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1 Intermolecular Forces / Liquids and SolidsI. 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 solidsis based on very little empty space – little to no compressibility Sometimes, liquids and solids are referred to as condensed statesof matter. A phaseis 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 watergas (water vapor)solid (ice)liquid
2 II. Intermolecular Forces Intermolecular forcesare forces that act between molecules. Intramolecular forcesare forces that act between atoms in the same molecule. These are the forces that hold molecules together – bonds. In general, intermolecular forces are weaker than intramolecular forces. A. Types of intermolecular forces Note:Although called intermolecular forces, some of these include ion/molecule forces. 1. Dipole-dipole forcesact 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 forcesact between ions and molecules possessing a permanent dipole moment. +–+–cationanionThese forces account for the solubility of ionic compounds. e.g.,NaCl(s) --> Na+(aq) + Cl–(aq) Hydration is an example of ion-dipole forces. HOHNa+Cl–HOHHOHHOHHOHHOHHOHHOHHClHClHClHClHClHClHClHClHCl+–+–+–+–+–+–+–+–+–
3 3. “Induced” forces a. An induced dipoleoccurs when an ion or polar molecule approaches a nonpolar species. e.g.,He atom nucleuselectron "cloud"A dipole can be induced in the symmetrical electron “cloud” ion-induced dipole dipole-induced dipole Na+Cl––+–+HCl!