Properties of Pure Substances Chapter 3

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

Pure Substance  In Chemistry you defined a pure substance as an element or a compound  Something that can not be separated  In Thermodynamics we’ll define it as something that has a fixed chemical composition throughout

Examples  Ice in equilibrium with pure water  Air  Air in equilibrium with liquid air is not a pure substance – Why?

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

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 Platinum Atoms

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 Norway/Norway_Jostedalsbreen_Glacier4.html

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 ONLINE_LESSONS/WEATHER/

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

Consider what happens when we heat water at constant pressure Piston cylinder device – maintains constant pressure

T v Liquid to Gas Phase Change

Two Phase Region Compressed Liquid Superheated Gas

Critical Point

 Above the critical point there is no sharp difference between liquid and gas!!

Pressure-volume diagram

Property Diagrams  So far we have sketched T – v diagram P – v diagram What about the P – T diagram?

Property Diagrams

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

3 Dimensional Phase Diagrams Expands on Freezing Contracts on Freezing

State Postulate The state of a simple compressible system is completely specified by two independent, intensive properties

State Postulate  Remember that during a phase change, Temperature and Pressure are not independent

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

A word about enthalpy  Enthalpy is a combination property h=u+Pv H=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.

Saturated Liquid and Saturated Vapor States

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.

Table A-4 and A-5  A-4 pg 890 Saturated water temperature table  A-5 pg 892 Saturated water pressure table

g stands for gas f stands for fluid fg stands for the difference between gas and fluid Transitions from liquid to gas

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

Quality X = 0X = 1

Average Properties When x = 0 we have all liquid, and y = y f 0 When x = 1 we have all gas, and y = y f + y fg = y g 1 = y g

Superheated Properties Table A-6, pg 894

Compressed Liquid We only need to adjust h if there is a big difference in pressure

Linear Interpolation AB X

Equations of State

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

Ideal Gas Law  PV=nRT Used in your Chemistry class From now on we will refer to the gas constant, R, as the universal gas constant, R u, and redefine R=R u /MW  PV=mRT R is different for every gas Tabulated in the back of the book PV=nR u T

Ideal Gas Law  v = V/m  Pv = RT This is the form we will use the most Relates 3 properties P, v and T

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

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

Is Steam an Ideal Gas?

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

Principle of Corresponding States  The Z factor is approximately the same for all gases at the same reduced temperature and reduced pressure

Comparison of z factors

What do you do when P or T is unknown? Check out Appendix A-15 pg 908

Other Equations of State Van der Waals

Beattie-Bridgeman

Benedict-Webb-Rubin

Percentage Error for Nitrogen

Summary  In this Chapter we learned How the state of a substance changes with Temperature and Pressure How to read and use property tables When we can use the ideal gas law Alternative equations of state