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I What will happen in this case -- 2 chambers connected by a tube...

  1. Dec 8, 2016 #1
    I want to put an imaginary experiment here. Suppose, there are two enclosed chambers side by side. One have saturated water-Ammonia solution in it and the other one is empty. The blank space of the two chambers are filled with Ammonia Vapour and both are connected by means of a tube. The temperature of the both chamber and its contents are 27°C and both are at same pressure level. Now, some liquid Ammonia has been sprayed inside the empty chamber. Both the chambers are well insulated. What will happen next?
    My knowledge of physics tell me that the Ammonia inside the empty chamber will quickly become cold and it will lower both the temperature and pressure inside the empty chamber and that will draw Ammonia out of the solution from the first chamber. This process will continue until all the liquid Ammonia in the second chamber will be evaporated. Am I right?
  2. jcsd
  3. Dec 8, 2016 #2
    Hmmm ... most of the parts sound right, but I'm not sure of the final result. The liquid ammonia is certainly cold, and in evaporating it certainly uses a lot of the heat energy, so I believe that chamber will get colder. However the evaporation also greatly increases the concentration of ammonia vapor, so I'm not sure the pressure has to drop. I don't know what the initial pressure was, but it was certainly below the vapor pressure of liquid ammonia at the initial temperature. If we are assuming it was in equilibrium with the solution then it was far below the vapor pressure of liquid ammonia at the initial temperature. Vapor pressure doesn't change too rapidly with temperature, so it seems likely even at a reduced temperature the vapor pressure of the liquid ammonia will be higher than the pressure you start with. (Not necessarily, but just guessing based on the description.) Also depending on how much liquid ammonia you add, the process doesn't have to continue until all the liquid evaporates. What is more likely is that the pressure will increase and the temperature will drop until the vapor pressure which drops slowly with the temperature and the rising pressure reach the same value. At that point the ammonia vapor at the new temperature is in equilibrium with the liquid ammonia. That box now has a lower temperature, but a higher pressure of ammonia vapor. Ammonia goes the other direction through the tube until the temperatures and pressures equilibrate. As the vapor pressure of ammonia above liquid ammonia is much higher even at the lower temperature than the vapor pressure of ammonia above the solution, more ammonia goes into solution.
  4. Dec 9, 2016 #3
    Thanks for replying. But, at least we can say that if the temperature of the liquid Ammonia will be below its boiling point, both the temperature and pressure will fall until and unless the liquid Ammonia will come to its boiling point, right?
  5. Dec 9, 2016 #4
    The temperature will fall. The pressure doesn't necessarily fall.

    Ignoring the tube for a moment, if you add enough liquid ammonia at equilibrium the system will be at a lower temperature (determined by how much liquid ammonia and how cold and how much evaporated) and the pressure will be the vapor pressure at the new temperature. The vapor pressure at the lower temperature is lower than the vapor pressure at the higher temperature. However the initial pressure was below the vapor pressure at the initial temperature (otherwise there would already have been a puddle of ammonia). There is nothing to say that the initial pressure was not lower than the final vapor pressure. How can the pressure go up when the temperature is going down you may ask? In the ideal gas law the evaporation is adding to n, so nRT can be going up or down.
  6. Dec 9, 2016 #5
    But, kindly think of the scenario. The hot Ammonia gas will enter in the second chamber and is turning cold and that means reduction in pressure too because the system is enclosed. That means drawing more Ammonia from the solution and chamber, right?
  7. Dec 9, 2016 #6
    I can only repeat what I have said before. The pressure may well go up. You are focused on the temperature and the idea that that will lower pressure, but you must remember you are adding ammonia vapor from the evaporation.
  8. Dec 9, 2016 #7
    You haven't noticed that I have told "until the temperature of the liquid Ammonia will reach the boiling point".
  9. Dec 9, 2016 #8
    I'm sorry, pranj. I don't think I've been very helpful. I'm afraid we are talking in circles. Perhaps someone else would like to jump in and talk this through with you?
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