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Simple question about ammonia/water solutions

  1. Jun 30, 2011 #1
    When ammonia dissolves in water some of it reacts to form ammonium and hydroxide ions:

    NH3 + H2O = NH4+ + OH-

    http://scifun.chem.wisc.edu/chemweek/pdf/ammonia.pdf" [Broken]

    What happens if you then heat or boil the solution, driving off the ammonia? Does all the NH4+ turn back into NH3?

    Just something I was wondering about, I can't seem to find the answer on Google.

    Thanks.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jun 30, 2011 #2
    The first problem you would have is separating the water and ammonia that didn't react from your desired products. I can't think of any way to do that which wouldn't encourage them to react back to water and ammonia (i.e. time, mixing).

    Nitrogen prefers 3 bonds and oxygen 2. Therefore, the amount of formation of the NH4+ and OH- is going to be extremely small so it would not be practical to distill. But if you were able to somehow do this I believe the hydroxyl group would vaporize first because it's lighter (17 vs 18 g/mol). Also, both of the components would probably boil at close to the same temperature (a guess based off molecular weight), making distillation nearly impossible.
     
  4. Jun 30, 2011 #3

    SpectraCat

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    Sorry, but most of that is pretty confused.

    First of all, the OP is exactly right. Ammonia is a gas at RT, so separation of the water and ammonia is trivial ... if you heat the solution in an open container, you will drive off NH3(g), pushing the equilibrium back to the left. So you would expect a temperature-dependent pH for the solution. However, you will never get quite all of the NH4+ converted back to NH3, at least until you boil the solution to dryness. Still, you will probably have some trace ammonium salts left in the residue.

    As a side note, the above phenomenon can have real consequences when storing basic waste products containing ammonia. If the storage area temperature rises, you will build pressure in your waste containers if they are not properly vented, which risks exposing people to the fumes, or in the worst cases, explosion of the containers. On the other hand, proper venting is not straightforward, since the ammonia vapor is highly pungent and must not be vented into a human air supply.

    Second, why would you think it is even theoretically possible to distill ions? What is the vapor pressure of the hydroxide ion in aqueous solution? If you were to "distill" the solution, you would end up boiling off the water until ionic crystals were left. Exactly what ionic compounds would be left depends on the trace ions in the water, but you would probably have a mixture consisting primarily of solid sodium hydroxide and ammonium chloride, since sodium and chloride ions are pretty much ubiquitous.
     
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