properties of pure substances chapter 3. why do we need physical properties? as we analyze...
TRANSCRIPT
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Properties of Pure Substances
Chapter 3
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Why do we need physical properties?
As we analyze thermodynamic systems we describe them using physical properties
Those properties become the input to the equations we’ll use to solve thermodynamic problems
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Pure Substance
In Chemistry you defined a pure substance as an element or a compound
Something that can not be separatedIn Thermodynamics we’ll define it as
something that has a fixed chemical composition throughout
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Examples
Ice in equilibrium with pure water
AirAir in equilibrium
with liquid air is not a pure substance – Why?
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Phases of Pure Substances
We all have a pretty good idea of what the three phases of matter are, but a quick review will help us understand the phase change process
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Solid
Long range order Three dimensional
pattern Large attractive
forces between atoms or molecules
The atoms or molecules are in constant motion – they vibrate in place
The higher the temperature – the more vibration This image is the property of IBM
http://www.kings.edu/~chemlab/vrml/
Platinum Atoms
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Liquid
When a solid reaches a high enough temperature the vibrations are strong enough that chunks of the solid break of and move past each other
Short range order Inside the chunks the
atoms or molecules look a lot like a solid
Ex. You only break 5% to 15% of the water hydrogen bonds to go from solid to liquid
http://www.earth-photography.com/Countries/Norway/Norway_Jostedalsbreen_Glacier4.html
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Gas
Molecules are far apartNo long or short range
orderHigh kinetic energyIn order to liquefy, lots
of that kinetic energy must be released
http://pisces.sdsu.edu/ONLINE_LESSONS/WEATHER/
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Solid to Liquid to Gas
On a molecular level, the difference between the phases is really a matter of degree
We identify melting points and vaporization points based on changes in propertiesEx – big change in specific volume
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Consider what happens when we heat water at constant pressure
Piston cylinder device – maintains constant pressure
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T
v
1
2
5
3 4 Liquid to Gas Phase Change
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Liquid to Gas Phase Change
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Liquid to Gas Phase Change
Two Phase Region
Compressed Liquid
Superheated Gas
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Critical Point
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Critical Point
Above the critical point there is no sharp difference between liquid and gas!!
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Pressure-volume diagram
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Property Diagrams
So far we have sketchedT – v diagramP – v diagramWhat about the P – T diagram?
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Property Diagrams
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Combine all three
You can put all three propertiesPTV
On the same diagram
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3 Dimensional Phase DiagramsExpands on Freezing Contracts on Freezing
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State Postulate
The state of a simple compressible system is completely specified by two independent, intensive properties
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State Postulate
Remember that during a phase change, Temperature and Pressure are not independent
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Property Tables
P - pressureT - temperaturev – specific volumeu – specific internal energyh – specific enthalpy h = u + Pvs – specific entropy -define in Chapter 7
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A word about enthalpy
Enthalpy is a combination propertyh=u+PvH=U+PV
It is useful because it makes some equations easier to solve
You could do all of thermodynamics without it – but its more convenient to use it.
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Saturated Liquid and Saturated Vapor States
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Saturation Properties
Saturation Pressure is the pressure at which the liquid and vapor phases are in equilibrium at a given temperature.
Saturation Temperature is the
temperature at which the liquid and vapor phases are in equilibrium at a given pressure.
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Table A-4 and A-5
A-4 pg 890Saturated water temperature table
A-5 pg 892Saturated water pressure table
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u u u
h h h
s s s
fg g f
fg g f
fg g f
g stands for gas
f stands for fluid
fg stands for the difference between gas and fluid
Transitions from liquid to gas
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Quality
xmass
mass
m
m msaturated vapor
total
g
f g
Fraction of the material that is gasx = 0 the material is all saturated liquid
x = 1 the material is all saturated gas
x is not meaningful when you are out of the saturation region
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Quality
X = 0 X = 1
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Average Properties
y y x y y
y x yf g f
f fg
( )
When x = 0 we have all liquid, and y = yf
0
When x = 1 we have all gas, and y = yf + yfg = yg
1= yg
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Superheated Properties
Table A-6, pg 894
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Compressed Liquid
y y f T @
h h v P Pf T f sat @ ( )
We only need to adjust h if there is a big difference in pressure
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Linear Interpolation
A B
100 5
200 10
130 X 510
5
100200
100130
x
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Equations of State
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Equations vs Tables
The behavior of many gases (like steam) is not easy to predict with an equation
That’s why we have tables like A-4, A-5 and A-6
Other gases (like air) follow the ideal gas law – we can calculate their properties
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Ideal Gas Law
PV=nRTUsed in your Chemistry classFrom now on we will refer to the gas
constant , R, as the universal gas constant, Ru , and redefine R=Ru/MW
PV=mRTR is different for every gasTabulated in the back of the book
PV=nRuT
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Ideal Gas Law
v = V/mPv = RT
This is the form we will use the most
Relates 3 properties
P, v and T
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When does the ideal gas law apply?
The ideal gas equation of state can be derived from basic principles if one assumes: 1. Intermolecular forces are small
2. Volume occupied by the particles is small
These assumptions are true when the molecules are far apart – ie when the gas is not dense
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Criteria
The ideal gas law applies when the pressure is low, and the temperature is high - compared to the critical values
The critical values are tabulated in the Appendix
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Is Steam an Ideal Gas?
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Compressibility Factor
You can adjust the ideal gas law with a fudge factor, called the compressibility factor
Pv = z RTz is just a value you put in to make it
work outz = 1 for ideal gases
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Principle of Corresponding States
The Z factor is approximately the same for all gases at the same reduced temperature and reduced pressure
TT
TP
P
PRcr
Rcr
and
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Comparison of z factors
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What do you do when P or T is unknown?
vvRTP
Ractual
cr
cr
Check out Appendix A-15 pg 908
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Other Equations of StateVan der Waals
( )( )Pa
vv b RT
2
aR T
Pb
RT
Pcr
cr
cr
cr
27
64 8
2 2
and
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Beattie-Bridgeman
PR T
v
c
vTv B
a
vu FHG
IKJ
2 3 21 ( )
A Aa
vB B
b
vo o FHG
IKJ F
HGIKJ1 1 and
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Benedict-Webb-Rubin
PR T
vB R T A
C
T v
bR T a
v
a
v
c
v T ve
uo u o
o u
v
FHG
IKJ
FHG
IKJ
2 2 3
6 3 2 2
1
12 /
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Percentage Error for Nitrogen
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Summary
In this Chapter we learnedHow the state of a substance changes
with Temperature and PressureHow to read and use property tablesWhen we can use the ideal gas lawAlternative equations of state