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THERMODYNAMICS

PROPERTIES OF PURE SUBSTANCES

Topics covered in this unit
(1)Thermodynamic properties of pure substances in solid, liquid and vapor
phases
(2) P-v T behavior of simple compressible substances, phase rule, thermodynamic
property tables and charts,
(3) Ideal and real gases, ideal gas equation of state and van der Waals equation
of state
(4) Law of corresponding states, compressibility factor and generalized
compressibility chart

Pure Substance
A substance that has a FIXED synthetic structure all throughout is called pure
substance
...
A combination of
phases of at least two or more substance is can be a pure substance in the event
that it is homogeneous, similar to ice and water (stolid and fluid) or water and
steam (liquid and gas)
...
In all the cases,
thermodynamics deals with the essential stages as it were
...

Solids are closely packed three dimensional crystals
...

Intermediate molecular bond strength
Liquid,
Molecular spacing is comparable to solids but their
Molecules can float about in groups
...

Their molecules are at higher energy levels, they must release
Large amounts of energy to condense or freeze
...

Think about water at room temperature (25°C) and typical atmospheric pressure
(1 atm) in a piston cylinder gadget
...

In the event that we heat the water, its temperature will increase; let us say until
50°C
...
As an outcome, the cylinder will move somewhat vertically to maintain
the consistent pressure (1 atm)
...
As of now, any extra expansion of intensity will

vaporize some water
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The main property that changes is the specific volume
...
This state
is called saturated vapor,
The state between saturated fluid (just fluid) and saturated vapor (only vapor)
where two stages exist is called saturated fluid vapor mixture
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Saturation Pressure and Saturation Temperature
Review that during a stage change, pressure and temperature are not
autonomous intensive properties
...
At the end of the day, water begins to
boil at 100 ºC yet just at 1 atm
...

At a given pressure, the temperature at which a pure substance changes phase is
known as the saturation temperature (Tsat)
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Property Diagrams for Phase Change Processes
T-v Diagram
In the event that we increase the pressure of water in the piston -cylinder
instrument, the process from compressed fluid to super-heated vapor will follow
a particular path that seems to be the interaction for P = 1 atm, the main
difference is just the width of the mixture area will be more limited
...
This point is known as the critical point
...

At the critical point, the properties of a substance are called critical properties
(critical temperature (Tcr), critical pressure (Pcr) and critical specific volume
(vcr)
...
09 MPa
Tcr = 374
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298 K
vcr = 0
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77 MPa

Tcr = 132
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65 K
vcr = 0
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On the off chance that we associate every one of the focuses addressing
saturated fluid we will acquire the saturated fluid line
...
The convergence of the two lines is the basic point
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The P-v diagram can be extended out to incorporate the strong stage, the solid
liquid also, the solid vapor saturation regions as certain substances, as water,
expand when they freeze, and the rest (the larger part) contracts during freezing
process, and we have two designs for the P-v chart with strong stage
...
In any
case, under specific circumstances, water can exist simultaneously as ice (solid),
fluid and vapor
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On a P-T diagram, these conditions are represented by a point
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Solid vapor sublimation
Solid liquid melting
Liquid vapor vaporization
Property Tables
Notwithstanding the temperature, pressure, and specific volume information,
tables contain information for the particular internal energy u, the specific
enthalpy h, furthermore, the specific entropy s
...
For simplicity this combination is defined
as a new property called enthalpy
...
Table A-4 has
temperature as the free property; Table A-5 has pressure as the autonomous
property
...

TABLE A-4
Saturated water-Temperature table
The last entry is the critical point at 373
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TABLE A-5
Saturated water-Pressure table
The last entry is the critical point at 22
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The subscript fg used in Tables A-4 and A-5 refers to the difference
between the saturated vapor value and the saturated liquid value region
...
It represents the amount of energy needed to vaporize a unit of
mass of saturated liquid at a given temperature or pressure
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Where,
P is the pressure,
V is the volume,
T is the temperature,
n is the number of moles of gases,
‘a’ and ‘b’ are the constants that are specific to each gas
...
Any mistakes committed in the process to deduce Van Der
Waals equation of state can be crucial and affect the whole process
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In the case of a real gas when students are using Van Der Waals equation, the
volume of a real gas is considered as (Vm - b), where b can be considered as the
volume occupied by per mole
...

( P+aV2
) ( Vm - b) = R

( P+an2V2
) ( V - nb) = nRT
Where,
Vm: molar volume of the gas
R: universal gas constant
T: temperature
P: pressure
V: volume
Thus, it is possible to reduce Van Der Waals equation to ideal gas law as PVm =
RT
...

The steps for derivation of the real gas equation for one gas are as follows
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Compressible fluids like polymers have fluctuating specific volumes and this can
be expressed as follows
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Benefits and Demerits of Van Der Waals Equation of State
Merits:
It can predict the behavior of gas much better and accurately than the ideal gas
equation
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The arrangement is made in a manner of cubic equation in volume
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Demerits:
It can only get accurate answers for real gases which are above the critical
temperature
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In the transition phase of gas, the equation is a failure
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In a recast worked on type of a constitutive condition, material constants that
differ for every sort of material are discarded
...
As per van der Waals, the hypothesis of
relating states (or standard/law of comparing states) expresses that all liquids
have generally a similar compressibility variable and all stray from ideal gas
conduct to about a similar degree when looked at a similar decreased
temperature and diminished pressure
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Accordingly, as the temperature increases, the variety from optimal way of
behaving decreases
...
At high tensions
and temperatures, a few gases submit to ideal gas laws
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