ch13 phase diagrams

7/30/2019 CH13 Phase Diagrams

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TRANSITIONS BETWEEN PHASES

See the phase diagram for water, Figure 13.37.

Lines connect all conditions of T and P whereEQUILIBRIUM exists between the phases on either side

of the line.

(At equilibrium particles move from liquid to gas as fast

as they move from gas to liquid, for example.)


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Copyright (c) 1999 by Harcourt Brace & Company

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Phase Diagram for Water

Animation of

solid phase.


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Animation of

equilibriumbetweensolid andliquid

phases.


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Animation of

liquid phase.


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Animation of

equilibriumbetweenliquid andgas phases.


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Animation of

gas phase.


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Animation of

equilibriumbetweensolid and gasphases.


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TRANSITIONSBETWEEN PHASES

As P and T increase, you finally reach the

CRITICAL T and P

.

LIQUID

GAS

Pcritical

HighPr

essure

High Temperature

Tcritical

Note that linegoes straight up


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TRANSITIONSBETWEEN PHASES

.

LIQUID

GAS

Pcritical

HighPr

essure

High Temperature

Tcritical

Note that linegoes straight up

Above critical T

no liquid exists

no matter how

high the

pressure.

As P and T increase, you finally reach the

CRITICAL T and P


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Critical T and P

COMPD Tc(oC) Pc(atm)H2O 374 218

CO2 31 73

CH4 -82 46Freon-12 112 41

(CCl2F2)

Notice that Tc and Pc depend onintermolecular forces.


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Solid-Liquid EquilibriaIn any system, if you increase P the DENSITY

will go up.

Therefore as P goes up, equilibrium favors

phase with the larger density (orSMALLER

volume/gram).Liquid H2O Solid H2O

Density 1 g/cm3 0.917 g/cm3

cm3/gram 1 1.09


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Solid-Liquid EquilibriaIn any system, if you increase P the DENSITY

will go up.

Therefore as P goes up, equilibrium favors

phase with the larger density (orSMALLER

volume/gram).Liquid H2O Solid H2O

Density 1 g/cm3 0.917 g/cm3

cm3/gram 1 1.09LIQUID H2OICE

favored atlow P

favored athigh P


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Solid-Liquid Equilbria

Raising the pressure at

constant T causes

water to melt.

The NEGATIVE SLOPEof the S/L line is

unique to H2O.

Almost everything

else has positiveslope.

P

T

freezingSolidH2O

LiquidH2O

760mmHg

0 C

Normal

point


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Solid-Liquid

Equilbria

The behavior of water

under pressure is an

example ofLE CHATELIERS

PRINCIPLE

At Solid/Liquidequilibrium, raising P

squeezes the solid.

It responds by going to

phase with greaterdensity, i.e., the liquid

phase.

Solid

H2O

LiquidH2O

P

T

760mmHg

0 C

Normalfreezingpoint


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Solid-Vapor Equilibrium

At P < 4.58 mmHg and T < 0.0098 C

solid H2O can go directly to vapor. This

process is called SUBLIMATION

This is how a frost-free refrigerator works.

ch13 phase diagrams

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