chemical thermodynamics - dypvp
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CHEMICAL
THERMODYNAMICS
Mrs.Vaishali Mahajan
Spontaneous Processes (changes):
Once the process begins, it proceeds
automatically without the need to do work on
the system.
The opposite of every Spontaneous Process is
a
Non-Spontaneous Process
that can only proceed if external work is done
on the system.
• In a
Reversible Process,the system undergoes changes such that the system plus it’s surroundings can be put back in their original states by exactly reversing the process.
• In a
Reversible Process,
changes proceed in
infinitesimally small steps, so
that the system is infinitesimally close to equilibrium at every step.
The First Law of Thermodynamics states that energy can be
converted from one form to another with the interaction of
heat, work and internal energy, but it cannot be created nor
destroyed, under any circumstances. Mathematically, this is
represented as
ΔU=q+w
ΔU is the total change in internal energy of a system,
q is the heat exchanged between a system and its
surroundings, and
w is the work done by or on the system.
First Law of Thermodynamics
LIMITATIONS OF FIRST LAW OF
THERMODYNAMICS It doesn’t tell the extent and direction of the convertibility
of one form of energy to another.
It doesn’t tell why chemical reactions do not proceed to completion.
It doesn’t tell why natural processes are unidirectional.
It doesn’t explain the feasibility and spontaneity of a process.
It says the equivalency of work and heat. But it has been observed that the work can be completely transformed into heat but heat cannot be transformed into work without permanent change in the system or surrounding which is not explained by the law.
CARNOT CYCLE
The Carnot cycle consists of the following four processes:
A reversible isothermal gas expansion process. In this process, the ideal gas in the system absorbs qin amount heat from a heat source at a high temperature Th, expands and does work on surroundings.
A reversible adiabatic gas expansion process. In this process, the system is thermally insulated. The gas continues to expand and do work on surroundings, which causes the system to cool to a lower temperature, Tl.
A reversible isothermal gas compression process. In this process, surroundings do work to the gas at Tl, and causes a loss of heat, qout.
A reversible adiabatic gas compression process. In this process, the system is thermally insulated. Surroundings continue to do work to the gas, which causes the temperature to rise back to Th.
Figure 1: An ideal gas-piston model of the Carnot cycle.
The P-V diagram of the Carnot cycle is shown in Figure 2.
In isothermal processes I and III, ∆U=0 because ∆T=0. In adiabatic processes
II and IV, q=0.
EFFICIENCY OF CARNOT ENGINE
Hence, the total work done by the gas on the environment in one complete cycle is given by:
W=W1→2+W2→3+W3→4+W4→1W=μRT1lnv2v1−μRT2lnv3v4
Net efficiency = Net workdone by the gas Heat absorbed by the gas
Net efficiency = WQ1=Q1−Q2Q1=1−Q2Q1=1−T2T1lnv3v4lnv2v1
Since the step 2–>3 is an adiabatic process, we can write T1V2Ƴ-1 =
T2V3Ƴ-1
Or,
V2/v3=(T2/T1)1γ−1
Similarly, for the process 4–>1, we can write
V1/v2=(T2/T1)1γ−1
This implies,
V2/v3=v1/v2
So, the expression for net efficiency of carnot engine reduces to:
Net efficiency=1−T2/T1
CONCLUSION FROM CARNOT CYCLE
The Carnot cycle provides a simple method to determine
the efficiency of any heat engine.There is no engine
available which when working between the same two
temperature can convert thermal to mechanical energy
with grater efficiency than the Carnot engine .Thus
Carnot cycle efficiency is used to find out the maximum
conversion of thermal mechanical energy that can be
expected for any real engine.
No heat engine can be constructed that can have unit
efficiency.
SECOND LAW OF THERMODYNAMICS
Rudolf Clausius was a German physicist, and he developed the Clausius statement, which says "Heat generally cannot flow spontaneously from a material at a lower temperature to a material at a higher temperature.“
Lord Kelvin, formulated the Kelvin statement, which states "It is impossible to convert heat completely in a cyclic process." This means that there is no way for one to convert all the energy of a system into work, without losing energy.
The second law of thermodynamics can be
understood through considering these
processes:
Hot pans cool down when taken out from the
stove.
Ice cubes melt in a warm room.
ENTROPY
Entropy just measures the spontaneous
dispersal of energy: or how much energy is
spread out in a process as a function of
temperature.
Entropy a measure of disorder in the physical system.
the second law of thermodynamics – the universe, or in any isolated system, the degree of disorder (entropy) can only increase.
the movement towards a disordered state is a spontaneous process.
Entropy = “ energy dispersed”/ T
Entropy couldn't be expressed without the
inclusion of absolute temperature.
Entropy change ΔS shows us exactly how
important to a system is a dispersion of a
given amount of energy.
THIRD LAW OF THERMODYNAMICS
“It is impossible to reach a temperature of
absolute zero.”
On the Kelvin Temperature Scale,
T = 0 Kis often referred to as
“Absolute Zero”
THIRD LAW OF THERMODYNAMICS
The entropy of a true equilibrium state of a system at T = 0 K is zero.”
(Strictly speaking, this is true only if the quantum mechanical ground state is non-degenerate. If it is
degenerate, the entropy at T = 0 K is a small constant, not 0!)
This is Equivalent to:“It is impossible to reduce the temperature of a system to
T = 0 K using a finite number of processes.”
you can pump heat out of a refrigerator (to
make ice cubes), but the heat is placed in
the house and the entropy of the house
increases, even though the local entropy of
the ice cube tray decreases.
“Energy can neither be created nor
destroyed. It can only be changed from
one form to another.”
• The 1st Law of Thermodynamics is
Conservation of Total Energy!!!!
• It says nothing about
The Direction of Energy Transfer!
Rudolf Clausius, 1850
Summary
VARIOUS STATEMENTS OF THE SECOND LAW
1. “No series of processes is possible whose
sole result is the absorption of heat from a
thermal reservoir and the complete
conversion of this energy to work.” That is
There are no perfect engines!
2. “All processes occuring in nature are
spontaneous and therefore irreversible
unless interfered by some external agency.”
3. “It is impossible to transfer heat from a cold to
hot reservoir without at the same time
converting a certain amount of work to heat.”-
4. By Clausius.
3. “It is impossible to extract an amount of heat QH
from a hot reservoir and use it all to do work W.
Some amount of heat QC must be exhausted to
a cold reservoir.”
The Kelvin-Planck
THIRD LAW OF THERMODYNAMICS
“The entropy of a true equilibrium state of a
system at T = 0 K is zero.”
The entropy of a perfect crystal of every
element or compound is zero at 0 K.
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