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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