If attempting the electrolysis of liquid ammonia using NaNH2 as an electrolyte. It is understood that Na+ will form solvated electrons. I understand that these solvated electrons can dramatically increase the conductivity of the solution. Will the movement of these electrons aid the production of H2 and N2 in electrolysis. Or will these electrons travel directly between the electrodes effectively short circuiting the ammonia and hampering the electrolysis.
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5$\begingroup$ Na+ can't and won't form solvated electrons, for it has no extra electrons to spare. Metallic Na is another story. $\endgroup$– Ivan NeretinCommented Feb 23 at 13:09
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1$\begingroup$ To get solvated electrons in solution, it is necessary to dissolve metallic sodium in liquid ammonia, and to be quick, because the solvated electrons will only exist before the following reaction : $$\ce{2Na + 2NH3 -> 2NaNH2 + H2}$$ or $$\ce{2 e^- + 2 NH3 -> 2 NH2^- + H2}$$ $\endgroup$– MauriceCommented Feb 23 at 20:03
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1 Answer
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Solvated electrons do not always short-circuit the electrolytic process. In some processes the solvated electrons accumulate in the ammoniacal medium instead, particularly if the anode which supplies the electrons in the first place is made of a soluble metal. As electrons are removed from the metal anode, the metal ions dissolve and intermingle with the electrons that accumulate in the ammonia. In effect we are observing what would ordinarily be an oxidation half-reaction of the anodic metal.
In many cases this metal is magnesium. As this answer describes, magnesium is difficult to dissolve directly into liquid ammonia because it tends to passivate with the formation of a poorly soluble amide, and the magnesium ions are sufficiently Lewis-acidic to solvolyze with the resulting ammonium ions then destroying the solvated electrons. With an electrolytic process we surmount these barriers. Pumping the electrons out of a magnesium anode promotes the otherwise hindered metal dissolution, and the supporting electrolyte stabilizes the magnesium ions against reacting with the solvated electrons (apparently by forming ion pairs). See Combellas et al1.
Reference
- C. Combellas, F. Kanoufi, A. Thiébault, "Solutions of solvated electrons in liquid ammonia: Part 1. Chemical properties of magnesium solutions", Journal of Electroanalytical Chemistry , 2001 499, 1, Pages 144-151. ISSN 1572-6657, https://doi.com/10.1016/S0022-0728(00)00504-0
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$\begingroup$ I think even inert metal electrodes in an ammonia solution containing an electrolyte (say LiCl) will release free electrons into the ammonia solution. Whether this eventually causes a short circuit is unclear, but the effect is clearly visible as the solution near the electrode releasing electrons goes blue. $\endgroup$ Commented Feb 23 at 23:19
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$\begingroup$ @matt_black note I am saying the anode dissolves. The electrons pass into the solution from the cathode which, in an electrolytic generation of solvated electrons, would be inert. $\endgroup$ Commented Feb 24 at 1:28