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Intermolecular Forces: Liquids, and Solids

Intermolecular Forces: Liquids, and Solids

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Intermolecular Forces: Liquids, and Solids

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  1. Intermolecular Forces: Liquids, and Solids Chapter 13 Chapter 13

  2. A Molecular Comparison of Liquids and Solids Chapter 13

  3. Intermolecular Forces Chapter 13

  4. Intermolecular Forces Ion-Dipole Forces • Interaction between an ion (Na+) and a dipole (water). • Strongest of all intermolecular forces Chapter 13

  5. Intermolecular Forces Chapter 13

  6. 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 13

  7. Intermolecular Forces Chapter 13

  8. Intermolecular Forces London Dispersion ForcesInduced 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 13

  9. Intermolecular Forces London Dispersion Forces Chapter 13

  10. 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 13

  11. Intermolecular Forces Hydrogen Bonding Chapter 13

  12. 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 13

  13. 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 13

  14. Some Properties of Liquids Surface Tension • Cohesive forces bind molecules to each other. • Adhesive forces bind molecules to a surface. Chapter 13

  15. 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 13

  16. 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 13

  17. 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 (DHovap) • 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 13

  18. 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 13

  19. 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 13

  20. 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 13

  21. 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 DHovap = enthalpy of vaporization Chapter 13

  22. 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 13

  23. Properties of Liquids Vapor Pressure and Boiling Point Chapter 13

  24. 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 13

  25. Structures of Solids Unit Cells Chapter 13

  26. Structures of Solids Unit Cells Chapter 13

  27. Structures of Solids Cell Occupancy Chapter 13

  28. Structures of Solids Cell Occupancy Chapter 13

  29. Structures of Solids Cell Occupancy Chapter 13

  30. Structures of Solids Cell Occupancy Zn4S4 ZnS Chapter 13

  31. 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 13

  32. Structures of Solids Close Packing of Spheres Chapter 13

  33. 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 13

  34. 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 13

  35. 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 13

  36. Phase Diagrams • Phase diagram: plot of pressure vs. temperature summarizing all equilibria between phases. Chapter 13

  37. Phase Diagrams Chapter 13

  38. 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 Chapter 13

  39. Homework 2, 16, 18, 24, 30, 32, 40, 44 Chapter 13