1. Points in the kinetic molecular theory
a. Molecules of a gas are widely separated
b. No forces of attraction between molecules
c. Molecules are in constant, random, rapid, straight line motion.
d. The kinetic energy of the molecules is determined only by the temperature of the gas.
2. Points a and b are significantly different for liquids and solids.
a. Gases can be compressed, but for all practical purposes liquids and solids can not be.
(1) Large volume change: gas<--->Liquid
Small volume change: liquid<--->solid
(2) Particles in a liquid are packed about as closely as are those in solids.
(3) Compressibility-change in volume with change in pressure-small for liquids or solids.
b. Strong intermolecular forces exist between the particles in the liquid or solid state.
(1) prevents expansion of liquids or solids
(2) allows the pouring of liquids and solids from one container to another
B. Intermolecular Forces-the attractive forces between molecules, between ions or between ions and molecules
1. Ion-ion forces-previously discussed
2. Ion-dipole forces-attractions between an ion and the oppositely charged end of a dipole. (40-600 kJ/mol)
a. Factors effecting attraction
(1) The distance between the ion and the dipole, as distances decreases attraction increases.
(2) The charge on the ion-greater charge=greater attraction
(3) The magnitude of the dipole-greater dipole=greater attraction
3. Dipole-dipole Forces-oppositely charged ends of polar molecules attract.
a. Energy is released when the molecules
interact.
gas (polar)--->Liquid(polar) + energy
b. Hydrogen bonding-a very strong dipolar attraction
(1) H covalently bonded to O,N,F
(2) H attracted to another electronegative atom(O,N,F)
(3) The presence of H-bonding increases the solubility of a solute in water and causes an increase in bp and mp of the substance.
c. The bp of structurally analogous molecules generally increases with an increase in molecular mass unless some special force is present.
CH4 < SiH4 < GeH4 <SnH4
When H-bonding is present, the compound with H- bonding is the one with the highest bp.
H2O >> H2S< H2Se <H2Te
4. Induced Dipoles
a. Dipole-induced dipole forces-a polar molecule induces a dipole in a nonploar molecule. Not as strong as dipole-dipole attractions.
(1) The ease with which a nonpolar molecule can be distorted by a polar molecule is called polarizability .
(a) Size of electron cloud about atom a mass
(b) Number of valence e- about the atom a mass
b. Induced dipole-induced dipole forces-London forces or dispersion forces-one nonpolar molecule induces a dipole in another nonpolar molecule. Very weak forces
(1) Increases as mass increases
(2) Caused by distortion of electron clouds- momentary dipoles
C. Kinetic Molecular Theory of Liquids and Enthalpy of Vaporization
1. The molecules in a liquid are not arranged in a regular pattern and move randomly -somewhat like gases.
a. The distribution of energies of molecules in a liquid depends only on the temperature of the liquid.
(1) As T increases-average KE increases.
(2) As T increases-more molecules have KE >E
b. In order for vaporization or evaporation to occur, molecules must overcome the forces in the liquid phase and move to the vapor phase.
(1) Heat required(at constant P)=molar enthalpy of vaporization, 食 vap, always endothermic.
(2) 食 process=(mol)(食vap)
(3) 食 condensation= -食vaporization
2. Vapor Pressure and Boiling Point
a. Equilibrium vapor pressure-liquid placed in a closed container.
At a constant temperature an equilibrium vapor pressure will be established.
b. Vapor pressure measures the tendency of molecules to enter the vapor phase. Volatile means high vapor pressure. The vapor pressure increases as the temperature increases.
Vapor pressure-temperature curves can be found on Page 411.
c. Boiling point-the temperature, at constant pressure, at which the vapor pressure of a liquid = the atmospheric pressure.
(1) Normal bp= bp at 1 atm (760 Torr)
(2) Boiling point increases as the pressure above the liquid increases, but it is not a linear increase.
3. Critical Temperature and Pressure
a. Critical temperature, Tc, = the temperature beyond which all liquid molecules have sufficient energy to separate from each other. No liquid can exist at anypressure past this temperature.
b. Critical Pressure-the vapor pressure at Tc.
c. Critical Point-the point at which the liquid and vapor phase are indistinguishable.
d. Supercritical fluid-the phase that exists at temperatures and pressures above the critical point.
4. Surface Tension and Viscosity
a. Surface tension is a measurement of the toughness of the surface of a liquid. Energy is required to break through the surface of a liquid.
Decreases as the temperature increases
(1) Cohesive forces-intermolecular forces between like molecules
(2) Adhesive forces-intermolecular forces between unlike molecules.
(3) Wet-spread out on a surface if adhesive forces are strong enough to overcome cohesive forces.
b. Viscosity-the resistance to flow of a liquid. As viscosity increases, the liquid flows more slowly. Decreases as the temperature increases.
D. Principles of Solid State Structures
1. The Unit Cell-the smallest repeating unit that has all of the symmetry characteristics of the way the atoms are arranged.
2. Crystal Lattice-A solid with a regular repeating unit cell containing atoms, ions or molecules.
3. The Cubic Unit Cell-cells that have equal edges that meet at 90o angles
a. Simple cubic (sc)
b. Body centered cubic (bcc)
c. Face centered cubic (fcc)
All have 8 identical atoms or ions at the corners of a cube, but body centered has one additional identical atom at the center of the cube, and face centered has in the center of each of the six cube faces an atom or ion of the same type as the corner atom or ion.
d. A simple cubic unit cell contains a net of 1 atom
e. A body centered unit cell of X atoms (or ions) will always contain 2 net atoms (or ions) within the unit cell.
f. A face centered cubic unit cell of X atoms (or ions) always contains 4 net atoms (or ions) within the unit cell.
Calculations Involving Unit Cells
