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Solvated electrons in ammonia solution

  1. Jan 18, 2010 #1
    Ammonia solution turns blue when alkali and alkaline earth metals are dissolved in it. Why is the blue colour common to all? Is it because of the dissolution of electrons that is common in all cases?
     
  2. jcsd
  3. Jan 19, 2010 #2

    Borek

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    Yes - they all behave similarly.

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  4. Jan 20, 2010 #3
  5. Jan 20, 2010 #4

    Borek

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    No, that was the answer.

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  6. Jan 21, 2010 #5
    Sorry, but what I meant was why do they all behave similarly?
     
  7. Jan 21, 2010 #6

    Borek

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    They have similar characteristic - low ionisation energy and after electron(s) are removed you are left with small cation, that easily attracts polar molecules. Not to mention fact that these small cations have filled all other orbitals, so there is no further fancy chemistry possible as is in the case of transition metals.

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  8. Jan 21, 2010 #7
    Thanks.
     
  9. Jan 21, 2010 #8
    Do you mean liquid ammonia, or a solution of alcali metals in ammonia, not "ammonia solutions".
    The reason is what Borek wrote: all those metals release an electron and the blue color is due to this ammonia-solvated electron, so the nature of the metal is not important anylonger.
     
  10. Jan 21, 2010 #9
    I meant liquid ammonia solution but pardon me, what's the difference?
     
  11. Jan 22, 2010 #10

    Borek

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    Ammonia solutions implies ammonia dissolved in water. What happens when you mix alkali metals with water?

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  12. Jan 22, 2010 #11
    You wrote:
    "Ammonia solution turns blue when alkali and alkaline earth metals are dissolved in it".
    It seems that you start from ammonia solutions of something and then you add the metals. I would instead have written: "ammonia solutions of alkali and alkaline earth metals..."
     
  13. Feb 4, 2010 #12
    Here's a a question to which I've been wondering the answer: why do electrons solvated in ammonia not react, whereas electrons solvated in water, even under basic conditions, DO react to form H2?
     
  14. Feb 4, 2010 #13

    chemisttree

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    Electrons solvated in ammonia DO react to produce hydrogen. It forms sodium amide. It does it slowly if the ammonia is extremely cold, however. Most 1 electron reductions (the most common use of sodium in ammonia) occur much more rapidly than the reaction of the electron with a hydrogen on ammonia at those temperatures. I use the stuff at -78oC when I have a need for it. And I generate it just before I need it. It has no shelf life.
     
  15. Feb 4, 2010 #14
    Are you saying that the reaction is reasonably fast under ambient conditions? If that were the case, then the blue color would start to disappear, right? I haven't dealt with the stuff.

    Barring the reaction with ammonia itself, why don't the solvated electrons react with each other to form H2, as in water (this was actually the question I was asking)? Is solvent reorganization too energetically costly (during reaction) in ammonia? I wonder if I can't find the solvation enthalpy for electrons in ammonia. . .

    What kind of shelf life are we talking? Minutes?
     
  16. Feb 5, 2010 #15

    DrDu

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    The solvation process of the electron both in water and in ammonia has been the topic of extensive research with femtosecond spectroscopy in the last decades. For example, the first stages of solvation are known as "wet electron".
     
  17. Feb 5, 2010 #16
    What does it mean? Electrons (solvated) reacting with each other? In case, electrons react with the solvent.
     
  18. Feb 5, 2010 #17

    chemisttree

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    At ambient temperature ammonia is a gas of course. It boils at something like -33C so solutions of sodium in ammonia are always pretty cold. The reaction of the sodium with the hydrogens of ammonia is exactly how sodium amide is made. I don't know the kinetics but whenever I use sodium in ammonia to reduce something, I always use it in a bit of excess to account for its decomposition to sodium amide, reaction with stuff on the glassware, stuff in the ammonia, stuff in my reaction not leading to product, etc... Just how much excess is dependent on the rate of reaction as well. If something were difficult to reduce, I would add a little more excess by trial and error.

    Of course I've never done one of these reactions on a large scale... usually 50-100 mg scale.
     
  19. Feb 26, 2010 #18
    There's something else that bothers me. The blue ammonia-alkali metal solutions are paramagnetic whereas the bronze ones are diamagnetic. Can anybody explain?
    Thanks in advance.
     
  20. May 31, 2010 #19
    I have a quick question someone, anyone...Is it possible for one to use ammonia gas, instead of liquid (i know about condensing the gas into dry ice container below temp -33 but the stuff smells awful! i live and share an apartment complex :( )? Lets say one was to put the ammonia gas into some kind of liquid absorbent it is soluble with and adding in the lithium to the solvent?
     
  21. May 31, 2010 #20

    Borek

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    No matter if you will work with liquid or gaseous ammonia, it will stink enough for your neighbors to call the police.
     
  22. Jun 4, 2010 #21
    I don't even want to know why you want to perform a Birch reduction in your apartment.

    No prizes for guessing, though.
     
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