are significant intermolecular forces in nonpolar molecules, while dipole-dipole forces are important in polar molecules.

Intermolecular attractions in hydrogen-containing compounds (e.g., hydrogen bonded to highly electronegative atoms) are unusually strong.
The strongly electronegative atoms (small size) exert a strong attraction on bond electrons, leaving the hydrogen with a significant positive charge (δ+).
The hydrogen atom is almost an exposed proton with no screening electrons.
The hydrogen atom of one molecule and an unshared electron pair of a second molecule can be strongly attracted, creating a hydrogen bond.
Molecules with hydrogen bonds usually have abnormally high boiling points, heats of vaporization, heats of fusion, and viscosities.
Hydrogen bonding is also significant in solutions, where molecules of different types can be hydrogen-bonded.