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11 - Liquids, Solids, and Materials

11 - Liquids, Solids, and Materials - Chapter 11 Liquids...

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Chapter 11: Liquids, Solids, and Materials 11.1The Liquid State Viscosity – resistance of a liquid to flow; higher viscosity = greater resistance; related to the intermolecular forces which determines how easily molecules can move past each other Viscosity decreases as temperature increases because they have enough kinetic energy to overcome some intermolecular forces Liquids have surface properties, the molecules at the surface have uneven attractive forces which causes the surface to contract Surface tension – energy required to expand a liquid surface; higher for liquids with stronger intermolecular attractions Water droplets are spheres because it minimizes surface area to volume ratio; fewer molecules are on the outside Capillary action – water rises in a tube because water forms hydrogen bonds with the surface of the tube which pulls water up; surface tension wants to minimize surface area which contributes to the rising water; force between surface and water is greater than water and water; water rises until the force of water-to-wall hydrogen bonds is balanced by gravity Meniscus – surface of a liquid in a tube; concave for water, convex for mercury (attractive forces between mercury atoms are greater than forces between mercury and surface) 11.2Vapor Pressure Volatility – tendency of a liquid to vaporize; increases as temperature increases At any time, some fraction of molecules in a liquid have enough energy to escape into gas phase; at higher temperatures, a larger fraction have enough energy so it speeds up evaporation Equilibrium vapor pressure or vapor pressure – pressure of gas is stable, dynamic equilibrium – molecules enter and leave liquid state at equal rates Liquid boils when vapor pressure = atmospheric pressure (boiling point) ; boiling occurs – vapor bubbles form and rise to the surface because of their lower density Normal boiling point is when atmospheric pressure is 1atm or 760mmHg Clausius-Clapeyron Equation o Clasius-Clapeyron equation shows relationship between vapor pressure and temperature o ln P = –ΔH vap / RT + C o can be used to calculate ΔH vap from two sets of data Vapor Pressure of Water and Relative Humidity o Water vapor is present in atmosphere at all times – relative humidity – ratio of partial pressure of water vapor in atmosphere to equilibrium partial pressure of water vapor in atmosphere o Equilibrium vapor pressure depends on temperature o Dew point – temperature at which actual partial pressure of water vapor equals the equilibrium vapor pressure; water vapor will condense as water droplets to form fog or dew 11.3Phase Changes: Solids, Liquids, and Gases Vaporization and Condensation o vaporization or evaporation is process in which a molecule leaves the liquid phase and enters the gas phase endothermic because high-energy molecules take energy as they leave the liquid enthalpy change is called enthalpy of vaporization (heat of vaporization)
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