Properties of a Pure Substance. 3-1 The Pure Substance A pure substance is one that has a...
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Transcript of Properties of a Pure Substance. 3-1 The Pure Substance A pure substance is one that has a...
Properties of a Pure Substance
3-1 The Pure Substance
A pure substance is one that has a homogeneous and invariable chemical composition. It may exist in more than one phase, but the chemical composition is the same in all phases.
3-2 Vapor-Liquid-Solid-Phase Equilibrium in a Pure Substance
Saturation Temperature and Saturation Pressure
The term saturation temperature designates the temperature at which vaporization takes place at a given temperature. This pressure is called the saturation pressure for the given temperature.
Saturated liquid or saturated vapor, subcooled liquid or compressed liquid, and superheated vapor.
Vapor-Pressure Curve of a Pure Substance
Temperature-Volume Diagram
Some Critical-Point Data
T-v Diagram
The Quality Specific Volume Relation (1/2)
gf xvvx 1
g
vap
f
liq
xv
m
mv
m
m
m
Vvv
fgf xvv
fgf vvxv
The Quality Specific Volume Relation (2/2)
m
m
mm
m liq
liqvap
liq
where
liqvap
vapvap
mm
m
m
mx
fgfg vvv
: quality or dryness
liqvap
vapvap
mm
m
m
mx
111
Some Triple-Point Data
P-T Diagram of Water
CO2 Phase Diagram
H2O Phase Diagram
3-3 Independent Properties of a Substance
The state of a simple pure substance is defined by two independent properties.
In a saturation state, pressure and temperature are not independent properties.
3-4 Tables of Thermodynamic Properties
3-5 Thermodynamic Surfaces
Expand on freezing Contract on freezing
P-V-T Surface for a Substance that Expands on Freezing (1/2)
P-V-T Surface for a Substance that Expands on Freezing (2/2)
P-V-T Surface for a Substance that Contracts on Freezing (1/2)
P-V-T Surface for a Substance that Contracts on Freezing (2/2)
3-6 The P-V-T Behavior of Low-and Moderate –Density Gases
At very low densities the average distances between molecules is so large that the intermolecular potential energy may effectively be neglected. In such a case, the particles would be independent of one another, a situation referred to as an ideal gas.
The Ideal Gas Equation of State (1/2)
TnRPV u
TRvP u
kmolkg
kg
M
mn
Kkmol
kJ
Kkmol
mkNR
u
3145.83145.8
uR : the universal gas constant
The Ideal Gas Equation of State (2/2)
mRTPV
RTPv
M
RR u
R : the gas constant
T-v Diagram for Water
Compressibility Factor Z
zRTpv
TR
vpz
u
TzRvp u
1z for ideal gas
Compressibility of N2
Observations from Figure 3.15
At all temperatures as . At the temperatures of 300K and above the
compressibility factor is near unity up to pressure of about 10MPa.
At lower temperatures or at very high pressure, the compressibility factor deviates significantly from the ideal-gas value.
1z 0P
Reduced Properties
cr P
PP
cr T
TT
rP : reduced pressure
rT : reduced temperature
cP : critical pressure
cT : critical temperature
The Benedict-Webb-Rubin Equation of State
2
223632
2000 1 ve
vTv
c
v
a
v
aRTb
v
TCARTB
v
RTP
Lee-Kesler Simple Fluid Compressibility Factor