JBruce Lecture 3 notes - 1/11/2008 Chem 106 Lecture 3...

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Unformatted text preview: 1/11/2008 Chem 106 Lecture 3 Chapter 13 Intermolecular Forces 1 Last time Molecular dipole prediction Electronegativity Molecular geometry l l Lewis dot structures Valence shell electron pair repulsion Ion-dipole interactions Ion Solubility Enthalpy of hydration Dipole-dipole interactions Dipole- Higher boiling point and enthalpy of vaporization Solubility "like dissolves like" 2 1 1/11/2008 There must be more... No charge, no dipole. Why are they soluble? 3 DIPOLEDIPOLE-INDUCED DIPOLES How can non How can nonpolar molecules such as O2 and I2 dissolve in water? The water dipole INDUCES a dipole in the O2 electric cloud. DipoleDipole-induced dipole Allows weak attraction between O2 and H2O 4 2 1/11/2008 DIPOLEDIPOLE-INDUCED DIPOLES Consider I2 dissolving in ethanol, CH3CH2OH. - I-I - O R H + The alcohol temporarily creates or INDUCES a dipole in I2. I-I + - O R H + 5 DIPOLEDIPOLE-INDUCED DIPOLES O2 is much larger than H2, more polarizable, and thus more soluble in a polar solvent. Process of inducing a dipole is polarization l Degree to which electron cloud of an atom or molecule can be distorted in its polarizability. 6 3 1/11/2008 Example 13.2 p.599 INDUCED DIPOLE FORCES Like dissolves like. I2 more soluble in CCl4. Forces involved? 7 London Dispersion Forces: Induced Dipole/Induced Dipole Forces Formation of a dipole in two nonpolar I2 molecules. 8 4 1/11/2008 London Dispersion Forces: Induced Dipole/Induced Dipole Forces The magnitude of the induced dipole depends on the tendency to be distorted. Higher MW > larger induced dipoles. Higher MW > larger induced dipoles Higher MW > larger induced dipoles. Boiling Points of Hydrocarbons C4H10 C3H8 C2H6 Molecule CH4 (methane) C2H6 (ethane) C3H8 (propane) C4H10 (butane) Boiling Point (oC) - 161.5 - 88.6 - 42.1 - 0.5 CH4 9 Intramolecular forces thus far... 1. . 2. 3. 4. Iondipole Dipoledipole ipole dipole Dipoleinduced dipole Induced dipoleinduced dipole (London dispersion) EXAMPLE: What IM forces are responsible for holding O2 in the liquid state? 10 5 1/11/2008 EXAMPLE: What IM forces are responsible for holding BF3 in the liquid state? Planar 1. 2. 3. 4. Iondipole Polar or Nonpolar? Nonpolar? Dipoledipole Dipoleinduced dipole Induced dipoleinduced dipole (London dispersion) 11 Intermolecular Forces: Hydrogen Bonding A special form of dipoledipole attraction, which A special form of dipoledipole attraction, which enhances dipoledipole attractions. enhances dipole H-bonding is strongest when X and Y are N, O, or F 12 6 1/11/2008 Boiling Points of Simple HydrogenHydrogen-Containing Compounds Active Figure 13.8 13 Hydrogen Bonding in Biology Hbonding is especially strong in biological systems -- systems -- such as DNA. y DNA -- DNA -- helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O. Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases. g p --adenine with thymine --guanine with cytosine 14 7 1/11/2008 Double helix of DNA Portion of a DNA chain 15 Hydrogen Bonding in H2O Hbonding is especially strong in water because strong in water because the O--H bond is very the O-- polar there are 2 lone pairs on the O atom Accounts for many of Accounts for many of water's unique properties. 16 8 1/11/2008 Hydrogen Bonding in Ice Ice has open lattice latticelike structure. Ice density is < liquid. And so solid floats on water. Snow flake: www.snowcrystals.com 17 Hydrogen Bonding in H2O Ice has open latticelike structure. Ice has open lattice y q Ice density is < liquid and so solid floats on water. Liquid molecules occupy smaller volume than solid. One of the VERY few substances where One of the VERY few substances where solid is LESS DENSE than the liquid. 18 9 1/11/2008 A consequence of hydrogen bonding 19 Intermolecular Forces Summary 20 10 1/11/2008 Intermolecular Forces Figure 13 13 13.13 21 Liquids Section 13.5 In a liquid molecules are in constant motion there are appreciable intermolec. forces intermolec. forces molecules close together Liquids are almost incompressible p Liquids do not fill the container 22 11 1/11/2008 Liquids The two key properties we need to describe are EVAPORATION and its opposite--CONDENSATION opposite-- pp evaporation---> evaporation---> LIQUID Add energy VAPOR break IM bonds make IM bonds Remove energy <---condensation ---condensation 23 Liquids-- Liquids--Evaporation To evaporate, molecules must have sufficient energy to break IM b k forces. Breaking IM forces requires energy. The p gy process of evaporation is endothermic. endothermic. 24 12 1/11/2008 Distribution of Energy in a Liquid Figure 13.14 25 When molecules of liquid are in the vapor state, they exert a VAPOR PRESSURE Liquids EQUILIBRIUM VAPOR PRESSURE is the pressure exerted by a vapor over a liquid in a closed container when the rate of evaporation = the rate of condensation. f d 26 13 1/11/2008 For Monday Read Chapter 13 sections 5 and 6 COMPLETE homework assignment 1 on OWL 27 14 ...
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