Entropy

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Entropy

• Chapter 8

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• The important point is that since entropy is a
  property, the change in the entropy of a
  substance in going from one state to another is
  the same for all processes, both reversible and
  irreversible, between these two states.
• From the third law of thermodynamics, which is
  based on observations of low-temperature chemical
  reactions, it is concluded that the entropy of
  all pure substances (in the appropriate
  structural form) can be assigned the absolute
  value of zero at the absolute zero of
  temperature. It also follows from the subject of
  statistical thermodynamics that all pure
  substances in the (hypothetical) ideal-gas state
  at absolute zero temperature have zero entropy.
• However, when there is no change of composition,
  as would occur in a chemical reaction, for
  example, it is quite adequate to give values of
  entropy relative to some arbitrarily selected
  reference state, such as was done earlier when
  tabulating values of internal energy and
  enthalpy. In each case, whatever reference value
  is chosen, it will cancel out when the change of
  property is calculated between any two states.

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8.3 The Entropy of Pure Substance

• In the steam tables the entropy of saturated
  liquid at 0.01C is given the value of zero.
• For many refrigerants, the entropy of saturated
  liquid at 400C is assigned the value of zero.
• 1/T serves as the integrating factor in
  converting the inexact differential dQ to the
  exact differential dQ/T for a reversible process.

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8.4 ENTROPY CHANGE IN REVERSIBLE PROCESSES
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Net work
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Eq.6.13
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( Gibbs equations )
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8.7 ENTROPY GENERATION
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Some Important Conclusions

• There are two ways in which the entropy of a
  system can be increasedby transferring heat to
  it and by having an irreversible process.
• Since the entropy generation cannot be less than
  zero, there is only one way in which the entropy
  of a system can be decreased, and that is to
  transfer heat from the system.
• For an adiabatic process, dQ 0, and therefore
  the increase in entropy is always associated with
  the irreversibilities.
• Finally, the presence of irreversibilities will
  cause the work to be smaller than the reversible
  work. This means less work out in an expansion
  process and more work into the control mass (dW
  lt0) in a compression process.

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• In fact, in many situations we are not certain
  of the exact state through which a system
  passes when it undergoes an irreversible
  process.
• The work for an irreversible process (fig.
  8.11a) is not equal to P dV, and the heat
  transfer is not equal to T dS.
• Therefore, the area underneath the path does not
  represent work and heat on the PV and T S
  diagrams, respectively.

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8.8
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For the control mass
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c.m.2
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• Thus we conclude that the net entropy change is
  the sum of a number of terms, each of which is
  positive, due to a specific cause of irreversible
  entropy generation, such that the net entropy
  change could also be termed the total entropy
  generation dSnet dScm dSsurr Sd Sgen
  0 --------- (8.16) where the equality holds
  for reversible processes and the inequality for
  irreversible processes.

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8.9 ENTROPY CHANGE OF A SOLID OR LIQUID
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8.10 ENTROPY CHANGE OF AN IDEAL GAS
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