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I Question about enthalpy

  1. May 12, 2018 #1
    Why is change of enthalpy equal to m L during a phase change?
    H=U+PV
    dH=dU+pdV+Vdp
    If temperature and pressure is unchanged during a phase change,
    dH=pdV, how does it lead to m L ? Thank you
     
  2. jcsd
  3. May 13, 2018 #2

    kith

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    Science Advisor

    How is L defined?
    Correct.
    That's not correct. How would you write dU in terms of temperature and pressure?
     
  4. May 13, 2018 #3
    dH= dU+pdV+Vdp
    dH= dQ-pdV+pdV+Vdp
    dH=dQ+Vdp
    Is the pressure unchange?
    L is defined as the energy need to vapourize/fuse 1kg of substance.
     
  5. May 13, 2018 #4

    kith

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    Science Advisor

    Yes, that's better. (dQ could also be written as dQ=TdS where dS is the change in entropy)

    In your first post, you stated that temperature and pressure should remain unchanged. This means that the change in pressure, dp, is equal to zero.

    Therefore, dH = dQ.
    Yes: L = Q/m (as a convention, pressure is assumed to stay constant throughout the phase change)
     
    Last edited: May 13, 2018
  6. May 13, 2018 #5
    Thanks for the explanation
    dH=dQ
    so if I integrate it, I can get
    ΔH=Q which is L for 1kg of substance
     
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