Ammonlum Hydroxlde: What Is Its Structure? TOthe Editor: In a recent note (I)i t is claimed that ammonium hydroxide does not exist. The evidence cited in support of this claim is a lifetime of the ammonia-water complex in liquid aqueous ammonia of 10-1° s. However, even though such a short lifetime is ohviously indicative of a highly labile complex, in itself the short lifetime tells us nothing about the thermodynamic stability of the complex. I t is this latter property that forms the proper basis for deciding whether a chemical compound exists.Fortunately, relevant thermodynamic data are available (2) for the reaction H,0(1) + NH,(l,sat)
-
NH,OH(I)
(1)
For this reaction a t 25 "C, AG, = -6.39 kJ/mol, which indicates that ammonium hdroxide indeed does exist, althoueh what its structure is remains an interesting, unanswered question. The short lifetime of the ammonium hydroxide in its aqueous solution can be interpreted to mean that the ammonia molecule or ammonium ion associates with a particular water molecule or hydroxide ion only for a short period of time, after which i t associates with a different water molecule or hvdroxide ion. The thermodvnamic stability of am. monium hydroxide requiresthat the ammonia in it.iaqueous solutions soend virtuallv all ot'the time in association with water, even though the exchange of one water, or hydroxide, for another may occur with great rapidity. Because i t does exist, the questions concerning the nature of the bonding in ammonium hydroxide are of considerable interest. Were the ammonium hydroxide composed of NH: and OH- ions, the solid compound would presumably he high-melting, A d its aqueou~solutionswould presumably dissociate almost completely like the alkali metal hydroxides do (3). However, its aqueous solutions dissociate only slightly. The Gibbs free energy for this dissociation is 27.1 (4). k,Jimoi . -,. , ,, and the solid comoound melts a t a low temperature, 194.15 K (2). consequently, the true structure or ammonium hvdroxide undoubtedly deviates appreciably from a simple cbmposite of the ammonium andhydroxide ions. Schematically we can write
iteof the univalent ions. In terms of the schematic representation of the structure aiven above, the value of 6 would be closer to zero than to inity, the H-.O distance would be close to the 0-H bond length in the water molecule and the N-.H distance might differ appreciably from the N-H bond leneth in the ammonium ion. In this light, the existence of aqieous tetramethylammonium hydroxide as a strong base ocrurs because no hydrogen bond between nitrogen and oxygen to stabilize the unionized form is possible. ~
~
Acknowledgment I am indebted t o Ralph J. Tykodi for his helpful comments. Llterature Clted 1. Yoke, J. J.Chrm.Edur. 1989,66.310. 2. Hildenbrand.0. L.; Giauque. W. F. J.Am.Chrm.Soc. 1953.75.2811. a. Baes. C. F., Jr.: Meomer. R. E. The Hydrulvrrs of Cations: Wiley: New York, 1978:
"..-""..-.
"L""*o.
4
4. wagman, D. D. e t d . J. Ph.W Chem. Re/. Dora 11. Supplement No. 2.
-
~~~
~
~
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where 6 is between zero and one and the intermediate hydroeen can be viewed as binding- the HsN and OH moieties together. Whatever the actual structure of ammonium hydroxide, i t should be experimentally determinable. Solid NH40H can he studied by X-ray crystallography. Possibly the gaseous molecule c& be studied usin~molecularbeam techniques. Certainly the molecule in its solutions with water and ammonia can he studied using the whole range of techniques available for studying chemical species in their solutions. - examole.,the enereetics of its aaueous solutions indicate For that ammonium hydroxide is stable with respect to dissociationinto ions or into neutral molecules hut that i t isenergetically closer to the molecules (-AG, = 6.4 kJ/mol) than it is to the dissociated solvated ions (27.1 kJ/mol). I t seems likely that this relative energetic propinquity is reflected in the closer resemblance of the actual molecular structure to a composite of the neutral molecules rather than to a compos-
.
Thomas R. Tuttle, Jr. Brandeis University Wailham, MA 02254
TOthe Editor:
+
-
"something", a t 25 "C the In the process Hz0 NH3 sienof AG isneeative. This does not prove that the "something" is ammoniim hydroxide. herei is clear evidence that it is not ammonium hydroxide under two important conditions: (1) in aqueous solution ( I ) ; (2) as a solid (which exists only a t a low temperature) (2, 3). There is every reason to suppose, in accord with modern theories of bonding, that the "something" consists of ammonia and water molecules engaged in hydrogen bonding. This cannot correctly he called ammonium hydroxide according to the rules of inorganic nomenclature. Literature Clted
John 1.Yoke Oregon State University Cawailis. OR 97331
To the Editor: There appears to be no disagreement between the author and mvself on the existence of a one-to-one intermolecular compound between ammonia and water. His claim of the nonexistence of ammonium hydroxide seems to constitute merely an objection to naming this compound ammonium hydroxide. This is an opinion of the author that may have some merit hut with which I do not agree. Certainly his "something" is hardly an acceptable alternative. There are many acceptable alternative names that are adequately informative and variously suggestive, among which are ammonia monohydrate, water monoammoniate, and hydronium amide. These names and others can he used to designate the comoound in ouestion unambieuouslv as well as the putative ammonium hidroxide. In any event, i shall continue to refer to ammonium hydroxide and ammonium oxide in confidence that such references will create no confusion, although they may create some irritation. Thomas R. Tuttle, Jr. Brandeis University Waltham. MA 02254
Volume 68
Number 6 June 1991
533
