2 Pure SubstanceA 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 SubstancePure 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 vaporCarbon DioxideNitrogenHomogeneous mixture of gases, such as air, as long as there is no change of phases.
5 Multiple phases mixture of a Pure Substance WaterAirvaporvaporliquidliquidNot pure, different condensation temperatures for different componentsPure
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, andIntensive 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 StateProperty 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 systemA simple system is defined as one for which only one quasiequilibrium work mode applies.Simple compressible systemsSimple elastic systemsSimple magnetic systemsSimple electrostatic systems, etc.
10 CompressibleIf 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), orP = 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 StateAny 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 heatP = 1 atmT = 300 oCP = 1 atmT = -10 oCP = 1 atmT = 0 oCP = 1 atmT = 20 oCP = 1 atmT = 100 oC
17 Phase-change Process Compressed liquid -- not about to evaporate Saturated liquid -- about to evaporateSaturated liquid-vapor mixture --two phaseSaturated Vapor -- about to condenseSuperheated Vapor -- not about to condense
18 Isobaric process P = 1 atm T-v DiagramoT, CIsobaric process P = 1 atm3005Superheated vapor23Saturated mixture1004Compressed liquid201v
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 Cp = 500 kPa, T = Coo*T and P are dependent during phase change*Allow us to control boiling temperature by controlling the pressure (i.e., pressure cooker).
25 Latent HeatLatent heat is the amount of energy absorbed or released during phase changeLatent heat of fusion -- melting/freezing =333.7 kJ/kg for 1 atm H2OLatent heat of vaporization --boiling/condensation = kJ/kg for 1 atm H2O
26 Two-phase or saturation region -- gas and liquid coexist P-v DiagramPSubcooled or compressed liquid regionCritical pointSuperheated region --substance is 100% vaporTwo-phase or saturation region -- gas and liquid coexistSaturated liquid lineSaturated vapor linev
27 P-v Diagram of a Pure Substance SUPERHEATEDIsothermal processv
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 PointPoint 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 PointA 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) DomeThe 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 tableTable A-1.3Superheated vapor
37 For Water P Subcooled or compressed liquid region superheated region v If T=Tsat, ppsatIf p=psat,TTsatsuperheated regionIf T=Tsat , ppsatIf p=psat , T>Tsatsaturation regionp=psat and T=Tsatv
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 QualityIn a saturated liquid-vapor mixture, the mass fraction (not volume fraction) of the vapor phase is called the quality and is defined asThe 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 + pVh = u + pvh = (1-x)hf + xhg = hf + x(hg - hf)= hf + xhfg
43 Saturated MixtureIn 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.