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Using Endothermic Solvation To Assist Distillation

  1. Sep 4, 2010 #1
    From time to time I find myself dissolving potassium nitrate in water (endothermic solvation) and a while back i considered that if i was boil a saturated solution of potassium nitrate that it would be easier than boiling distilled water; as evaporation would have to be exothermic, which would assist boiling.

    If distilling sea water, i would consider that it would more efficient energy wise to saturate the solution with potassium nitrate and then distill. Compared to just simply distilling the sea water.

    Distillation is not a practical method of purifying water, considering the existence of reverse osmosis technology. Distillation is still used however as the primary method of obtaining ethanol, from a fermented solution.

    Energy efficiency is important in in this process, as much of the ethanol produced these days is for renewable energy purposes.

    Potassium nitrate does not dissolve in ethanol, but other compounds with the same endothermic solvation property might be able to be used. Does anyone know of such compounds and consider that this concept may have merit?
  2. jcsd
  3. Sep 4, 2010 #2


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    Staff: Mentor

    Water evaporation would be not exothermic, and amount of energy required to evaporate water would not change much. But solution will boil at higher temperature than pure water (google boiling point elevation).
  4. Sep 4, 2010 #3
    I could understand that if it were an exothermic solvation, like NaCl.

    As your absorbing energy from the solvation of potassium nitrate, conservation of energy would require it to be liberated when it returns to a solid, otherwise i would consider you would be destroying energy.
  5. Sep 5, 2010 #4


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    Staff: Mentor

    This all happens, but it has nothing to do with the enthalpy of vaporization.
  6. Sep 5, 2010 #5


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    I remember getting this question in high school chemistry. "If it requires heat to dissolve ammonium nitrate, why does it dissolve?" The answer to this is the answer to why your suggestion doesn't work. Because you're conflating an endothermic reaction with an endergonic one. If the solvation of ammonium (or potassium) nitrate required energy, it would not dissolve spontaneously. That's basic thermodynamics - things don't spontaneously go to a higher energy. It dissolves because solvation lowers the total energy. Since that energy's obviously not coming from enthalpy (heat), it's coming from entropy. It requires heat to dissolve, but more than compensates for this in increased entropy.

    If you want to re-crystallize it, then you have to use at least as much energy as the change in entropy - which was more work than it gained from the heat. If you intend to do this at at a higher temperature (the boiling point) you'll have to put in even more energy, since entropy is temperature dependent. (About a difference of 60 C, means around 60/300 = 20% more energy, assuming constant deltaH and deltaS)

    Of course if you don't believe me, just try it yourself. Put two beakers of water (or water-ethanol, if you like) on identical stoves, saturate one with KNO3, and see which one evaporates faster.
    Last edited: Sep 5, 2010
  7. Sep 5, 2010 #6
    using ethanol in such an experiment would be a waste of time, because KNO3 doesn't dissolve in it. From what you both have posted i would consider that:

    (a) dissolving KNO3 in water, reduces the temperature of the solution.
    (b) heat from the environment returns the solution to its original temperature.
    (c) when evaporated heat energy is released as the KNO3 precipitates, this realease of energy does assist evaporation, but the fact that one is evaporating KNO3 solution you require more energy to evaporate the solution than you would pure water.
    (d) the hindrance of the need for more energy is greater than the assistance of the energy released and no advantage exists.

    Why does it always work out this way, or is it just in this instance?
    Last edited: Sep 5, 2010
  8. Sep 6, 2010 #7
    have a look at my thread " solvation heat pump" its more thermodynamics based.
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