Presentation on theme: "Properties of Pure Substances. Pure Substance w A substance that has a fixed (homogeneous and invariable) chemical composition throughout is called a."— Presentation transcript:
Properties of Pure Substances
Pure Substance w A substance that has a fixed (homogeneous and invariable) chemical composition throughout is called a pure substance. w It may exist in more than one phase, but the chemical composition is the same in all phases.
Pure Substance w Pure means “…of uniform and invariable chemical composition (but more than one molecular type is allowed).” This allows air to be a pure substance. w 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.
Examples of Pure Substance w Water (solid, liquid, and vapor phases) w Mixture of liquid water and water vapor w Carbon Dioxide w Nitrogen w Homogeneous mixture of gases, such as air, as long as there is no change of phases.
Multiple phases mixture of a Pure Substance vapor liquid vapor liquid WaterAir Pure Not pure, different condensation temperatures for different components
Thermodynamic Properties w We have discussed extensive properties such as m, U, and V (for volume) which depend on the size or extent of a system, and w 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.
Important Questions.…. w How many properties are needed to define the state of a system? w How do we obtain those properties? Equation of State Property Tables
Review - State Postulate w 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. w 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.
Simple system w A simple system is defined as one for which only one quasiequilibrium work mode applies. w Simple compressible systems w Simple elastic systems w Simple magnetic systems w Simple electrostatic systems, etc.
Compressible w If we restrict our system to being compressible, we define what that quasiequilibrium work mode is:
For a simple system, w We may write P = P(v,T) w or v = v(P,T) w or perhaps T = T(P,v)
For a simple, pure substance w y 0 = y(y 1,y 2 ), or w P = P(v,T), v = v(P,T), and T = T(P,v) w What do these equations define, in space? w Equations used to relate properties are called “Equations of State”
Equation of State w Any two independent, intrinsic properties are sufficient to fix the intensive state of a simple substance. w One of the major task of Thermodynamics is to develop the equations of state which relate properties at a give state of a substance.
Ideal gas “law” is a simple equation of state R u = universal gas constant = 8.3144 (kPa-m 3 )/(kgmol-K) = 1.545 (ft-lbf)/(lbmol-R)
Phases of a Pure Substance w Solid phase -- molecules are arranged in a 3D pattern (lattice). w Liquid phase -- chunks of molecules float about each other, but maintain an orderly structure and relative positions within each chunk. w Gas phase -- random motion, high energy level.
Phase Equilibrium p heat ice p liquid ice P = 1 atm T = -10 o C liquid p p vapor p P = 1 atm T = 0 o C P = 1 atm T = 20 o C P = 1 atm T = 100 o C P = 1 atm T = 300 o C
Phase-change Process w Compressed liquid -- not about to evaporate w Saturated liquid -- about to evaporate w Saturated liquid-vapor mixture --two phase w Saturated Vapor -- about to condense w Superheated Vapor -- not about to condense
T-v Diagram 1 Compressed liquid Saturated mixture 2 Superheated vapor Isobaric process P = 1 atm 3 4 5 v T, C o 20 100 300
P-T Diagram (Phase Diagram) of Pure Substances
Superheated Vapor Compressed Liquid Isothermal Process
Superheated Vapor Subcooled Liquid Isobaric Process a
Water Expands on Freezing! w Ice floats on top of the water body (lakes, rivers, oceans, soft drinks, etc.). w If ice sinks to the bottom (contracts on freezing), the sun’s ray may never reach the bottom ice layers. w This will seriously disrupt marine life.
Saturation Temperature and Pressure w T sat -- Temperature at which a phase change takes place at a given pressure. w P sat -- Pressure at which a phase change takes place at a given temperature.
Saturation Temperature T sat = f (P sat ) p = 1atm = 101.3 kPa, T = 100 C p = 500 kPa, T = 151.9 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).
Latent Heat w Latent heat is the amount of energy absorbed or released during phase change w Latent heat of fusion -- melting/freezing =333.7 kJ/kg for 1 atm H 2 O w Latent heat of vaporization -- boiling/condensation =2257.1 kJ/kg for 1 atm H 2 O
P v Superheated region -- substance is 100% vapor Two-phase or saturation region -- gas and liquid coexist Subcooled or compressed liquid region Saturated liquid line Saturated vapor line Critical point P-v Diagram
P-v Diagram of a Pure Substance w SUPERHEATED v Isothermal process
T-v Diagram of a Pure Substance v
Critical & Supercritical w The state beyond which there is no distinct vaporization process is called the critical point. w At supercritical pressures, a substance gradually and uniformly expands from the liquid to vapor phase. w Above the critical point, the phase transition from liquid to vapor is no longer discrete.
Critical Point w Point at which the saturated vapor and saturated liquid lines coincide. w If T T c or P P c there is no clear distinction between the superheated vapor region and the compressed liquid region.
Critical Point w A point beyond which T T c and a liquid-vapor transition is no longer possible at constant pressure. If T T c, the substance cannot be liquefied, no matter how great the pressure. w Substances in this region are sometimes known as “fluids” rather than as vapors or liquids.
Vapor (Steam) Dome w The dome-shaped region encompassing the two-phase, vapor-liquid equilibrium region. w 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. w The region below the vapor dome is also called: saturated liquid-vapor region, wet region, two-phase region, or saturation region.
STEAM IS NOT AN IDEAL GAS!
Steam Tables w Table A-1.1 w Saturation water -- temperature table w Table A-1.2 w Saturation water -- pressure table w Table A-1.3 w Superheated vapor
P v saturation region p=p sat and T=T sat superheated region If T=T sat, p p sat If p=p sat, T>T sat Subcooled or compressed liquid region If T=T sat, p p sat If p=p sat,T T sat For Water
Two properties are not independent in the vapor dome (the two-phase region) w The temperature and pressure are uniquely related. Knowing a T defines the P and vice versa. w Use quality to determine the state in two-phase region.
Quality is related to the horizontal differences of P-v and T-v Diagrams w
QualityQuality w In a saturated liquid-vapor mixture, the mass fraction (not volume fraction) of the vapor phase is called the quality and is defined as w 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.
What is v for something in the two-phase region? v = (1-x)v f + xv g = v f + x(v g - v f ) = v f + xv fg
Enthalpy in Two-Phase Region w H = U + pV w h = u + pv h = (1-x)h f + xh g = h f + x(h g - h f ) = h f + xh fg
Saturated Mixture w 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.
v = v f + x(v g - v f ) = v f + xv fg u = u f + x(u g - u f ) = u f + xu fg h = h f + x(h g - h f ) = h f + xh fg s = s f + x(s g - s f ) = s f + xs fg Saturated Mixture f : saturated liquid g : saturated vapor
Saturated Water (temperature)
Examples 3-2 thru 3-7 Steam Tables A-1.1 and A-1.3
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(m 3 /kg)x (if appl.) 3001.0 0.150.65 0.50300
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.) 7.00.0 7.01.0 7.00.05 7.0600 7.0100 7.0460