1

CHE 131

Phases and Phase Diagrams

Chapter 11: pp. 488-492.pp. 494-507.

Prof. S. Wong

Phase ChangesRed arrows – endothermic processes (require heat)

Blue arrows – exothermicprocesses (require cooling)

2

Solids- ordered- tightly packed- Epot > Ekin

Liquids- disordered- tightly packed- Epot > Ekin

Gases- disordered- separated- Epot < Ekin

Water

Boiling Liquids

Temperature at which the equilibriumvapor pressure equalsthe atmospheric pressure is designated as the boiling pointof the liquid.

At 1 atm, normal boiling point.

3

Properties of Liquids

boiling point

• the temperature at which the vapor pressure of a liquid equals the applied pressure

normal boiling point• the temperature at which the vapor

pressure of the liquid is equal to one atmospheric pressure

Properties of Liquids

liquid-vapor equilibrium• both liquid and vapor of the liquid present

in the same container user stable conditions

vapor pressure • The pressure exerted by a vapor in

equilibrium with its solid or liquid phase.

4

Properties of Liquids

enthalpy of vaporization – The amount of heat required to convert a liquid at its boiling point into vapor without an increase in temperature.

Vaporization is endothermic and process increases with temperature. As high-energy molecules leave liquid, they take some energy with them.

Phase Changes

Evaporationphase change from liquid to gas

- Thermal energy absorbed during evaporation is latent heat of vaporization.

Condensationphase change from gas to liquid

ΔHvaporization = -ΔHcondensation

5

Molar Enthalpies of Vaporization

Phase Changes

Meltingphase change from solid to liquid

Freezingphase change from liquid to solid

melting point (freezing point)temperature at which a liquid converts into the solid state

6

Phase ChangesMelting and Freezing

enthalpy of fusion – heat absorbed by the substance in changing solid into a liquid without raising its temperature.

Freezing is always an exothermic process.Melting points and heats of fusion increase with

increasing number of electrons (and molar mass) as London forces become stronger. Ionic compounds have highest melting points.

Molar Enthalpies of Fusion

7

Fig. 11-10, p.497

Melting of Naphthalene

Table 11-3, p.498

8

Fig. 11-12, p.500

Sublimation of Ice

Phase Changes

sublimation – conversion of a solid directly to a gas

deposition – conversion of a gas directly to a solid

ΔHsublimation = -ΔHdeposition

9

Q uestio n : W h ich g ro u p inc lu d es o n lyen d o th erm ic p rocesses?

a . freez ing , va po r iza tio n , d epo sitio nb . freezin g , co n d en sa tio n , dep o sitio nc . m e ltin g , co n d en sa tio n , d epo sitio nd . m e ltin g , eva po ra tio n , su b lim a tio ne . m e ltin g , co n d en sa tio n , su b lim a tio n

Heating Curve

10

Generic Phase DiagramTriple point : point at which all three phases are in equilibrium.

Phase Diagram for WaterTriple point for water is at P = 4.58 mm Hg and T = 0.01ºC.

11

Unusual Properties of Water

p.506

12

Fig. 11-16, p.503

p.507

Open Cage Structure of Ice

Regular array of hydrogen bonding

13

•9 forms of ice are known, each with a different structure.•None is stable below 2000 atm. Ice III is illustrated.

D

Fig. 11-11, p.499

Dry IceSublimation of CO2

14

Phase Diagram for Carbon Dioxide

Critical Point

critical temperature - the temperature above which there is no distinction between the gas and liquid states

critical pressure - the pressure above which there is no distinction between the gas and liquid states

15

Where is:

a. critical point.b. triple point.c. absolute freezing point.d. normal freezing point.e. normal boiling point.