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Overview of Chapter 13

• Intermolecular Forces• Liquids and their Properties

• Solids and Their Properties

• Phase diagrams

Today’s Topics

• Heats of vaporization and condensation

• Vapor Pressure

• Pressure-Temperature relationships in liquids

• Phase diagrams

Questions to consider:

• What happens when liquids boil?

• What is a triple point?

• How are the phase diagrams for water andCO2 different?

LiquidsHEAT OF VAPORIZATION is the heat needed

(at constant P) to vaporize the liquid.Liquid + heat ---> Vap or

CompoundCompound ∆∆HHvapvap (kJ/mol) (kJ/mol) IM ForceIM ForceHH22OO 40.7 (10040.7 (100 ooCC)) H-bondsH-bondsSOSO22 26.8 (-4726.8 (-47 ooCC)) dipoledipoleXeXe 12.6 (-10712.6 (-107 ooCC)) induced dipoleinduced dipole

LiquidsAt higher T, more

molecules haveenough energy tobreak IM forces andmove from liquid tovapor.

High E moleculescarry away E.

==> You cool downwhen sweating orafter swimming.

# m

olec

ules

LiquidsMolecules in the vapor

state exert a VAPORPRESSURE

EQUILIBRIUM VAPORPRESSURE is thepressure exerted by avapor over a liquid in aclosed container whenthe rate of evaporation =the rate of condensation.

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Measuring Equilibrium Vapor Pressure

Liquid in flask evaporates and exerts pressure on manometer.

Liquids: Boiling

Liquid boils when itsvapor pressureequals atmosphericpressure.

Equilibrium Vapor Pressure Equilibrium Vapor Pressure1. The curves show all conditions of P and T where

liquid and vapor are in equilibrium.2. The VP rises with T.3. When VP = external P, the liquid boils. (The boiling points of liquids change with altitude.)4. If external P = 760 mm Hg, T of boiling is the

normal boiling point.5. VP of a molecule at a given T depends on

intermolecular forces.

Equilibrium Vapor Pressure:The Clausius-Clapeyron Equation

•• Clausius-Clapeyron Clausius-Clapeyron equation isequation isused to find used to find ∆∆HH˚̊vapvap..

•• The The ln ln (=natural log) of the vapor(=natural log) of the vaporpressure P is proportional topressure P is proportional to∆∆HHvaporiationvaporiation and to 1/T.and to 1/T.

•• ln ln P = P = ––((∆∆HH˚̊vapvap/RT/RT) + C) + C

(Recall y = (Recall y = mx mx + b for a straight line:+ b for a straight line:

Here, the slope m =Here, the slope m = –– ∆∆HH˚̊vapvap/R /R ))

(C = a constant depending on the substance)

Clausius-Clapeyron equation

!

lnP2 =-"Hvap

RT2

#

$ %

&

' ( + c

!

" lnP1 =-#Hvap

RT1

$

% &

'

( ) + c

!

lnP =-"Hvap

RT

#

$ %

&

' ( + c

!

lnP2 - lnP1 =-"Hvap

RT2

#

$ %

&

' ( )

-"Hvap

RT1

#

$ %

&

' (

!

" lnP2

P1

#

$ %

&

' ( =

-)Hvap

R

#

$ %

&

' ( 1

T2*1

T1

#

$ %

&

' (

!

lnP2

P1

"

# $

%

& ' =

(Hvap

R

"

# $

%

& ' 1

T1)1

T2

"

# $

%

& '

Subtract two of these equations at different temperatures:

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Phase Diagrams

In the shaded regions, only 1 phase exists.On the boundary lines, 2 phases are in equilibrium.At the triple point, 3 phases are in equilibrium.

Phase Diagrams—Important Points for Water

T(T(˚̊C)C) P(mmHg)P(mmHg)Normal boil point Normal boil point 100100 760760Normal freeze pointNormal freeze point 00 760760Triple point Triple point 0.00980.0098 4.584.58

Critical T and P

Above critical TAbove critical Tno liquid existsno liquid existsno matter howno matter howhigh thehigh thepressure.pressure.

As P and T increase, you finally reachAs P and T increase, you finally reachthe CRITICAL T and Pthe CRITICAL T and P

CO2 Phase Diagram

CO2 critical point

http://www.chem.leeds.ac.uk/People/CMR/criticalpics.html

CO2 critical point

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CO2 critical point