4. (a) “All naturally occurring processes proceed in one direction only. Such
spontaneous one-way processes are “irreversible”.
In reversible process, the system and environment will return to their
Reversible: dissolution of a salt into water, reaction of O2 and H2 to form water, phase
changes like freezing or boiling of water
Irreversible: hydrocarbon combustion like the burning of wood or oil, radioactive decay
5. Processes that are usually idealized as reversible include:
•Restrained compression or expansion
•Energy transfer as heat due to infinitesimal temperature non-uniformity
•Electric current flow through a zero resistance
•Restrained chemical reaction
•Mixing of two samples of the same substance at the same state.
Processes that are irreversible include:
•Movement with friction
•Energy transfer as heat due to large temperature non uniformities
•Electric current flow through a non zero resistance
•Spontaneous chemical reaction
•Mixing of matter of different composition or state.
6. Physical properties of matter are categorized as either Intensive or Extensive:
•Intensive - Properties that do not depend on the amount of the matter present.
•Luster - How shiny a substance is.
•Malleability - The ability of a substance to be beaten into thin sheets.
•Ductility - The ability of a substance to be drawn into thin wires.
•Conductivity - The ability of a substance to allow the flow of energy or electricity.
•Hardness - How easily a substance can be scratched.
•Extensive - Properties that do depend on the amount of matter present.
•Mass - A measurement of the amount of matter in a object (grams).
•Weight - A measurement of the gravitational force of attraction of the earth acting on an object.
•Volume - A measurement of the amount of space a substance occupies.
7. The second law of thermodynamics introduces the notion of entropy (S), which
is a measure of system disorder (messiness)
U is the quantity of a system’s energy, S is the quality of a system’s energy.
law of thermodynamics
Law can be stated that:
heat flows spontaneously from a hot object to a cold object
(spontaneously means without the assistance of external work)
8. Direction of a Process
• The 2nd
Law helps determine the preferred direction of a
• A reversible process is one which can change state and then
return to the original state
• This is an idealized condition
• ( all real processes are irreversible)
10. Entropy is a physical quantity that controls the
direction of irreversible processes.
It is a property of the state of a system; like T, P, V, U.
Entropy principle: “If an irreversible process occurs in a closed system,
the entropy of that system always increases; it neverdecreases.”
Entropy (S )
Another statement of the second law of thermodynamics:
(The total entropy of an isolated system never decreases).
Entropy is a measure of the disorder of a system. This gives us yet another
statement of the second law:
Natural processes tend to move toward a state of greater disorder.
Entropy is an extensive
thermodynamic property. In
other words, the entropy of a
complex system is the sum of
the entropies of its parts
11. – The greater the number of possible arrangements, the greater the
entropy of a system, i.e., there is a large positional probability.
– The positional probability or the entropy increases as a solid
changes from a liquid or as a liquid changes to a gas
Ex. Choose the substance with the higher positional entropy:
– CO2(solid) or CO2(gas)?
– N2(gas) at 1 atm and 25o
C or N2(gas) at .010 atm and 25o
Ssolid < Sliquid < Sgas
12. Entropy of the System
Is greater in:
Gases than solids.
Larger volumes of gases than smaller volumes.
Larger number of gas molecules than smaller number of
Example: The second law of thermodynamics leads us to conclude that
(A) the total energy of the universe is constant.
(B) disorder in the universe is increasing with the passage
(C) it is theoretically possible to convert heat into work
with 100% efficiency.
(D) the average temperature of the universe is increasing
with the passage of time.
13. In fig. below, it can be shown that the gas
and the reservoir form a closed system.
The entropy change of this closed system is
found to be zero: .
The second law of thermodynamics and Entropy
When the systems for the irreversible processes
are closed, the corresponding .0>∆S
} 0, =∆ closedtotS
For expansion process
14. Probability of Disorder
• Entropy is defined as a
measure of disorder.
• A system (such as a room) is
in a state of high entropy
when its degree of disorder is
• As the order within a system
increases, its entropy
•Is there a higher probability your room will be messy or neat as
time goes on?
15. Entropy change for reversible processes
The definition of entropy change for a reversible process:
Here dQ is the increment of heat energy that is transferred into (or
out) of the closed system at temperature T.
If the process is isothermal,
the entropy of that system increases
the entropy of that system constant
0 0Q S= ⇔ ∆ =
16. Divided by T, we obtain
dU dQ dW= +
vdU nC dT=
dQ nC dT PdV nC dT dV
= + = +
dQ dT dV
T T V
Entropy as a state property
Integrate between an arbitrary initial state i and an final state f :
f f f
S nC ln nR ln
T T V
∫∆ = = +
from the first law
What’s the entropy change for ideal gases?
Only related to initial and final states
A vessel containing 1.8kg of water is placed on a hot plate. Both the water and hot
plate being initially at . The temperature of the hot plate is raised very slowly
to at which point the water begins to boil. Find of the water during this
T T f
S mc mcln 1818 J/K
T T T
Δ ∫ ∫= = = =
c= 4186 J kg¯1
18. Processes can be discussed profitably using the entropy concept.
For a reversible process:
• If the reversible process is isothermal:
AB ∆=⇒==−=∆ ∫δ
S increases if the system absorbs heat, otherwise S decreases
00 =∆⇒==−=∆ ∫ S
Reversible isothermal processes are isentropic but in irreversible ones the entropy may change
• If the reversible process is adiabatic:
19. Example: A heat source at 800 K loses 2000 kJ of heat to a sink
at (a) 500 K and (b) 750 K. Determine which heat transfer
process is more irreversible.
(a) For the heat transfer process to a sink at 500 K:
(b) Similarly, for process at 750 K:
Process (b) is less irreversible since it involves a smaller T difference (smaller
TdSQIn a finite process: (depends on the trajectory)
In a cycle:
=−== ∫ WTdSQ work done by the system in the cycle
heat absorbed by system
∆S = 0
21. Entropy and the performance of engines
• A heat engine is a device that extracts energy from its environment in the
form of heat and does useful work.
• At the heart of every engine is a working substance (gas).
• For an engine to do work on a sustained basis, the working substance must
operate in a cycle.
This will be discussed in details in next module
22. Energy is conserved
o FIRST LAW OF THERMODYNAMICS
o Examples: Engines (Internal -> Mechanical)
Friction (Mechanical -> Internal)
All processes must increase entropy
o SECOND LAW OF THERMODYNAMICS
o Entropy is measure of disorder
o Engines can not be 100% efficient