Unformatted text preview: Chapter 16 Objectives
16.1 Describe the differences between intermolecular (between molecules) and intramolecular (within molecules) forces with an emphasis on the relative energies of each type of attraction. 16.2 Discuss the influence of these forces on physical properties such as freezing point and boiling point. 16.3 Provide real-life applications with biological systems such as DNA or proteins, and other systems as appropriate. 05/11/09 Zumdahl Chapter 16 1 Intermolecular Forces: Why Condensed Phases Exist Intramolecular Forces, i.e., Chemical bonds Strong Directional Short Range (relative) Intermolecular Forces Weaker than chemical bonds, usually much weaker Less directional than covalent bonds, more directional than ionic bonds Longer range than covalent bonds but at shorter range than ionic bonds Condensed Phases Solids and Liquids Intermolecular forces: Mutual attractions hold the molecules closer together than gases
Zumdahl Chapter 16 2 05/11/09 Types of Intermolecular (Non-bonding) Interactions 05/11/09 Zumdahl Chapter 16 3 Dipole - Dipole Interactions and Hydrogen Bonding What is a hydrogen bond? A particularly strong dipole-dipole interaction where hydrogen is "bound" to a highly electronegative second row atom, i.e., N, O, or F. The strength of the interaction results from the great polarity of the bond and the closer than normal approach of the dipoles.
05/11/09 Zumdahl Chapter 16 4 The Boiling Points of the Covalent Hydrides of Elements in Groups 4A, 5A, 6A, and 7A. 05/11/09 Note Importance of Hydrogen Bonding Zumdahl Chapter 16 5 Ion - Dipole Interactions 05/11/09 Zumdahl Chapter 16 6 Ion - Induced Dipole Interactions
Ar Ar 05/11/09 Zumdahl Chapter 16 7 London Dispersion Forces Instantaneous fluctuations in electron distribution in atoms or molecules with no permanent dipole moment
05/11/09 Zumdahl Chapter 16 8 Kinetic Theory of Liquids and Solids Intermolecular distances Solids about 3 X 10 -10 m = .3nm Liquids about 5 X 10 -10 m = .5nm Gases: much larger than liquids unless P > 100 atm Intramolecular bonds About 0.05 to 0.25 nm
Zumdahl Chapter 16 05/11/09 9 The Liquid State
Characteristics of the Liquid State Low compressibility High density, relative to gases Surface tension Exhibit capillary action Cohesive forces Adhesive forces Viscosity water is "thin", having a lower viscosity vegetable oil is "thick" having a higher viscosity.
05/11/09 Zumdahl Chapter 16 10 A molecule in the interior of a liquid is attracted to the molecules surrounding it, whereas a molecule at the surface of liquid is attracted only by molecules below it and on each side of it.
Cohesive forces Adhesive forces bind molecules of the same type together bind a substance to a surface 05/11/09 Zumdahl Chapter 16 11 Adhesive Forces and Capillary Action X Concave Meniscus
Adhesive forces > Cohesive Example: X X Convex Meniscus
Cohesive forces > Adhesive Example: Hg X X X X x 05/11/09 Zumdahl Chapter 16 12 Introduction to Structures and Types of Solids
Crystalline Solids Highly regular arrangement "Lattice": 3D arrangement, with unit cell structures Simple cubic (Po metal) Body-centered cubic (Ur metal) Face-centered cubic (Au metal) Amorphous Solids 05/11/09 Disordered structures Non-crystalline E.g., window glass Zumdahl Chapter 16 13 Three cubic unit cells and the corresponding lattices 05/11/09 Zumdahl Chapter 16 14 X-rays scattered from two different atoms may reinforce (constructive interference) or cancel (destructive interference) one another d = =
05/11/09 Distance between atoms wavelength Zumdahl Chapter 16 15 Reflection of X rays
Bragg Equation n L=2 d sin O 05/11/09 Zumdahl Chapter 16 16 Examples of Three Types of Crystalline Solids
Atomic Solids Ionic Solids Molecular Solids 05/11/09 Zumdahl Chapter 16 17 Interacting molecules or ions Consider the composition, elcetronegativies and structure of the species Please also review Chapter 13 such concepts as ions, ionic, covalent bonding and dipole moments NO Are polar molecules involved? NO Are ions involved? YES Are polar molecules and ions present? NO YES (Dipole-Dipole interactions) YES YES Are hydrogen atoms bonded to N, O, or F atoms? NO Dispersion forces only (induced dipoles) Examples: Ar (l), I2 (s) Dipole-Dipole forces Examples: H2S, CH3Cl Hydrogen Bonding (specialized type of dipole-dipole) Examples: liquid and solid H2O, NH3, HF Ion-Dipole forces Examples: KBr in water Ionic Bonding Examples: NaCl, NH4NO3 Van der Waals forces highest Strength of Interspecies Forces Generally Increases from Left to Right
Notes 1. Dispersive forces are found in all substances. The strength of dispersive forces increases with Molar Mass. 2. Dipole-Dipole forces add to the effect of dispersive forces and are found in polar molecules. 3. Hydrogen bonds, which require H atoms bonded to F, O, or N, also add to the effect of dispersion forces. Hydrogen bonds tend to be the strongest type of intermolecular forces. 4. Ordinary Ionic (and covalent) bonding forces are the strongest. 05/11/09 Zumdahl Chapter 16 18 Chapter 16 Liquids and Solids
16.1 Intermolecular Forces 16.2 The Liquid State 16.3 An Introduction to Structures and Types of Solids 16.4 Structure and Bonding in Metals 16.5 Carbon and Silicon: Network Atomic Solids 16.6 Molecular Solids 16.7 Ionic Solids 16.8 Structures of Actual Ionic Solids 16.9 Lattice Defects 16.10 Vapor Pressure and Changes of State 16.11 Phase Diagrams 05/11/09 Zumdahl Chapter 16 19 ...
View Full Document