CHEMISTRY THE CENTRAL SCIENCE

11 LIQUIDS AND INTERMOLECULAR FORCES

CHAPTER SUMMARY AND KEY TERMS

INTRODUCTION AND SECTION 11.1 Substances that are gases or liquids at room temperature are usually composed of molecules. In gases the intermolecular attractive forces are negligible compared to the kinetic energies of the molecules; thus, the molecules are widely separated and undergo constant, chaotic motion. In liquids the intermolecular forces are strong enough to keep the molecules in close proximity; nevertheless, the molecules are free to move with respect to one another. In solids the intermolecular attractive forces are strong enough to restrain molecular motion and to force the particles to occupy specific locations in a three-dimensional arrangement.

SECTION 11.2 Three types of intermolecular forces exist between neutral molecules: dispersion forces, dipole–dipole forces, and hydrogen bonding. Dispersion forces operate between all molecules (and atoms, for atomic substances such as He, Ne, Ar, and so forth). As molecular weight increases, the polarizability of a molecule increases, which results in stronger dispersion forces. Molecular shape is also an important factor. Dipole–dipole forces increase in strength as the polarity of the molecule increases. Hydrogen bonding occurs in compounds containing O—H, N—H, and F—H bonds. Hydrogen bonds are generally stronger than dipole–dipole or dispersion forces. Ion–dipole forces are important in solutions in which ionic compounds are dissolved in polar solvents.

SECTION 11.3 The stronger the intermolecular forces, the greater is the viscosity, or resistance to flow, of a liquid. The surface tension of a liquid also increases as intermolecular forces increase in strength. Surface tension is a measure of the tendency of a liquid to maintain a minimum surface area. The adhesion of a liquid to the walls of a narrow tube and the cohesion of the liquid account for capillary action and the formation of a meniscus at the surface of a liquid.

SECTION 11.4 A substance may exist in more than one state of matter, or phase. Phase changes are transformations from one phase to another. Changes of a solid to liquid (melting), solid to gas (sublimation), and liquid to gas (vaporization) are all endothermic processes. Thus, the heat of fusion (melting), the heat of sublimation, and the heat of vaporization are all positive quantities. The reverse processes (freezing, deposition, and condensation) are exothermic. A gas cannot be liquefied by application of pressure if the temperature is above its critical temperature. The pressure required to liquefy a gas at its critical temperature is called the critical pressure. When the temperature exceeds the critical temperature and the pressure exceeds the critical pressure, the liquid and gas phases cannot be distinguished and the substance is in a state called asupercritical fluid.

SECTION 11.5 The vapor pressure of a liquid indicates the tendency of the liquid to evaporate. The vapor pressure is the partial pressure of the vapor when it is in dynamic equilibrium with the liquid. At equilibrium the rate of transfer of molecules from the liquid to the vapor equals the rate of transfer from the vapor to the liquid. The higher the vapor pressure of a liquid, the more readily it evaporates and the more volatile it is. Vapor pressure increases nonlinearly with temperature. Boiling occurs when the vapor pressure equals the external pressure. The normal boiling point is the temperature at which the vapor pressure equals 1 atm.

SECTION 11.6 The equilibria between the solid, liquid, and gas phases of a substance as a function of temperature and pressure are displayed on a phase diagram. A line indicates equilibria between any two phases. The line through the melting point usually slopes slightly to the right as pressure increases, because the solid is usually more dense than the liquid. The melting point at 1 atm is the normal melting point. The point on the diagram at which all three phases coexist in equilibrium is called the triple point.

SECTION 11.7 A liquid crystal is a substance that exhibits one or more ordered phases at a temperature above the melting point of the solid. In a nematic liquid crystal the molecules are aligned along a common direction, but the ends of the molecules are not lined up. In a smectic liquid crystal the ends of the molecules are lined up so that the molecules form layers. In smectic A liquid crystals the long axes of the molecules line up perpendicular to the layers. In smectic C liquid crystals the long axes of molecules are inclined with respect to the layers. A cholesteric liquid crystal is composed of molecules that align parallel to each other within a layer, as they do in nematic liquid crystalline phases, but the direction along which the long axes of the molecules align rotates from one layer to the next to form a helical structure. Substances that form liquid crystals are generally composed of molecules with fairly rigid, elongated shapes, as well as polar groups to help align molecules through dipole-dipole interactions.

KEY SKILLS

• Identify the intermolecular attractive interactions (dispersion, dipole–dipole, hydrogen bonding, ion–dipole) that exist between molecules or ions based on their composition and molecular structure and be able to compare the relative strengths of these intermolecular forces. (Section 11.2)

• Explain the concept of polarizability and how it relates to dispersion forces. (Section 11.2)

• Explain the concepts of viscosity and surface tension in liquids. (Section 11.3)

• Know the names of the various changes of state for a pure substance. (Section 11.4)

• Interpret heating curves and be able to calculate quantities related to temperature and enthalpies of phase changes. (Section 11.4)

• Define critical pressure, critical temperature, vapor pressure, normal boiling point, normal melting point, critical point, and triple point. (Sections 11.5 and 11.6)

• Be able to interpret and sketch phase diagrams. Explain how water's phase diagram differs from most other substances, and why. (Section 11.6)

• Understand how the molecular arrangements characteristic of nematic, smectic, and cholesteric liquid crystals differ from ordinary liquids and from each other. Be able to recognize the features of molecules that favor formation of liquid crystalline phases. (Section 11.7)