2
CHAPTER
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Thermodynamics
Properties of
Pure Substances
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Constant-Pressure Phase-Change
Process
(fig. 2-16)
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Thermodynamics
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T-v Diagram of a Pure Substance
Energy, not mass, crosses closed-system boundaries
(Fig. 2-18)
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P-v Diagram of a Pure
Substance
(Fig. 2-19)
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SUPERHEATED
Thermodynamics
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P-v Diagram of Substance that
Contracts on Freezing
(Fig. 2-21)
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P-v Diagram of Substance that
Expands on Freezing
(Fig. 2-22)
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Thermodynamics
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P-T Diagram of Pure
Substances
(Fig. 2-25)
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Thermodynamics
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P-v-T Surface of a Substance that
Contracts on Freezing
(Fig. 2-26)
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Thermodynamics
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P-v-T Surface of a Substance that
Expands on Freezing
(Fig. 2-27)
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Partial List of Table A-4
(Fig. 2-35)
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Thermodynamics
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Quality Shown in P-v and T-v
Diagrams
Quality is related to the horizontal differences of P-V and T-v diagrams
(Fig. 2-41)
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Partial List of Table A-6
(Fig. 2-45)
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Thermodynamics
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Pure Substances
can Exist as Compressed Liquids
At a given P and T, a pure substance will exist
as a compressed liquid if T<T sat @ P
(Fig. 2-49)
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Thermodynamics
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The Region Where Steam
can be Treated as an Ideal Gas
(Fig. 2-54)
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Thermodynamics
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Comparison of Z Factors
for Various Gases
(Fig. 2-57)
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Percent of Error in Equations
for the State of Nitrogen
(Fig. 2-66)
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Thermodynamics
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Chapter Summary
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Thermodynamics
• A substance that has a fixed chemical
composition throughout is called a pure
substance.
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Chapter Summary
Thermodynamics
• A pure substance exists in different phases
depending on its energy level. In the liquid phase,
a substance that is not about to vaporize is called
a compressed or subcooled liquid.
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Chapter Summary
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Thermodynamics
• In the gas phase, a substance that is not about to
condense is called a superheated vapor.
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Chapter Summary
Thermodynamics
• During a phase-change process, the temperature
and pressure of a pure substance are dependent
properties. At a given pressure, a substance
changes phase at a fixed temperature, called the
saturation temperature. At a given temperature,
the pressure at which a substance changes phase
is called the saturation pressure. During a boiling
process, both the liquid and the vapor phases
coexist in equilibrium, and under this condition
the liquid is called saturated liquid and the vapor
saturated vapor.
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Thermodynamics
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Chapter Summary
• In a saturated liquid-vapor mixture, the mass
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.
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Chapter Summary
• In the saturated mixture region, the average value
of any intensive property y is determined from
Thermodynamics
where f stands for saturated liquid and g for
saturated vapor.
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Chapter Summary
Thermodynamics
• In the absence of compressed liquid data, a
general approximation is to treat a compressed
liquid as a saturated liquid at the given
temperature, that is,
where y stands for v, u, or h.
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Chapter Summary
Thermodynamics
• 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.
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Chapter Summary
Thermodynamics
• All three phases of a substance coexist in
equilibrium at states along the triple line
characterized by triple-line temperature
and pressure.
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Chapter Summary
Thermodynamics
• Various properties of some pure sub-stances are
listed in the appendix. As can be noticed from
these tables, the compressed liquid has lower v, u,
and h values than the saturated liquid at the same
T or P. Likewise, superheated vapor has higher v,
u, and h values than the saturated vapor at the
same T or P. is a major application area of
thermodynamics.
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Chapter Summary
Thermodynamics
• Any relation among the pressure, temperature,
and specific volume of a substance is called an
equation of state. The simplest and best-known
equation of state is the ideal-gas equation of state,
given as
where R is the gas constant. Caution should be
exercised in using this relation since an ideal gas
is a fictitious substance. Real gases exhibit idealgas behav-ior at relatively low pressures and high
temperatures.
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Chapter Summary
Thermodynamics
• The deviation from ideal-gas behavior can be
properly accounted for by using the
compressibility factor Z, defined as
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Chapter Summary
Thermodynamics
• The Z factor is approximately the same for all
gases at the same reduced temperature and
reduced pressure, which are defined as
where Pcr and Tcr are the critical pressure and
temperature, respectively. This is known as the
principle of corresponding states.
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Chapter Summary
(Continued from previous slide)
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Thermodynamics
• When either P or T is unknown, Z can be
determined from the compressibility chart with the
help of the pseudo-reduced specific volume,
defined as
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Chapter Summary
• The P-v-T behavior of substances can be
represented more accurately by the more complex
equations of state. Three of the best known are
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Thermodynamics
van der Waals:
where
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Chapter Summary
• Beattie-Bridgeman:
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Thermodynamics
where
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Chapter Summary
• Benedict-Webb-Rubin:
Thermodynamics
Third Edition
The constants appearing in the Beattie-Bridgeman and Benedict-WebbRubin equations are given in Table A-29 for various substances.
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