C121 11-3 lec30GL

C121 11-3 lec30GL - Lecture 30 November 21, 2011...

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Unformatted text preview: Lecture 30 November 21, 2011 Intermolecular Forces Last Day: II Molecular Orbital Theory (14.2-14.4) Examples Today: Intermolecular Forces (Chapter 16) Introduction/Definitions Dipole-Dipole Forces Hydrogen Bonding Dispersion Forces Physical Properties and Intermolecular Forces Liquids, Solids and Intermolecular Forces What are Intermolecular Forces? (IMF) • the attractive forces (electrostatic in nature) that exist between molecules, atoms and ions • intermolecular forces weaker than intramolecular forces (between molecules) (within molecules) 2 – 20 kJ/mol 100’s-1000’s kJ/mol ie. it takes less energy to vaporize water (overcome intermolecular forces) than it does to break the O-H bonds (overcome intramolecular forces) For 1 mole UH vap (H 2 O) = 40.7 kJ D(O-H) = 2(467 kJ ) Many physical properties of solids, liquids (condensed phases) and gases can be explained in terms of the types and relative strengths of the intermolecular forces present ie. state changes m.p., b.p., UH fusion , UH vaporization surface tension viscosity vapour pressure Types of Intermolecular Forces A Dipole-Dipole Forces (between polar molecules) Recall: polar bonds polar molecules P F δ + δ- due to differences in electronegativity PF 3 P F F F : δ- δ- δ- 3δ + net dipole moment Σμ ≠ 0 δ- F F δ- P : F δ- 3δ + P : δ- F δ- 3δ + P : F δ- F δ- 3δ + P : F δ- F δ- F F δ- F δ- δ- 3δ + P : F F δ- F δ- Polar covalent molecules are electrostatically attracted to each other through their dipoles ex. in the condensed (liquid) phase :C Ξ O: δ + δ- :C Ξ O: δ + δ- :C Ξ O: δ + δ- : C Ξ O : δ + δ- : O Ξ C : δ- δ + Dipole-dipole forces result in semi-organized arrangement whereas CCl 4 tetrahedral Note: • the strength of the dipole-dipole forces depends on the magnitude of the dipole moment • dipole-dipole forces operate over shorter distances (ie. they are weaker) than ion-ion interactions C Cl Cl Cl Cl Σμ = O , non-polar no dipole-dipole forces even though there are polar bonds B Hydrogen-Bonding (a special case of dipole-dipole forces) • ONLY occurs in compounds where H is directly bonded to a...
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This note was uploaded on 12/10/2011 for the course CHEM 121 taught by Professor Leech during the Fall '11 term at Simon Fraser.

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C121 11-3 lec30GL - Lecture 30 November 21, 2011...

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