chem chp summaries - Chapter 11 Solids Liquids and...

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Chapter 11 Solids, Liquids, and Intermolecular Forces (11.1, 11.2, 11.3) The forces that hold molecules or atoms together in a liquid or solid are called intermolecular forces. The strength of the intermolecular forces in a substance determines its phase. Dispersion forces are always present because they result from the fluctuations in electron distribution within atoms and molecules. These are the weakest intermolecular forces, but they are significant in molecules with high molar masses. Dipole-dipole forces, generally stronger than dispersion forces, are present in all polar molecules. Hydrogen bonding occurs in polar molecules that contain hydrogen atoms bonded directly to fluorine, oxygen, or nitrogen. These are the strongest intermolecular forces. Ion-dipole forces occur in ionic compounds mixed with polar compounds, and they are especially important in aqueous solutions. Surface Tension, Viscosity, and Capillary Action (11.4) Surface tension results from the tendency of liquids to minimize their surface area in order to maximize the interactions between their constituent particles, thus lowering their potential energy. Surface tension causes water droplets to form spheres and allows insects and paper clips to "float" on the surface of water. Viscosity is the resistance of a liquid to flow. Viscosity increases with increasing strength of intermolecular forces and decreases with increasing temperature. Capillary action is the ability of a liquid to flow against gravity up a narrow tube. It is the result of adhesive forces, the attraction between the molecules and the surface of the tube, and cohesive forces, the attraction between the molecules in the liquid. Vaporization and Vapor Pressure (11.5, 11.7) Vaporization, the phase transition from liquid to gas, occurs when thermal energy overcomes the intermolecular forces present in a liquid. The opposite process is condensation. Vaporization is endothermic and condensation is exothermic. The rate of vaporization increases with increasing temperature, increasing surface area, and decreasing strength of intermolecular forces. The heat of vaporization is the heat required to vaporize one mole of a liquid. In a sealed
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