Properties of Pure Substances

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Properties of Pure Substances

• Chapter 2

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Pure Substance

• In Chemistry you defined a pure substance as an
  element or a compound
• Something that can not be separated
• In Thermodynamics well define it as something
  that has a fixed chemical composition throughout

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Examples

• Ice in equilibrium with water
• Air
• Air 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

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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

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Gas

• Molecules are far apart
• No long or short range order
• High kinetic energy
• In order to liquefy, lots of that kinetic energy
  must be released

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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 properties
• Ex 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
Liquid Water
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Superheated Gas
Compressed Liquid
Two Phase Region
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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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Pv Diagram of a Substance that Contracts on
Freezing
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Pv Diagram of a Substance that Expands on Freezing
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Property Diagrams

• So far we have sketched
• T v diagram
• P v diagram
• What about the P T diagram?

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Combine all three

• You can put all three properties
• P
• T
• V
• On the same diagram

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Contracts on Freezing
Expands on Freezing
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Property Tables

• P - pressure
• T - temperature
• v specific volume
• u specific internal energy
• h specific enthalpy h u Pv
• s specific entropy -define in Chapter 7

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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.
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• 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 936
• Saturated water temperature table
• A-5 - pg 938
• Saturated water pressure table

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g stands for gas
f stands for fluid
fg stands for the difference between gas and fluid
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Quality
Fraction of the material that is gas
x 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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Average Properties
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Superheated Properties
Table A-6 Page 834
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Compressed Liquid

• If the liquid is highly compressed, properties
  like specific volume will change
• Use Table A-7 pg 838
• However, if the compression is moderate, the
  properties do not vary significantly with
  pressure
• They do vary with temperature

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Compressed Liquid
We only need to adjust h if there is a big
difference in pressure
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Linear Interpolation
Try example 2-9, page 85
130
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
• Thats 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

• PVnRT
• Used in your Chemistry class
• From now on we will refer to the gas constant ,
  R, as the universal gas constant, Ru , and
  redefine RRu/MW
• PVmRT
• R is different for every gas
• Tabulated in the back of the book

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Ideal Gas Law

• v V/m
• Pv RT
• This is the form we will use the most

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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?
It depends!!
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Compressibility Factor

• You can adjust the ideal gas law with a fudge
  factor, called the compressibility factor
• Pv z RT
• z is just a value you put in to make it work out
• z 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

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Comparison of z factors
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What do you do when P or T is unknown?
Check out Appendix A-30
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Combined Gas Law
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Other Equations of StateVan der Waals
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Beattie-Bridgeman
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Benedict-Webb-Rubin
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Virial Equation of State
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Percentage Error for Nitrogen