Intermolecular Forces and Liquids and Solids Chapter 11

A phase – the homogeneous part of the system in contact with other parts of the system but separated from them by a well- defined boundary. Fig 11.1 Molecular-level comparison of gases, liquids, and solids

States of Matter Because in the solid and liquid states particles are closer together, we refer to them as condensed phases. Fig 11.1 Molecular-level comparison of gases, liquids, and solids

Intermolecular Forces Intermolecular forces - attractive forces between molecules. Intramolecular forces - hold atoms together in a molecule. “Measure” of intermolecular force boiling point melting point  H vap  H fus  H sub Generally, intermolecular forces are much weaker than intramolecular forces.

State a substance at a particular temperature and pressure depends on two antagonistic entities: – the kinetic energy of the particles; – the strength of the attractions between the particles.

These intermolecular forces as a group are referred to as van der Waals forces Ion-dipole forces Dipole-dipole interactions Hydrogen bonding London dispersion forces

Ion-dipole interactions Important in solutions of ions Attractive forces between an ion and a polar molecule Strength of these forces makes it possible for ionic substances to dissolve in polar solvents. Fig 11.3 Intermolecular Forces

Interaction of the water molecule with two different ions

Intermolecular Forces Dipole-dipole forces Attractive forces between polar molecules Molecules with permanent dipoles are attracted to each other Important when the molecules are close to each other Fig 11.4

Intermolecular Forces London dispersion forces Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules These forces are present in all molecules, whether they are polar or nonpolar Fig 11.5

Intermolecular Forces Dispersion forces continued Polarizability - ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass.

S O O What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH 4 CH 4 is nonpolar: dispersion forces. SO 2 SO 2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO 2 molecules.

Intermolecular Forces Hydrogen bond Hydrogen bond - special dipole-dipole interaction between the hydrogen atom in a polar N−H, O−H, or F−H bond and an electronegative O, N, or F atom on another molecule A H … B A H … A or A & B are N, O, or F Fig 11.8

Hydrogen Bond A special type of dipole-dipole interaction HCOOH

Why is the hydrogen bond considered a “special” dipole-dipole interaction? Decreasing molar mass Decreasing boiling point Fig 11.7 Boiling point as function of molecular weight