Phase Changes

Copyright 1999, PRENTICE HALL Chapter 11 1

Phase ChangesPhase 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 ChangesEnergy Changes Accompanying Phase Changes• Energy change of the system for the above processes

are:


Intermolecular Forces Bulk and SurfaceIntermolecular Forces Bulk and Surface


Phase ChangesPhase Changes

Energy Changes Accompanying Phase ChangesEnergy Changes Accompanying Phase Changes– Sublimation: Hsub > 0 (endothermic).

– Vaporization: Hvap > 0 (endothermic).

– Melting or Fusion: Hfus > 0 (endothermic).

– Deposition: Hdep < 0 (exothermic).

– Condensation: Hcon < 0 (exothermic).

– Freezing: Hfre < 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.



Energy Changes Accompanying Phase ChangesEnergy 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.



Energy Changes Accompanying Phase ChangesEnergy Changes Accompanying Phase Changes



Heating CurvesHeating 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 Hfus and Hvap.

• 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.



Heating CurvesHeating Curves


Heating Curve IllustratedHeating Curve Illustrated



Critical Temperature and PressureCritical 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.


Critical Temperature, TCritical Temperature, Tcc


Supercritical COSupercritical CO22 Used to Decaffeinate Coffee Used to Decaffeinate Coffee


Vapor PressureVapor Pressure

Explaining Vapor Pressure on the Molecular Explaining Vapor Pressure on the Molecular LevelLevel

• 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.


Gas-Liquid EquilibrationGas-Liquid Equilibration



Explaining Vapor Pressure Explaining Vapor Pressure on the Molecular Levelon 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.



Volatility, Vapor Pressure, and TemperatureVolatility, 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.


Liquid Evaporates when no Equilibrium is EstablishedLiquid Evaporates when no Equilibrium is Established



Volatility, Vapor Pressure, and TemperatureVolatility, Vapor Pressure, and Temperature


Vapor PressureVapor PressureVapor Pressure and Boiling PointVapor 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).


Phase DiagramsPhase 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.


Phase DiagramsPhase Diagrams• Any temperature and pressure combination not on a

curve represents a single phase.


Phase DiagramsPhase DiagramsThe Phase Diagrams of HThe Phase Diagrams of H22O and COO and CO22

• Water:– The melting point curve slopes to the left because ice is less

dense than water.

– Triple point occurs at 0.0098C and 4.58 mmHg.

– Normal melting (freezing) point is 0C.

– Normal boiling point is 100C.

– Critical point is 374C and 218 atm.

• Carbon Dioxide:– Triple point occurs at -56.4C and 5.11 atm.

– Normal sublimation point is -78.5C. (At 1 atm CO2 sublimes it does not melt.)

– Critical point occurs at 31.1C and 73 atm.


Phase DiagramsPhase DiagramsThe Phase Diagrams of HThe Phase Diagrams of H22O and COO and CO22

Phase Changes

Documents

liquid gas

gas liquid

gas phase molecules

solid liquid

liquid solid

phase changesall phase

bulk liquid

gas phase increases