Chapter11notes part1 - 1 Intermolecular Forces Liquids and Solids I Kinetic Molecular Theory(KMT of Liquids and Solids KMT of gases has been discussed

Chapter11notes part1 - 1 Intermolecular Forces Liquids and...

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1 Intermolecular Forces / Liquids and Solids I. Kinetic Molecular Theory (KMT) of Liquids and Solids KMT of gases has been discussed. This is based on no appreciable forces between molecules mostly empty space – very compressible KMT of liquids and solids is based on very little empty space – little to no compressibility Sometimes, liquids and solids are referred to as condensed states of matter. A phase is a homogeneous part of a system in contact with other parts of the system but separated from the other parts by a well-defined boundary. Water is an example of a substance where all three phases are commonly observed. 2. Gas/Liquid/Solid Comparisons Phase Volume/Shape Intermolecular Force Strength Density Ability to be compressed Molecular Motion Gas Assumes volume and shape of its container Low to none Low Very high Very free Liquid Definite volume / assumes shape of its container High High Slight Molecules slide past one another freely Solid Definite volume and shape High High Almost none Very little – only vibration about fixed positions Sealed container containing water gas (water vapor) solid (ice) liquid
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2 II. Intermolecular Forces Intermolecular forces are forces that act between molecules. Intramolecular forces are forces that act between atoms in the same molecule. These are the forces that hold molecules together – bonds. In general, inter molecular forces are weaker than intra molecular forces. A. Types of intermolecular forces Note: Although called intermolecular forces, some of these include ion/molecule forces. 1. Dipole-dipole forces act between molecules possessing a permanent dipole moment. e.g., HCl Solid: maximum alignment Liquid: less rigid Gas: virtually no alignment In general, stronger dipole-dipole interactions lead to higher melting points. Note: Dipole-dipole forces act between all molecules with permanent dipole moments. 2. Ion-dipole forces act between ions and molecules possessing a permanent dipole moment. + + cation anion These forces account for the solubility of ionic compounds. e.g., NaCl( s ) --> Na + ( aq ) + Cl ( aq ) Hydration is an example of ion- dipole forces. H O H Na + Cl H O H H O H H O H H O H H O H H O H H O H H Cl H Cl H Cl H Cl H Cl H Cl H Cl H Cl H Cl + + + + + + + + +
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3 3. “Induced” forces a. An induced dipole occurs when an ion or polar molecule approaches a nonpolar species. e.g., He atom nucleus electron "cloud" A dipole can be induced in the symmetrical electron “cloud” ion-induced dipole dipole-induced dipole Na + Cl + + H Cl !
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  • Fall '08
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