CH131_5A

CH131_5A - Homework #5 Read Chap 10.2 - 10.6 Problems: 10 -...

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Homework #5 Read Chap 10.2 - 10.6 Problems: 10 - 15, 17, 21, 23, 25, 27, 31, 35, 37, 39, 43, 45, 47
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Intermolecular forces (Chap. 10, sections 2-6) Gases (Chap. 9) - molecules far apart on average (at least at low P, ideal gases). For solids & liquids - inter molecular forces are crucial to consider: are more important because molecules are closer together, on average in liquids and solids than in gas; d liq ~d solid >> d gas hence they dominate the character of bulk properties of liquids Unlike gases; solids have definite shape We call solids and liquids the “condensed phases” since molecules are so much closer together than in the gas phase So, on a molecular scale, where do these intermolecular forces come from?
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Types of intermolecular forces 1. Dipole-dipole forces : (electrostatic interaction) some covalent molecules possess positive & negative ends, i.e. they have an electrostatic dipole charge distribution H—Cl δ + δ H—Cl Given this asymmetric charge distribution, 2 HCl molecules can orient themselves when near each other to minimize electrostatic repulsions and maximize electrostatic attractions Cl H Better; δ + nearer δ than …… δ – nearer δ or better yet <= two attractive interactions (head-to-tail) Energy lowering due to dipole-dipole attract. makes molecules “sticky”
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What about molecules without permanent dipoles? 1. Induced dipole interactions electric charges in nonpolar or polar molecules experience constant fluctuations for example consider the nonpolar molecule Cl 2 : Cl—Cl μ = 0 momentarily, there maybe an instantaneous dipole Cl δ + induces a dipole in neighboring Cl 2 The instantaneous dipoles created by the charge fluctuation can now cause intermolecular attractions. Induced dipole-induced dipole interaction leads to electrostatic attraction between molecules!
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Induced dipole-induced dipole forces: weaker than forces due to (permanent) dipole-dipole interactions. all molecules/atoms subject to induced dipole-induced dipole forces (London dispersion forces) whether they have a permanent dipole or not. Analogy: Like a tennis game between 2 equally good players; on average, over the course of the game, the average position of the ball is at the center (net) => however, at any given instant the ball is either on one side or the other.
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3. ion-ion forces: already discussed in the context of ionic bonding
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This note was uploaded on 04/17/2008 for the course CAS CH131 taught by Professor Zigler during the Spring '08 term at BU.

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CH131_5A - Homework #5 Read Chap 10.2 - 10.6 Problems: 10 -...

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