Chapter 136 Intermolecular Forces Dipole-Dipole Forces -Interaction between an dipole on one molecule and a dipole on an adjacent molecule. -Dipole-dipole forces exist between neutral polar molecules. -Weaker than ion-dipole forces
Chapter 138 Intermolecular Forces London Dispersion Forces Induced Dipole – Induced Dipole -Weakest of all intermolecular forces. -It is possible for two adjacent nonpolar molecules to affect each other. -The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). -This attraction causes the electron clouds become distorted. -In that instant a polar molecule (dipole) is formed (called an instantaneous dipole).
Chapter 139 Intermolecular Forces London Dispersion Forces
Chapter 1310 Intermolecular Forces Hydrogen Bonding -A special case of dipole-dipole forces. -This intermolecular force is very strong. -Strongest of the three Van der Waal’s forces (Hydrogen bonding, Dipole-dipole, London forces,) -H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N).
Chapter 1312 Some Properties of Liquids Viscosity -Viscosity is the resistance of a liquid to flow. -A liquid flows by sliding molecules over each other. -The stronger the intermolecular forces, the higher the viscosity.
Chapter 1313 Some Properties of Liquids Surface Tension -The surface of a liquid behaves as a membrane or barrier. -This is due to the unequal attractive forces on molecules as the surface. -Surface molecules are only attracted inwards towards the bulk molecules.
Chapter 1314 Some Properties of Liquids Surface Tension -Cohesive forces bind molecules to each other. -Adhesive forces bind molecules to a surface.
Chapter 1315 Some Properties of Liquids Surface Tension -Meniscus is the shape of the liquid surface. –If adhesive forces are greater than cohesive forces, the liquid surface is attracted to its container more than the bulk molecules. Therefore, the meniscus is U-shaped (e.g. water in glass). –If cohesive forces are greater than adhesive forces, the meniscus is curved downwards.
Chapter 1316 Some Properties of Liquids Surface Tension Capillary Action - When a narrow glass tube is placed in water, the meniscus pulls the water up the tube.
Chapter 1317 Properties of Liquids Vaporization Also called evaporation –A process in which a substance is transfromed from a liquid to a gas. Standard molar enthalpy of vaporization ( H o vap ) –The energy required to convert one mole of a liquid at its boiling point to a gas. The resulting gas will exert a pressure on a system.
Chapter 1318 Properties of Liquids Vapor Pressure This is the pressure exerted by a substance in the gas phase. As a liquid’s temperature increases, its vapor pressure increases.
Chapter 1319 Properties of Liquids Vapor Pressure Volatile – A substance which has a low boiling point Or A substance which has a high vapor pressure at a low temperature
Chapter 1320 Properties of Liquids Vapor Pressure and Boiling Point -Liquids boil when the external pressure equals the vapor pressure. -Two ways to get a liquid to boil: increase temperature or decrease pressure. -Normal boiling point is the boiling point at 760 mmHg (1 atm).
Chapter 1321 Properties of Liquids Vapor Pressure and Boiling Point -Vapor pressure, temperature and enthalpy of vaporization can be related to each other using: Clausius-Clapeyron equation: P = pressure T = temperature R = gas law H o vap = enthalpy of vaporization
Chapter 1322 Properties of Liquids Vapor Pressure and Boiling Point The Clausius-Clapeyron equation makes more sense when it is rearranged into the slope intercept form.
Chapter 1323 Properties of Liquids Vapor Pressure and Boiling Point
Chapter 1324 Structures of Solids Unit Cells -Crystalline solid: well-ordered, definite arrangements of molecules, atoms or ions. -Crystals have an ordered, repeated structure. -The smallest repeating unit in a crystal is a unit cell. -Three-dimensional stacking of unit cells is the crystal lattice.
Chapter 1330 Structures of Solids Cell Occupancy Zinc (grey)4 Center4 atoms Sulfur (yellow)8 corners 6 faces 1 atom 3 atoms Zn 4 S 4 ZnS
Chapter 1331 Structures of Solids Close Packing of Spheres -A crystal is built up by placing close packed layers of spheres on top of each other. -There is only one place for the second layer of spheres. -There are two choices for the third layer of spheres: -Third layer eclipses the first (ABAB arrangement). This is called hexagonal close packing (hcp). -Third layer is in a different position relative to the first (ABCABC arrangement). This is called cubic close packing (ccp).
Chapter 1332 Structures of Solids Close Packing of Spheres
Chapter 1333 Structures of Solids Close Packing of Spheres -Each sphere is surrounded by 12 other spheres (6 in one plane, 3 above and 3 below). -Coordination number: the number of spheres directly surrounding a central sphere.
Chapter 1334 Structures of Solids Other Kinds of Solid Materials Molecular Solids These are crystalline substances in which the “building blocks” are composed of molecules in place of ions. Example: Table Sugar
Chapter 1335 Structures of Solids Other Kinds of Solid Materials Network Solids These are crystalline substances in which the “building blocks” are atoms and all the atoms are connected by covalent bonds. Example: Diamond
Chapter 1336 Phase Diagrams -Phase diagram: plot of pressure vs. temperature summarizing all equilibria between phases.
Chapter 1338 Phase Diagrams Triple point - Temperature and pressure at which all three phases are in equilibrium. Critical point – Point above which the liquid and gas phases are indistinguishable. Critical temperature - The minimum temperature for liquefaction of a gas using pressure Critical pressure - Pressure required for liquefaction