Download Phases of Pure Substance

-Properties of Pure Substances
-first law of thermodynamics
and applications
-Definition of Pure Substance
A pure substance has a homogeneous and invariable
chemical composition and may exist in more than
one phase.
-Phases of Pure Substance
Solid
Liquid
Gas
Phase Change Process of Pure Substance
T
c
2”
1”
1’
2’
2
3”
T=constant
p
5”
c
5’
4”
3’
3
5
1” 2”
4’
1’
1 2
4
2’
3”
3’
3
4”
5”
4’
5’
4
5
1
v
Critical point
Liquid
p
vapor
Liquid +Vapor
Solid+Vapor
Triple line
T
v
P-V-T Surface
- Critical point
- Triple point (line)
Extract on
freezing
Expand on
P freezing
Critical
point
Melting line
liquid
Vaporization
Solid
Vapor
Sublimation
T
Property
In addition to the temperature, pressure, and volume
data, contain the data for the specific internal energy u
the specific enthalpy h and the specific entropy s. The
enthalpy is a convenient grouping of the internal energy,
pressure, and volume and is given by
H  U  PV
The enthalpy per unit mass is
h  u  Pv
x
masssaturated vapor
masstotal

mg
m f  mg
v  (1  x)v f  xvg
Solid i,
h  h f  xhfg
Vapour g
v  v f  xv fg
v  xvg  (1  x)v f
Liquid f,
First law of thermodynamics
-Energy can be changed from one form to another, but
it cannot be created or destroyed.
The total amount of energy and matter in the Universe
remains constant, merely changing from one form to
another.
Q  W  E
W is net work done by the system
Q is net heat
Qa,out
Qb,in
Wb,out
Wa,in
If the work is done by the system on the surroundings, e.g.,
when a fluid expands pushing a piston outwards, the work is
said to be positive.
i.e., Work output of the system = + W
If the work is done on the system by the surroundings, e.g.,
when a force is applied to a rotating handle, or to a piston to
compress a fluid, the work is said to be negative.
i.e., Work input to system = – W
Heat received by the system = + Q
Heat rejected or given up by the system = – Q.
If properties of system do not change in time
Ein  Eout
Or
Cycle is a process at the end of which system returns to an
original state.
Q W
Power plant cycle
Refrigeration/Heat-pump cycle
2-5 The Ideal-Gas Equation of State
2-5-1 The Ideal-Gas
The molecules of ideal-gas have no volume
There are no attraction among molecules of ideal-gas
2-5-2 The Ideal-Gas Equation of State
(1). pV = mRT
pv=RT
R------The gas constant
(2)
pVm = μRμT
pvm=RμT
Rμ-----The universal gas constant
= 8.314kJ/kmol.K
The First Law of Thermodynamics. Application to a particular case:
A gas confined in a cylinder with a movable piston
Work done on the system, Won , is the energy transferred as work to the system.
When this energy is added to the system its value will be positive.
The work done on the gas in an
expansion is
V2
Won gas    P dV
V1
Won gas  Wby gas
P- V diagrams
Constant pressure
V2
Won gas    P dV  P(V1  V2 )
V1
If 5 L of an ideal gas at a pressure of 2 atm is cooled
so that it contracts at constant pressure until its
volume is 3 L what is the work done on the gas?
[405.2 J]
The First Law of Thermodynamics. P-V diagrams
P- V diagrams
Conecting an initial state and a final state
by three paths
Isothermal
V2
Constant pressure
Constant Volume
Constant Temperature
Won gas    P dV  P(V1  V2 )
V1
V2
Won gas    P dV  0
V1
V2
Won gas   
V1
n RT
V2
dV  n R T ln
V
V1
The First Law of Thermodynamics. Processes. P-V Diagrams
Adiabatic Processes. No heat flows into or out of the system
The First Law of Thermodynamics. Processes. P-V Diagrams
Adiabatic Processes. No heat flows into or out of the system
Qin  0
Adiabatic process
then Eint  Won,adiabatic  n cV T
The equation of curve describing the adiabatic
process is
P V   const ;  
T V  1  const
T  P1  const
A quantity of air is compressed adiabatically
and quasi-statically from an initial pressure of
1 atm and a volume of 4 L at temperature of
20ºC to half its original volume. Find (a) the
final pressure, (b) the final temperature and (c)
the work done on the gas.
cP = 29.19 J/(mol•K); cV = 20.85 J/(mol•K).
M=28.84 g
CP
CV
adiabatic coefficient
We can use the ideal gas to rewrite
the work done on the gas in an
adiabatic process in the form
Won gas,adiab 
Pf V f  Pi Vi
 1
Thank you