Phase Changes ? Vaporization (Evaporation): Liquid to Gas Phase ? Sublimation: Solid to Gas Phase ? Fusion (Melting): Solid to Liquid Phase ? Vapor Pressure: The partial pressure exerted by a substance in the vapor phase in equilibrium with that same substance in the liquid phase. And a related concept: Distribution of Molecular Speeds of Nitrogen Kinetic Molecular Theory for Gases: Allows the calculation of the distribution of kinetic energies of gas molecules at various temperatures. Energies of Molecules Energies of Molecules in the Condensed Phases. Evaporation, Condensation, and Vapor Pressure Kinetic-Molecular Theory of Liquids: Similar distribution of kinetic energies as seen for gases. Evaporation proceeds until rate equals that of condensation. Molecules of a particular substance. Temperature Dependence of Vapor Pressure: The Clausius-Clapeyon Equation p vap ' ? × e &?H vap /RT ln p vap ' ln ? & ?H vap RT ln p vap,1 p vap,2 ' &?H vap R 1 T 1 & 1 T 2 Thermodynamics of Vaporization: Vaporization is an endothermic process. H 2 O(l) ? H 2 O(g) )H vap = + 40.7 kJ/mol (at 100 EC) Thermodynamics of Melting: Melting (fusion) is an endothermic process. H 2 O(s) ? H 2 O(l) )H fus = + 6.02 kJ/mol (at 0 EC) Thermodynamic Definitions of Normal Phase Transition Temperatures: The Normal Boiling Point occurs at a temperature where the vapor pressure of the liquid equals one atmosphere. The Normal Melting Point occurs at a temperature where the vapor pressure of both the solid and liquid phases are equal under conditions where the total pressure is equal to one atmosphere. Heating Curve for 1 Mole Water, from B25 EC to 70 EC Phase Diagrams A phase diagram allows us to predict the state (or phase) of a substance given its temperature and pressure. We can use the Clausius-Clapeyron equation to determine the phase boundary between condensed (solid or liquid phases) and the vapor phase for a substance. Important points on a phase diagram: ? Triple point: where three phases are in equilibrium. ? Critical point: where the phase boundary between the liquid and vapor phases ceases to exist; beyond this point the substance exists as a super-critical fluid. Phase Diagram, CO 2 Phase Diagram, H 2 O Ice-I (Normal ice) dhm1h Microsoft PowerPoint - 00-Chapter-11-print.ppt
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