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Properties of Pure Substances

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1 Properties of Pure Substances

2 Pure Substance A substance that has a fixed (homogeneous and invariable) chemical composition throughout is called a pure substance. It may exist in more than one phase, but the chemical composition is the same in all phases.

3 Pure Substance Pure means “…of uniform and invariable chemical composition (but more than one molecular type is allowed).” This allows air to be a pure substance. All our substances will be pure. We will drop the use of the word. When we refer to a simple system we mean one filled with a pure substance--a simple, pure system.

4 Examples of Pure Substance
Water (solid, liquid, and vapor phases) Mixture of liquid water and water vapor Carbon Dioxide Nitrogen Homogeneous mixture of gases, such as air, as long as there is no change of phases.

5 Multiple phases mixture of a Pure Substance
Water Air vapor vapor liquid liquid Not pure, different condensation temperatures for different components Pure

6 Thermodynamic Properties
We have discussed extensive properties such as m, U, and V (for volume) which depend on the size or extent of a system, and Intensive properties such as u, v, T, and P (sometimes we write a “p” for pressure, using P and p interchangeably) which are independent of system extent.

7 Equation of State Property Tables
Important Questions .…. How many properties are needed to define the state of a system? How do we obtain those properties? Equation of State Property Tables

8 Review - State Postulate
The number of independent intensive properties needed to characterize the state of a system is n+1 where n is the number of relevant quasiequilibrium work modes. This is empirical, and is based on the experimental observation that there is one independent property for each way a system’s energy can be independently varied.

9 Simple system A simple system is defined as one for which only one quasiequilibrium work mode applies. Simple compressible systems Simple elastic systems Simple magnetic systems Simple electrostatic systems, etc.

10 Compressible If we restrict our system to being compressible, we define what that quasiequilibrium work mode is:

11 For a simple system, We may write P = P(v,T) or v = v(P,T)
or perhaps T = T(P,v)

12 For a simple, pure substance
y0 = y(y1,y2), or P = P(v,T), v = v(P,T), and T = T(P,v) What do these equations define, in space? Equations used to relate properties are called “Equations of State”

13 Equation of State Any two independent, intrinsic properties are sufficient to fix the intensive state of a simple substance. One of the major task of Thermodynamics is to develop the equations of state which relate properties at a give state of a substance.

14 Ideal gas “law” is a simple equation of state
Ru = universal gas constant = (kPa-m3)/(kgmol-K) = (ft-lbf)/(lbmol-R)

15 Phases of a Pure Substance
Solid phase -- molecules are arranged in a 3D pattern (lattice). Liquid phase -- chunks of molecules float about each other, but maintain an orderly structure and relative positions within each chunk. Gas phase -- random motion, high energy level.

16 Phase Equilibrium p p p p p vapor liquid liquid vapor liquid ice ice
heat P = 1 atm T = 300 oC P = 1 atm T = -10 oC P = 1 atm T = 0 oC P = 1 atm T = 20 oC P = 1 atm T = 100 oC

17 Phase-change Process Compressed liquid -- not about to evaporate
Saturated liquid -- about to evaporate Saturated liquid-vapor mixture --two phase Saturated Vapor -- about to condense Superheated Vapor -- not about to condense

18 Isobaric process P = 1 atm
T-v Diagram o T, C Isobaric process P = 1 atm 300 5 Superheated vapor 2 3 Saturated mixture 100 4 Compressed liquid 20 1 v

19 P-T Diagram (Phase Diagram) of Pure Substances

20 Isothermal Process Compressed Liquid Superheated Vapor

21 Isobaric Process Subcooled Liquid a Superheated Vapor

22 Water Expands on Freezing!
Ice floats on top of the water body (lakes, rivers, oceans, soft drinks, etc.). If ice sinks to the bottom (contracts on freezing), the sun’s ray may never reach the bottom ice layers. This will seriously disrupt marine life.

23 Saturation Temperature and Pressure
Tsat -- Temperature at which a phase change takes place at a given pressure. Psat -- Pressure at which a phase change takes place at a given temperature.

24 Saturation Temperature
Tsat = f (Psat) p = 1atm = kPa, T = 100 C p = 500 kPa, T = C o o *T and P are dependent during phase change *Allow us to control boiling temperature by controlling the pressure (i.e., pressure cooker).

25 Latent Heat Latent heat is the amount of energy absorbed or released during phase change Latent heat of fusion -- melting/freezing =333.7 kJ/kg for 1 atm H2O Latent heat of vaporization --boiling/condensation = kJ/kg for 1 atm H2O

26 Two-phase or saturation region -- gas and liquid coexist
P-v Diagram P Subcooled or compressed liquid region Critical point Superheated region --substance is 100% vapor Two-phase or saturation region -- gas and liquid coexist Saturated liquid line Saturated vapor line v

27 P-v Diagram of a Pure Substance
SUPERHEATED Isothermal process v

28 T-v Diagram of a Pure Substance

29 Critical & Supercritical
The state beyond which there is no distinct vaporization process is called the critical point. At supercritical pressures, a substance gradually and uniformly expands from the liquid to vapor phase. Above the critical point, the phase transition from liquid to vapor is no longer discrete.

30 Critical Point Point at which the saturated vapor and saturated liquid lines coincide. If T  Tc or P  Pc there is no clear distinction between the superheated vapor region and the compressed liquid region.

31 Critical Point A point beyond which T  Tc and a liquid-vapor transition is no longer possible at constant pressure. If T  Tc , the substance cannot be liquefied, no matter how great the pressure. Substances in this region are sometimes known as “fluids” rather than as vapors or liquids.

32 Vapor (Steam) Dome The dome-shaped region encompassing the two-phase, vapor-liquid equilibrium region. It is bordered by the saturated liquid line and the saturated vapor line, both of which end at the triple line and end at the critical point. The region below the vapor dome is also called: saturated liquid-vapor region, wet region, two-phase region, or saturation region.




36 Steam Tables Table A-1.1 Saturation water -- temperature table
Saturation water -- pressure table Table A-1.3 Superheated vapor

37 For Water P Subcooled or compressed liquid region superheated region v
If T=Tsat, ppsat If p=psat,TTsat superheated region If T=Tsat , ppsat If p=psat , T>Tsat saturation region p=psat and T=Tsat v

38 Two properties are not independent in the vapor dome (the two-phase region)
The temperature and pressure are uniquely related. Knowing a T defines the P and vice versa. Use quality to determine the state in two-phase region.

39 Quality is related to the horizontal differences of P-v and T-v Diagrams

40 Quality In a saturated liquid-vapor mixture, the mass fraction (not volume fraction) of the vapor phase is called the quality and is defined as The quality may have values between 0 (saturated liquid) and 1 (saturated vapor). It has no meaning in the compressed liquid or superheated vapor regions.

41 What is v for something in the two-phase region?
v = (1-x)vf + xvg = vf + x(vg - vf) = vf + xvfg

42 Enthalpy in Two-Phase Region
H = U + pV h = u + pv h = (1-x)hf + xhg = hf + x(hg - hf) = hf + xhfg

43 Saturated Mixture In the saturated mixture region, the average value of any intensive property y is determined from where f stands for saturated liquid and g for saturated vapor.

44 v = vf + x(vg - vf) = vf + xvfg u = uf + x(ug - uf) = uf + xufg
Saturated Mixture v = vf + x(vg - vf) = vf + xvfg u = uf + x(ug - uf) = uf + xufg h = hf + x(hg - hf) = hf + xhfg s = sf + x(sg - sf) = sf + xsfg f : saturated liquid g : saturated vapor

45 Saturated Water (temperature)

46 Superheated Vapor

47 Examples 3-2 thru 3-7 Steam Tables A-1.1 and A-1.3

48 TEAMPLAY Complete the table below as a team. The substance is water. Make sure everybody understands how to do it! P (MPa) T(C) v(m3/kg) x (if appl.) 300 1.0 0.15 0.65 0.50

49 TEAMPLAY Complete the table below as a team. The substance is water. Use linear interpolation if needed. P(MPa) T(C) u (kJ/kg) x (if appl.) 7.0 0.0 1.0 0.05 600 100 460


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