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Ammonia forced to boil because H2 is at 200psi ?

  1. Jun 24, 2013 #1
    Comes now the web page


    which says that the ammonia-based absorption-cycle refrigerators ==do== operate only because droplets of liquid ammonia dripping into the ambient of the evaporator (= 200psi of pure hydrogen gas) can only continue to exist (as a liquid) if there would ALSO arise na ammonia-vapor-pressure of the same 200psi.

    (which can never happen, so all of the ammonia ==doesl== evaporate, trying to get there)

    and the webpage further says that all of this is due to the truth of Dalton's Law.

    However comma, I can't find anywhere else, statements about Dalton's Law which say that the pure-ammonia-liquid-droplets, are even able to sense what the hydrogen-pressure is; much less think that that ought to equilibrate to it.

    What am I missing?
  2. jcsd
  3. Jun 24, 2013 #2

    D H

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    You are rejecting the concept of pressure.

    Suppose you go out into a hot, dry desert. A very, very dry desert, one with 0% relative humidity. Pour some water in a pan. Does the water instantly come to a boil because the H2O partial pressure is essentially vacuum? Of course not. It might evaporate at a hefty rate, but it won't boil. That's because it's total pressure rather than the H2O partial pressure that dictates the boiling point.

    The exact same concept applies to that ammonia.
  4. Jun 24, 2013 #3


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    I am not sure what they are telling on that web page but it is known that the vapour pressure rises with increasing total pressure. To vapourise a drop of ammonia you need some more or less fixed amount of energy ΔH proportional to the size of the drop V. However the hydrogen gas will gain an energy pV which increases with increasing pressure.
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