Chapter 3 The Laws of Thermodynamics

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Chapter 3The Laws of Thermodynamics

• Notes on
• Thermodynamics in Materials Science
• by
• Robert T. DeHoff
• (McGraw-Hill, 1993).

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The Laws of Thermodynamics

• 0th Law --- There is a temperature scale and
  energy (heat) flows down hill on that scale.
• 1st Law --- Energy is conserved.
• 2nd Law --- Entropy is not conserved. Entropy
  always changes in one direction (increases).
• 3rd Law --- There is an absolute zero
  temperature, and the entropy of all substances is
  the same at absolute zero.

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Temperature Scales
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Energy The ability to do work.Can be converted
or transported.Cannot be created or destroyed.

• Kinetic Energy --- Energy of motion.
• K.E. 1/2 mV2
• Potential Energy --- Energy of position in a
  potential field.
• P.E. mg.h ...
• Internal Energy (U) --- Energy associated with
  the condition of matter, not its motion or
  position.

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First LawEnergy is a property of the universe
which cannot change no matter what process occurs.

• The change in internal energy of a system must be
  equal to the sum of all energy transfers across
  the boundary of the system.
• DU increase in the internal energy of the
  system.
• Q quantity of heat that flows into the system.
• W mechanical work done on the system by
  external pressure exerted by the surroundings.
• W/ all other kinds of work done on the system.
• For a process DU Q W W/
• For an infinitesimal step dU dQ dW dW/

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WorkA process variable. Value depends on path.

• An increment of work (dW) equals an increment of
  motion (dx) of a point on a body under an applied
  force (F).
• dW F.dx
• The work done by a process is integrated over the
  path.

Substituting P F/A and dV -A.dx
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Second Law

• Entropy is a property of the universe which
  always changes in the same direction no matter
  what process occurs.
• DSt, DSt/ entropy transferred across the
  boundary during the process for system
  surroundings, respectively.
• DSp , DSp/ entropy production within the system
  surroundings, respectively, during the process.

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Entropy --- a State Function

• Entropy is transferred across a boundary
  created within the system the surroundings.
• Entropy production results from dissipative
  processes.
• Entropy production increases as the rate of the
  process increases system is further from
  equilibrium.
• Entropy processes carried out infinitesimally
  close to equilibrium, ideally, have zero entropy
  production are reversible.
• Real processes, carried out at finite rates, are
  irreversible, create entropy there is some
  permanent change in the universe.

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3.6 Give five examples of the operation of the
second law of thermodynamics in your daily
experience (different from those given in the
text).

• The morning coffee cools with time.
• Sugar dissolves in hot coffee.
• Left to itself a pendulum will slow and stop.
• Organisms die.
• An expanding gas cools.

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3.6 Why is irreversible so appropriate in its
application to the description of processes in
thermodynamics? Suggest 2 or 3 alternate phrases.

• Incapable of being reversed.
• The reverse process cannot happen.
• Process producing a permanent change.
• Irretraceable.
• All real processes are accompanied by permanent
  change in the universerse. By definition, the
  permanent changes cannot be undone by reversing
  the influences driving the system.

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Combined 1st 2nd Laws

• Applying,
• dWrev -PdV
• and
• dQrev TdS
• then
• dU TdS - PdV dW/

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The Third Law

• There is a lower limit to the temperature that
  can be attained by matter, absolute zero.
• The entropy of all substances at that
  temperature, at absolute zero, is the same.

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Entropy Change for a Reaction
aA bB cC dD
DSO cSOC dSOD - aSOA bSOB
As example 3/2 SiO2 2Al 3/2 Si Al2O3
SO (J/mole-K) 42.09 11.55 18.83
50.99
DSO 3/2 (18.83) (50.99) - 3/2 (42.09) 2
(11.55)
DSO -7.0 J/mole-K