University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Second law of thermodynamics
Thermodynamics is concerned with transformations of energy . The first law
reflects the observation that energy is conserved, but it imposes no restriction on
the process direction.
The differences between the two forms of energy, heat and work, provide some
insight into the second law.
Work is readily transformed into other form of energy :
 Potential due to elevation change of weight
 Kinetic due to acceleration of a mass
 Electrical energy e.g. generator
Conversion efficiency can be approach 100 by elimination friction or by some
other means.
First law: showed the equivalence of work and heat
Second law:
 Puts restrictions on useful conversion of work and heat
 Follows from observation of a directionality to natural or spontaneous
processes
 Provides a set of principles for
- determining the direction of spontaneous change
-determining equilibrium state of system
Statements of the 2nd law
Statement 1: No device can operate in such a way that its only effect (in system
and surroundings) is to convert heat absorbed by a system completely into work
done by the system.
 Statement 1 a: It is impossible by a cyclic process to convert the heat
absorbed by a system completely into work done by the system.
Statement 2: No process is possible which consists solely in the transfer of heat
from one temperature level to a higher one.
Heat engine
The device that convert heat into work is called "heat engine" e.g. power plant , In
such a power plant the cycle (in its simplest form) consists of the following steps:
 Liquid water at ambient temperature is pumped into a boiler at high
pressure.
 Heat from a fuel (heat of combustion fuel) or heat from a nuclear reaction is
transferred in the boiler to the water, converting it to high-temperature steam
at the boiler pressure.
University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
 Energy is transferred as shaft work from the steam to the surroundings by a
device such as a turbine, in which the steam expands to reduced pressure and
temperature.
 Exhaust steam from the turbine is condensed by transfer of heat to the
surroundings , producing liquid water for return to the boiler , thus
completing the cycle.
Essential to all heat-engine cycles are the absorption of heat at high temperature ,
the rejection of heat at lower temperature and the production of work.
Heat reservoir Definition: A very large system of uniform T, which does not change
regardless of the amount of heat added or withdrawn.
In heat engine there are two temperature level
 High temperature TH reservoir where the working fluid of heat engine
absorbs heat QH and produce work.
 Low temperature TC where the working fluid of heat engine discard
heatQC and return to its original state .
The first law therefore reduce to :
W  Q  QH  QC
Thermal efficiency  :ratio between net work output to heat input

Q  QC
Q
W
 H
 1 C
QH
QH
QH
University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Absolute value signs with heat to make equations independent of sing convention
for Q .
 equal unity (100%) when QC = 0 .No engine ever been built for which his is
true.
Carnot engine
The characteristics of such ideal engine were first described by N. Carnot in 1824,
the four steps that make up a Carnot cycle
Step 1: A system at the temperature of a cold reservoir T C undergoes a reversible
adiabatic process that causes its temperature to rise to that of a hot reservoir at T H.
Step 2: The system maintains contact with the hot reservoir at T H, and undergoes a
reversible isothermal process during which heat QH is absorbed from the hot
reservoir.
Step 3: The system undergoes a reversible adiabatic process in the opposite
direction of step 1 that brings its temperature back to that of the cold reservoir at
TC.
Step 4: The system maintains contact with the reservoir at TC, and undergoes a
reversible isothermal process in the opposite direction of step 2 that returns it to its
initial state with rejection of heat QC to the cold reservoir.
Since a Carnot engine is reversible, it may be operated in reverse; the Carnot cycle
is then traversed in the opposite direction, and it becomes a reversible refrigeration
cycle for which the quantities QH , QC , and W are the same as for the
engine cycle but are reversed in direction.
Ideal-Gas Temperature Scale; Carnot's Equations
The cycle traversed by an ideal gas serving as the working fluid in a Carnot engine
is shown by a below PV diagram. It consists of four reversible steps:
 a → b Isothermal expansion to arbitrary point b with absorption of heat QH .
 b → c Adiabatic expansion until the temperature decreases to T2 .
 c → d Isothermal compression to the initial state with rejection of heat QC .
 d → a Adiabatic compression until the temperature rises from T 2 to T1.
University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Step 1 : U  0 and Q  W  RT1 ln
PA
PB
Step 2: Q  0 and W  CV (T2  T1 )
T2 PC

]
Reversible adiabatic process
T1 PB
Step 3: U  0 and Q  W  RT2 ln
Step 4: Q  0 and W  CV (T1  T2 )
( 1)

PC
PD
T 
  1 
 T2 

(  1)

PB
PC
University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Reversible adiabatic process
Eff . 
Q1  Q2

Q1
RT1 ln
T2 PD

]
T1 PA
( 1)

T 
  1 
 T2 

( 1)

PA
PD
PA
P
 RT2 ln D
PB
PC
P
RT1 ln A
PB
PA PD

PB PC
 
T1  T2
T
1 2
T1
T1
Where T1 and T2 in Kelvin scale
This equation are known as Carnot`s equation .The thermal efficiency of a Carnot
engine approaches unity only when TH (T1) approaches infinity , or TC
(T2)approaches zero.
Conclusions:
 no engine operating between two heat reservoir each having a fixed
temperature can be more efficient than a reversible one operate between the
same temperature.
 All reversible engine operation between two heat reservoirs at the same
temperature , each having the same efficiency.
 The efficiency of any reversible engine operate between two reservoirs is
independent of the nature of the working but depend on the temperature of
reservoirs.