Liquids and Solids PDF

## Chapter 2: Liquids and Solids ### Introduction - The kinetic energies of gas molecules decrease when the temperature is lowered. - Intermolecular attractive forces cause the gas molecules to condense into a liquid when the gas has been cooled sufficiently. - The molecules are closer together a...

## Chapter 2: Liquids and Solids ### Introduction - The kinetic energies of gas molecules decrease when the temperature is lowered. - Intermolecular attractive forces cause the gas molecules to condense into a liquid when the gas has been cooled sufficiently. - The molecules are closer together and the attractive forces exert a greater influence in a liquid than in a gas. - Molecular motion, therefore, is more restricted in the liquid state than in the gaseous state. - Additional cooling causes the kinetic energies of the molecules to decrease further and ultimately produces a solid. - In a crystalline solid the molecules assume positions in a crystal lattice, and the motion of the molecules is restricted to vibration about these fixed points. ### 1. Intermolecular Forces of Attraction - Atoms are held together in molecules by covalent bonds. - Intermolecular attractive forces hold molecules together in the liquid and solid states. - Two types of intermolecular attractive forces are discussed in this section, and a third is presented in the next section. #### Dipole-dipole Forces - Dipole-dipole forces occur between polar molecules. - Molecules have dipoles and line up in an electric field. - Dipole-dipole forces are caused by the attractions of the positive and negative poles of the molecules for one another. - Electronegativity differences between atoms can be used to predict the degree of polarity of a diatomic molecule as well as the positions of the positive and negative poles. #### London Forces - London forces (dispersion forces) attract nonpolar molecules to each other in the liquid and solid state. - The existence of London forces is postulated. - These forces are thought to arise from the motion of electrons. #### Hydrogen Bond - The intermolecular attractions of certain hydrogen-containing compounds are unusually strong. - These attractions occur in compounds in which hydrogen is covalently bonded to highly electronegative elements of small atomic size. - The atom of the electronegative element exerts a strong attraction on the bonding electrons, resulting in a significant positive charge on the hydrogen atom. - The hydrogen atom of one molecule and a pair of unshared electrons on the electronegative atom of another molecule are mutually attracted and form a hydrogen bond. - Hydrogen bonding increases the difficulty in separating the molecules from the liquid state. ### 2. The Liquid State - In gases, the molecules move rapidly in a completely random manner. - In solids, the molecules are held together in the orderly arrangements typical of crystals. - The liquid state is intermediate between the gaseous state and the solid state. ### 3. The Solid State - The normal freezing point is the temperature at which solid and liquid are in equilibrium under a total pressure of 1 atm. - Some supercooled liquids can exist for long periods, or even permanently, in this state. - Substances of this type have complex molecular forms for which crystallization is difficult. ### 4. Enthalpy of Vaporization - The enthalpy of vaporization is the quantity of heat that must be supplied to vaporize a mole of a liquid at a specified temperature. - It is usually recorded at the normal boiling point in kilojoules per mole. ### 5. Vapor Pressure - When a liquid in a closed container evaporates, the vapor molecules cannot escape from the vicinity of the liquid. - At equilibrium, the pressure that the vapor exerts is a constant. - The pressure of vapor in equilibrium with a liquid at a given temperature is called the equilibrium vapor pressure of the liquid. ### 6. Boiling Point - The temperature at which the vapor pressure of a liquid equals the external pressure is called the boiling point of the liquid. - The normal boiling point of a liquid defined as the temperature at which the vapor pressure of the liquid equals 1 atm. ### 7. Surface Tension - The surface tension of a liquid is a measure of the inward force on the surface of a liquid, the force which must be overcome to expand the surface area. ### 8. Evaporation - The kinetic energies of the molecules of a liquid follow a Maxwell-Boltzmann distribution. - Some of the molecules have kinetic energies high enough to overcome the attractive forces of surrounding molecules and escape from the liquid and enter the gas phase. ### 9. Freezing Point - The normal freezing point is the temperature at which solid and liquid are in equilibrium under a total pressure of 1 atm. - The freezing point of the solid-liquid system remains constant until all of the liquid is frozen. - The quantity of heat that must be removed to freeze a mole of a substance at the freezing point is called the molar enthalpy of crystallization. - Liquids that are undercooled or supercooled can usually be caused to revert to the freezing temperature and the stable solid-liquid equilibrium by scratching the interior walls of the container with a stirring rod or by adding a seed crystal.