This document discusses entropy and the laws of thermodynamics. It covers:
1) How entropy changes during different thermodynamic processes like constant volume, pressure, temperature, and adiabatic processes.
2) How entropy changes for different phases of pure substances like ice heating and melting.
3) The third law of thermodynamics which states that the entropy of a pure crystalline substance is zero at absolute zero temperature, providing an absolute reference point for determining entropy.
2. TopicTopic
Entropy Change During
Thermodynamic Process
Entropy Change For Pure Substances
Third Law Of Thermodynamics (Nernst
Law)
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3. Entropy Change DuringEntropy Change During
Thermodynamic ProcessThermodynamic Process
Let m Kg of gas at a pressure P , volumeV ,₁ ₁
absolute temperature T and entropy S , be₁ ₁
heated by any thermodynamic process.
Its final pressure, volume, temperature and
entropy are P ,V ,T and S respectively.₂ ₂ ₂ ₂
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4. Law Of Conservation Of EnergyLaw Of Conservation Of Energy
Q = dU + W,δ δ
Where,
Q = small change in heatδ
dU = small change in internal energy
W = small change of work doneδ
dT = small change in temperature
dv = small change in volume
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8. Change Of Entropy Of DifferentChange Of Entropy Of Different
ProcessesProcesses
1. Constant Volume Process
2. Constant Pressure Process
3. Constant Temperature Process
4. Reversible Adiabatic Process
5. Irreversible Adiabatic Process
6. Polytropic Process
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9. Heating A Gas At ConstantHeating A Gas At Constant
Volume ProcessVolume Process
Consider a m Kg of perfect gas being
heated at a constant volume process.
Heat supplied at constant volume
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11. Heating A Gas At ConstantHeating A Gas At Constant
PressurePressure
Consider a m Kg of perfect gas being
heated at a constant Pressure process.
Heat supplied at constant Pressure
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13. Isothermal Process Or ConstantIsothermal Process Or Constant
Temperature ProcessTemperature Process
Consider a m Kg of perfect gas being
heated at a constant temperature during
expansion process.
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15. Reversible Adiabatic ProcessReversible Adiabatic Process
In the adiabatic process, heat is neither supplied to system or
rejected by the system.
This shows that the change of entropy during a reversible
adiabatic process is zero, the T-S graph is shown by line 1-2.
The entropy of the gas remains constant during reversible
adiabatic expansion or compression of the gas, this process is
said to be isentropic process.
It is frictionless adiabatic process.
In this, change in internal energy is equal to the work done by
the gas during expansion .
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17. Irreversible Adiabatic ProcessIrreversible Adiabatic Process
If adiabatic process with internal friction, it is known as irreversible adiabatic
process.
If the irreversible adiabatic expansion process takes place between same
temperature limits T₁ and T₂ then due to internal friction, the internal energy of
the gas at the end of expansion will be more than at point 2 of reversible process.
Suppose, δQ’ heat gain by gas due to internal friction then the increase of
entropy will be given by,
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18. It means that higher entropy at the end of expansion of
irreversible process, therefore work done by gas will be
less than that of reversible process.
Mathematically,
U₁ - U₂ = W, dS = 0 for reversible adiabatic process
U₁ - U’₂ = W’ dS > 0 for irreversible adiabatic process
Where,
U’₂ = internal energy of gas at the end of irreversible
expansion
U₂ = internal energy of gas at the end of reversible
expansion
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27. Entropy Change For PureEntropy Change For Pure
SubstancesSubstances
Consider (M) Kg of ice is
heated continuously at
constant atmospheric
pressure.
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29. T₁ = initial temperature of ice
T₂ = melting temperature of ice = 0C =
273 K
T = boiling temperature of water =100C
T = superheated steam temperatureᵤ
Cᵨ = specific heat of iceᵢ
Cᵨ = specific heat of waterᵥᵥ
Cᵨ = specific heat of steam
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30. Process 1-2 Sensible Heating Of IceProcess 1-2 Sensible Heating Of Ice
Temperature of ice increase from T to₁
T . Change of entropy during 1-2₂
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31. Process 2-3 Melting Of IceProcess 2-3 Melting Of Ice
On further heating of ice is converted
into water at constant temperature T₂
(0C).
The heat supplied is utilized to change
phase called latent heat of fusion of ice
(h ).ᵢ
Therefore Q = mhᵢ
Change of entropy during process
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32. Process 3-4 Sensible Heating OfProcess 3-4 Sensible Heating Of
WaterWater
Water from T is heated to water at T₂ .
Change of entropy during process
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33. Process 4-5 Boiling Of WaterProcess 4-5 Boiling Of Water
On further heating of water, is
converted into steam at constant
temperature T .
The heat supplied is utilized to
change phase called latent heat of
evaporation.
Change of entropy during process.
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34. Process 5-6 Sensible Heating OfProcess 5-6 Sensible Heating Of
SteamSteam
Temperature of steam increases from Ts
to Tsup .
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35. Third Law Of ThermodynamicsThird Law Of Thermodynamics
(Nernst Law)(Nernst Law)
Entropy is a measure of molecular randomness
At absolute zero temperature molecules
become completely motionless.
The entropy of a pure crystalline substance at
absolute zero temperature is zero since there is
no uncertainly about the state of the molecules at
that instant.
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36. This is third law of thermodynamics which states that
“ The entropy of all perfect crystalline substance
(Solid) is zero at absolute zero temperature”.
In equilibrium crystalline state, its atoms are arranged in a
pattern that represents the maximum degree of order, and
if it also at absolute zero temperature there must be a
minimum of disordering thermal motion.
Mathematically,
The third law of thermodynamics provides an absolute
reference point for the determination of entropy.
The entropy determined relative to this reference point
is called absolute entropy.
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37. ApplicationApplication
1. Provides an absolute reference point for the
determination of entropy.
2. Explaining the behaviour of solids at very low
temperature.
3. Measurement of action of chemical forces of the
reacting substances.
4. Analysing the chemical and phase equilibrium.
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