Unit 3 · Intermolecular Forces and Properties

London dispersion forces (LDF) ● (core concept) — Intermolecular forces arising from temporary, fluctuating dipoles in electron clouds. They increase with contact area, number of electrons, and polarizability, and are often the strongest IMF between large molecules.

Polarizability ● (core concept) — How easily an electron cloud is distorted; it increases with more electrons and larger atoms, and is enhanced by pi bonding.

Dipole-induced dipole forces ● (core concept) — Attractive forces between a polar molecule and a nonpolar molecule, caused by the polar molecule inducing a temporary dipole in the nonpolar one; strength depends on the dipole's size and the polarizability.

Dipole-dipole forces ● (core concept) — Attractive forces between the permanent dipoles of two polar molecules.

Ion-dipole forces ● (core concept) — Attractive forces between an ion and a polar molecule.

Hydrogen bonding ● (core concept) — An especially strong dipole-dipole interaction in which hydrogen bonded to N, O, or F is attracted to N, O, or F on another molecule (or another part of the same molecule).

van der Waals forces ○ — A general term for all intermolecular forces; do not use it as a synonym for London dispersion forces, which are only one type.

Noncovalent interactions in biomolecules ● (core concept) — In large biomolecules and polymers, noncovalent interactions can occur between different molecules or between different regions of the same molecule; the molecule's shape, and therefore its function, is largely set by these interactions.

Boiling point and IMFs ● (core concept) — The boiling point of a substance increases with the strength of its intermolecular forces, since more energy is needed to separate the particles.

Vapor pressure ● (core concept) — The pressure exerted by a vapor in equilibrium with its liquid or solid; substances with stronger IMFs have lower vapor pressure.

Ionic solids ● (core concept) — Solids made of ions; they have low vapor pressure, high melting and boiling points, are brittle, and conduct electricity only when molten or dissolved in water.

Covalent network solids ● (core concept) — Solids in which nonmetal or metalloid atoms are linked by covalent bonds in a continuous network — three-dimensional (e.g., diamond, silicon dioxide, silicon carbide) or two-dimensional layers (e.g., graphite). They are hard and have high melting points, except that layered networks like graphite are soft because the layers slide.

Molecular solids ● (core concept) — Solids made of discrete molecules held together by weak intermolecular forces; they have low melting points and do not conduct electricity.

Metallic solids (properties) ● (core concept) — Solids with mobile delocalized electrons; they are good conductors of heat and electricity and are malleable and ductile, while alloys are less malleable because the foreign atoms distort the lattice.

Crystalline solid ● (core concept) — A solid whose particles are arranged in a regular, repeating three-dimensional pattern.

Amorphous solid ● (core concept) — A solid whose particles have no regular arrangement.

Molar volumes of solids and liquids ● (core concept) — A substance's solid and liquid phases typically have similar molar volumes, because in both phases the particles are in close contact at all times.

Ideal gas law ● (core concept) — PV = nRT, relating the pressure, volume, amount (moles), and Kelvin temperature of an ideal gas.

Partial pressure ● (core concept) — The pressure a single gas in a mixture would exert on its own: P_A = P_total × X_A, where X_A is the mole fraction of gas A.

Dalton's law of partial pressures ● (core concept) — The total pressure of a gas mixture equals the sum of the partial pressures of the individual gases: P_total = P_A + P_B + P_C + …

Mole fraction ● (core concept) — The moles of one component divided by the total moles in the mixture: X_A = n_A / n_total.

Kinetic molecular theory (KMT) ● (core concept) — The theory that relates the macroscopic properties of gases to the motion of their particles: gas particles move randomly, collide elastically, and have kinetic energy proportional to Kelvin temperature.

Maxwell-Boltzmann distribution ● (core concept) — The distribution of particle kinetic energies in a gas at a given temperature; as temperature rises, the average energy rises and the distribution spreads.

Average kinetic energy of gas particles ● (core concept) — KE = ½mv² per particle, and it is proportional to the Kelvin temperature of the gas.

Deviations from ideal gas behavior ● (core concept) — Real gases deviate from PV = nRT when intermolecular attractions matter (near conditions where the gas would condense) or when particle volumes are significant (at high pressure).

Homogeneous mixture (solution) ● (core concept) — A mixture with uniform macroscopic properties throughout; solutions can be solids, liquids, or gases.

Heterogeneous mixture ● (core concept) — A mixture whose properties vary depending on where in the sample you look.

Molarity ● (core concept) — Concentration in moles of solute per liter of solution: M = n_solute / L_solution.

Particulate representations of solutions ● (core concept) — Diagrams of solutions at the particle level show the relative concentrations of components and the interactions between solute and solvent particles.

Filtration ● (core concept) — A separation method that passes a mixture through a porous barrier; it cannot separate dissolved components from a liquid solution.

Chromatography ● (core concept) — A separation technique (paper, thin-layer, or column) in which components separate based on how strongly they interact with a mobile phase versus a stationary phase; the resulting chromatogram lets you infer relative polarities.

Distillation ● (core concept) — A separation method that exploits differences in vapor pressure (boiling point) between components of a mixture.

"Like dissolves like" ● (core concept) — A substance tends to dissolve in a solvent with similar intermolecular forces: polar solutes in polar solvents, nonpolar in nonpolar.

Electromagnetic spectrum and transitions ● (core concept) — Regions of the electromagnetic spectrum correspond to different molecular energy changes: microwaves cause rotational transitions, infrared causes vibrational transitions, and ultraviolet/visible light causes electronic transitions.

Photon absorption and emission ● (core concept) — When a species absorbs or emits a photon, its energy changes by exactly the photon's energy.

Wave equation ● (core concept) — c = λν, relating the speed of light to the wavelength and frequency of an electromagnetic wave.

Planck's equation ● (core concept) — E = hν, giving the energy of a photon from its frequency.

Beer-Lambert law ● (core concept) — A = εbc: the absorbance of a solution is proportional to the molar absorptivity (ε), the path length (b), and the concentration (c), so absorbance can be used to find concentration.

Spectrophotometer ● (core concept) — An instrument that measures absorbance; it is set to the wavelength of maximum absorbance for the most sensitive concentration measurements.

Molar absorptivity (ε) ● (core concept) — In A = εbc, the molar absorptivity ε describes how intensely a chemical species absorbs light of a specific wavelength.