C121 11-3 lec31GL

C121 11-3 lec31GL - Lecture 31 November 23, 2011...

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Lecture 31 November 23, 2011 Intermolecular Forces Last Day: Intermolecular Forces (Chapter 16) Introduction/Definitions Dipole-Dipole Forces Hydrogen Bonding Dispersion Forces Physical Properties Today: Intermolecular Forces Physical Properties Trends in Physical Properties Vapour Pressure Temperature Dependence of Vapour Pressure
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Types of Intermolecular Forces A Dipole-Dipole Forces (between polar molecules)
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B Hydrogen-Bonding (a special case of dipole-dipole forces) ONLY occurs in compounds where H is directly bonded to a small atom of high electronegativity (ie. only in compounds with F-H, N-H or O-H bonds) ex. H 2 O, NH 3 , CH 3 COOH etc.
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C London Dispersion Forces All molecules, atoms, ions exhibit dispersion forces i.e. all are POLARIZABLE can have their e- cloud distorted to create a dipole (i.e. a dipole can be induced) Two Types 1 dipole-induced dipole (in response to external dipole) polar molecule comes close enough to a non-polar molecule to induce a dipole ex. O 2 (g) in H 2 O (l) non-polar polar O 2 H - δ + δ + δ - δ + induced
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2 instantaneous dipole-induced dipole momentary distortion of e- cloud on one atom / molecule can induce a dipole moment in a nearby atom /molecule
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Some Properties of Bulk Matter Solid Liquid Gas m.p. UH fusion f.p. Fairly Strong IMF (moderate motion) Viscosity Surface tension Vapour pressure raise T raise T lower T lower T b.p. UH vaporization Strong IMF (little motion) Weak IMF (lots of motion) UH sublimation
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Many physical properties of liquids are related to their intermolecular forces: (i) Compressibility (ii) Lack of rigidity (iii) High densities (iv) Properties involving changes of state (v) Viscosity Physical Properties and Intermolecular Forces
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(vi) Surface Tension • tendency of liquids to maintain a ( γ ) [J/m 2 ] minimum surface area Within the liquid, intermolecular forces act in all directions no net pull At the surface, imbalance of intermolecular forces net pull inwards and together at the surface
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C121 11-3 lec31GL - Lecture 31 November 23, 2011...

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