CHAPTER 11 Brady

CHAPTER 11 Brady - CHAPTER 11 INTERMOLECULAR FORCES AND...

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CHAPTER 11 INTERMOLECULAR FORCES AND LIQUIDS AND SOLIDS Liquid-Vapor Equilibrium Phase Diagrams Molecular Substances: Intermolecular Forces Network Covalent, Ionic, and Metallic Solids Crystal Structures 1
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Gases at ordinary temperatures and pressures follow the ideal gas law PV = nRT This is an equation of state What about liquids and solids? Is there a simple equation of state for liquids and solids? What are the reasons if the answer to this question is either yes or no? 2
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The answer is no. There is no simple equation that can be written to correlate the properties of liquids and solids. There are two reasons 1. Molecules are much closer together in liquids and solids. In a gas molecules are generally separated by 10 or more molecular diameters. In liquids and solids they are touching one another. This explains why liquids and solids are: Much more dense Virtually incompressible 2. Intermolecular forces play a much larger role in liquids and solids, but are negligible in gases Molecular Substances; Intermolecular Forces Molecules are the characteristic structural units of gases , most liquids and many solids As a class they tend to have the following characteristics: 3
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Nonconductors of electricity when pure Soluble in nonpolar solvents such as carbon tetrachloride (CCl 4 ) or benzene (C 6 H 6 ) Insoluble in water Low melting and boiling Why or what makes molecular substances have low melting and boiling points? This general low melting and boiling points of molecular substances comes from weak intermolecular forces The stronger the intermolecular forces the higher the boiling point Dispersion (London) Forces : Induced dipoles: rapid shifting of electrons back and forth between atoms in molecule. 4
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All molecules have dispersion forces, the strength of these forces depends on two factors: The number of electrons in the atoms that make up the molecule The ease with which electrons are dispersed to form temporary dipoles. Both factors increase with increasing molecular size Outer electrons are farther from the nucleus and easier to disperse. In general: As molar mass increases, dispersion forces become stronger and boiling points of nonpolar molecular substances increase 5
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Dipole Forces : (a) Dipole-Dipole; (b) Ion- Dipole (a)Dipole-Dipole Polar molecules like all other molecules experience dispersion forces In addition, the experience dipole forces δ + δ - δ + δ - ICl I --- Cl I --- Cl (b) Ion-Dipole: Attraction between an ion (either cation or anion) and a polar molecule Boiling points of polar compounds are usually higher 6
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Hydrogen bonds : Substances or molecules containing F, O, N are the basis for hydrogen bonding Substances that can form hydrogen bonds HF, H 2 O NH 3 , have unusually high boiling points Two reasons why hydrogen bonds are stronger then ordinary dipole forces: Difference in electronegativity between F and H is quite large. The
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This note was uploaded on 05/10/2011 for the course CHEM 111 taught by Professor Lent during the Spring '11 term at Allen University.

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CHAPTER 11 Brady - CHAPTER 11 INTERMOLECULAR FORCES AND...

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