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Question on entropy in adiabatic phase change

  1. Apr 11, 2016 #1
    1. The problem statement, all variables and given/known data

    Consider a closed, adiabatic system consisting of a mixture of liquid and solid substance Z at equilibrium at its melting point.

    Z (solid) <---------> Z (liquid)

    Which of the following statements is true regarding the system?

    A) The entropy of the system is at a maximum
    B) The entropy of the system is at a minimum
    C) The entropy of the system will increase over time.
    D) The entropy of the system is zero
    E) the entropy of pure substances does not change if at a constant temperature.

    2. Relevant equations

    none
    3. The attempt at a solution

    I put down my answer as being E. The entropy change for a reversible process is zero according to the second law of thermodynamics, so because my temperature is the same while melting, this answer choice makes sense.

    However, my review book gave me A as being the correct explanation and then did not bother to explain this at all to me why the entropy of the system is at a maximum.

    Could anyone please explain why this is the case and share their thoughts with me?

    Thanks!
     
  2. jcsd
  3. Apr 12, 2016 #2

    Ygggdrasil

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    You are correct that at equilibrium, the change in entropy over time is zero. From your knowledge of calculus, if dS/dt = 0, what does that tell you about the entropy (S) of the system?
     
  4. Apr 12, 2016 #3
    Well it could be at a max or at a min. I could take the second derivative, but that is just complicating things.

    So I can choose between A or B now. Thoughts?
     
  5. Apr 12, 2016 #4

    Ygggdrasil

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    What do the laws of thermodynamics say?
     
  6. Apr 14, 2016 #5
    Entropy of universe always increases for a spontaneous process except for equilibrium. In that case delta S = 0.
     
  7. Apr 14, 2016 #6

    Ygggdrasil

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    Right, so if entropy always increases until the system reaches equilibrium, then equilibrium represents a...
     
  8. Apr 14, 2016 #7
    Maximum. Right?
     
  9. Apr 14, 2016 #8

    Ygggdrasil

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    Yes. Reactions will always seek to increase the entropy of the universe until it reaches the point where the entropy is at its maximum and can't increase any more. At this point, the system is at equilibrium.
     
  10. Apr 14, 2016 #9
    Aha that makes sense! Thanks!
     
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