Copyright 1999, PRENTICE HALLChapter 111 Phase Changes Surface molecules are only attracted inwards towards the bulk molecules. Sublimation: solid  gas. Vaporization: liquid  gas. Melting or fusion: solid  liquid. Deposition: gas  solid. Condensation: gas  liquid. Freezing: liquid  solid. Energy Changes Accompanying Phase Changes Energy change of the system for the above processes are:

Copyright 1999, PRENTICE HALLChapter 112 Intermolecular Forces Bulk and Surface

Copyright 1999, PRENTICE HALLChapter 113 Phase Changes Energy Changes Accompanying Phase Changes –Sublimation:  H sub > 0 (endothermic). –Vaporization:  H vap > 0 (endothermic). –Melting or Fusion:  H fus > 0 (endothermic). –Deposition:  H dep < 0 (exothermic). –Condensation:  H con < 0 (exothermic). –Freezing:  H fre < 0 (exothermic). Generally heat of fusion (enthalpy of fusion) is less than heat of vaporization: –it takes more energy to completely separate molecules, than partially separate them.

Copyright 1999, PRENTICE HALLChapter 114 Phase Changes Energy Changes Accompanying Phase Changes All phase changes are possible under the right conditions (e.g. water sublimes when snow disappears without forming puddles). The sequence heat solid  melt  heat liquid  boil  heat gas is endothermic. The sequence cool gas  condense  cool liquid  freeze  cool solid is exothermic.

Copyright 1999, PRENTICE HALLChapter 115 Phase Changes Energy Changes Accompanying Phase Changes

Copyright 1999, PRENTICE HALLChapter 116 Phase Changes Heating Curves Plot of temperature change versus heat added is a heating curve. During a phase change, adding heat causes no temperature change. –These points are used to calculate  H fus and  H vap. Supercooling: When a liquid is cooled below its melting point and it still remains a liquid. Achieved by keeping the temperature low and increasing kinetic energy to break intermolecular forces.

Copyright 1999, PRENTICE HALLChapter 117 Phase Changes Heating Curves

Copyright 1999, PRENTICE HALLChapter 118 Heating Curve Illustrated

Copyright 1999, PRENTICE HALLChapter 119 Phase Changes Critical Temperature and Pressure Gases liquefied by increasing pressure at some temperature. Critical temperature: the minimum temperature for liquefaction of a gas using pressure. Critical pressure: pressure required for liquefaction.

Copyright 1999, PRENTICE HALLChapter 1110 Critical Temperature, T c

Copyright 1999, PRENTICE HALLChapter 1111 Supercritical CO 2 Used to Decaffeinate Coffee

Copyright 1999, PRENTICE HALLChapter 1112 Vapor Pressure Explaining Vapor Pressure on the Molecular Level Some of the molecules on the surface of a liquid have enough energy to escape the attraction of the bulk liquid. These molecules move into the gas phase. As the number of molecules in the gas phase increases, some of the gas phase molecules strike the surface and return to the liquid. After some time the pressure of the gas will be constant at the vapor pressure.

Copyright 1999, PRENTICE HALLChapter 1113 Gas-Liquid Equilibration

Copyright 1999, PRENTICE HALLChapter 1114 Vapor Pressure Explaining Vapor Pressure on the Molecular Level Dynamic Equilibrium: the point when as many molecules escape the surface as strike the surface. Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium.

Copyright 1999, PRENTICE HALLChapter 1115 Vapor Pressure Volatility, Vapor Pressure, and Temperature If equilibrium is never established then the liquid evaporates. Volatile substances evaporate rapidly. The higher the temperature, the higher the average kinetic energy, the faster the liquid evaporates.

Copyright 1999, PRENTICE HALLChapter 1116 Liquid Evaporates when no Equilibrium is Established

Copyright 1999, PRENTICE HALLChapter 1117 Vapor Pressure Volatility, Vapor Pressure, and Temperature

Copyright 1999, PRENTICE HALLChapter 1118 Vapor Pressure Vapor Pressure and Boiling Point Liquids boil when the external pressure equals the vapor pressure. Temperature of boiling point increases as pressure increases. Two ways to get a liquid to boil: increase temperature or decrease pressure. –Pressure cookers operate at high pressure. At high pressure the boiling point of water is higher than at 1 atm. Therefore, there is a higher temperature at which the food is cooked, reducing the cooking time required. Normal boiling point is the boiling point at 760 mmHg (1 atm).

Copyright 1999, PRENTICE HALLChapter 1119 Phase Diagrams Phase diagram: plot of pressure vs. Temperature summarizing all equilibria between phases. Given a temperature and pressure, phase diagrams tell us which phase will exist. Features of a phase diagram: –Triple point: temperature and pressure at which all three phases are in equilibrium. –Vapor-pressure curve: generally as pressure increases, temperature increases. –Critical point: critical temperature and pressure for the gas. –Melting point curve: as pressure increases, the solid phase is favored if the solid is more dense than the liquid. –Normal melting point: melting point at 1 atm.

Copyright 1999, PRENTICE HALLChapter 1120 Phase Diagrams Any temperature and pressure combination not on a curve represents a single phase.

Copyright 1999, PRENTICE HALLChapter 1121 Phase Diagrams The Phase Diagrams of H 2 O and CO 2 Water: –The melting point curve slopes to the left because ice is less dense than water. –Triple point occurs at  C and 4.58 mmHg. –Normal melting (freezing) point is 0  C. –Normal boiling point is 100  C. –Critical point is 374  C and 218 atm. Carbon Dioxide: –Triple point occurs at  C and 5.11 atm. –Normal sublimation point is  C. (At 1 atm CO 2 sublimes it does not melt.) –Critical point occurs at 31.1  C and 73 atm.

Copyright 1999, PRENTICE HALLChapter 1122 Phase Diagrams The Phase Diagrams of H 2 O and CO 2