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J o m a oa ~ CHEMICAL EDUCATION
OCTOBER, 1925
LIQUID AMMONIA AS A SOLVENT* R. H. BULLILRD, HOBART COLLEGE, GENEVA,N. Y.
The chemistry with which we are ordinarily acquainted is almost exclusively the chemistry of aqueous solutions. We speak of a substance as being "soluble" or "insoluble" and in so saying we refer to water as the solvent. If the substance is soluble we treat i t with some other soluble suhstance obtaining perhaps a precipitate. It is these reactions which go to make up a large part of our ordinary chemistry. For example, an aqueous solution of sodium. iodide is added to a solution of silver nitrate and we obtain the yellow precipitate of silver iodide. And so accordingly silver iodide is classed as an insoluble substance and we carry out almost no further reactions with it, certainly no metathetic reactions. But in liquid ammonia silver iodide is one of our soluhlk salts and is dissociated into its constituent ions. If this solution is now treated with a solution of barium nitrate we obtain a precipitate of barium iodide. By such metathetic reactions we can obtain, with ammonia as a solvent, such substances as aluminium sulfide, chromium sulfide, and many others unobtainable by use of aqueous solution. In water we have no such thing as a solution of a pure metal. Any solvent action which water may have on a metal is a case of reaction and not of simple solution. We all know what happens when we put sodium in water. But in liquid ammonia we can obtain solutions of such metals i s sodium, potassium, barium, caltium, and the like. These are true solutions and not cases of reactions, for oa evaporation of the ammonia the metal is recovered in its original state. The alkali metals are very soluble, sodium dissolving to the extent of about 168 grams in a liter of ammonia while potassium dissolves to the extent of about 320 grams per liter. The alkaline earth metals are not soluble to such a considerable extent but they are, however, appreciably soluble. Now when the metal is dissolved i t ionizes into the positive metallic ion and the negative electron. For example : Na--+NaC
+ e-.
Sodium ion is of course colorless, the blue color of the solution being due to the negative electron. This has been proven by electrolysis of the solution in which the positive sodium ion goes to the cathode and the negative electron to the anode. Now solutions of these metals give us valuable reagents. They are, of course, excellent reducing agents since the electron is so readily available, and these solutions are now used extensively for reduction reactions in organic chemistry. Further, these metals will replace oth& more electronegative metals from solution, as for example: * Paper read before the New England Association of Chemistry Teachers at Brown University, March 14, 1925.
VOL. 2, No. 10
LIQUIDAMMONIAAS Na
A
SOLVB~
925
+ AgI --+ NaI + Ag
whereby the silver is precipitated out in a finely divided form. Now the question naturally arises, do we have acids and bases in liquid ammonia solution? In the water system, water ionizes into positive hydrogen and negative hydroxyl ions. Substances giving hvdrogen ions we call acids and those giving hydroxyl ions we call bases. And so we say that acids are substances which, when in solution, yield positive ions in common with the solvent, and bases are substances which yield negative ions in common with the solvent. Now these ions are more or less solvated and the complex formed may be more or less stable depending on the case. Ammonia will ionize, only slightly to be sure, according to the equation: HNH,
3 H + fNHs-.
Hence we see that acids in liquid ammonia are substances yielding NHl+ ions (the solvated or ammonated hydrogen ion) and bases are substances yielding NH2- ions. That is, NHnC1, NHaNOa, or any ammonium salt is an acid in liquid ammonia and is called an ammono acid. KNH*, NaNHz, or any amide is then an ammono base. The following will illustrate the action of ammonium salts as acids: One of the typical reactions of an acid in water solution is its action on metals forming the salt of the metal and liberating hydrogen. In ammonia solution ammonium salts will attack metals in the same manner. N H C l added to our solution of sodium will form sodium chloride and liberate hydrogen according to the equation: 9 2NH.CI
+ 2Na -+2NaCI + H1+ 2NHa.
Magnesium, zinc, and the other more electropositive metals are acted on similarly, only of course not as rapidly, since these, unlike sodium, are insoluble in liquid ammonia. By the use of liquid ammonia as a solvent we have come to recognize the presence of metals in the electronegative condition. For example, we have the salt NarPbgwhich when dissolved in ammonia ionizes according to the equation: Na4Pbr --t 4Na+ f Pbo----, where the Pb9 ion is a complex negative ion analogous to our complex sulfide S, ion. Now if a solution of this salt is treated with a solution of a lead salt as PbClz where we have lead as a positive ion Pb++ we have the reaction: NadPbn
+ 2PbCIe +4NaCI + PbZbe
and ordinary metallic lead is precipitated out. Here a positive lead ion has combined with a negative lead ion, or a t least they have neutralized each other, forming free metallic lead. The reason for our not having
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JOURNALCHEMICAL EDUCATION
OCTOBER, 1925
recognized these salts of metals in the negative conditions is that they do not exist in aqueous solutions. The following reactions will illustrate this. I n water solution we have metallic tin reacting with potassium hydroxide according to the equation: Now just what is the mechanism of this reaction? Certainly potassium hydroxide is not acting here as an acid. By looking into the analogous reaction in liquid ammonia we can obtain an intelligent explanation of the above reaction. I n liquid ammonia we have stated that KNH2 is a base. If we treat tin with our base we have the following reaction: The amphoteric hase Sn(NH& will further react with KNH2 to form an ammono salt. Now let us consider our reaction in water in the light of this reaction. The first reaction of tin with the hase will be: 3Sn
+ 4KOH +K6Sn + 2Sn(OH)..
The amphoteric base Sn(0H)z will react with KOH to form a salt: Sn(OH)a f 2KOH
But our K4Sn in water is not stable. equation: K,Sn
+Sn(0K). + 2H20. It reacts with water according to the
+ 4HOH +.4KOH + H,Sn ~s
and HISn then decomposes according to the equation: On summing up the above series of equations it is seen that the resulting equation is: Sn f 2KOH +Sn(OKh Hx.
+
Thus we see that the chemistry of liquid ammonia solutions is in reality exceedingly extensive, and the use of ammonia as a solvent has given us a deeper insight into the chemistry of many substances and reactions than was possible before its use. General Organic Chemistry Symposium. Further arrangements are being made for the general symposium on organic chemistry to be held a t Rochester, N. Y., on December 29, 30. and 31, 1925. The program should prove most interesting and helpful. The following people will attend the meeting and have consented t o present papers: R. R. Renshaw, Morris Kharasch. Harry L. Fisher, Homer B. Adkins, B. Johnson, Treat B. Johnson, W. J. Hale, E. H. Volwiler, A. W. Browne, L. E. Wise, Roger Adams, Franli C. Whitmore, R. J. Anderson, W. Lee Lewis, James F. Norris, A. J. Hill, W. L. Evans, M. T . Bogert, J. B. Conant, John Johnston, and Charles H. Herty.
