Chemical Education Today
Letters Formula for Ammonia Monohydrate The reality of NH4OH was argued in J. Chem. Educ. and elsewhere a decade ago (1–5). Further evidence is now available. My colleague Darrah Thomas has calculated the geometry and bond lengths of H5NO using Gaussian (6). The calculation was done using the D95 basis set and the B3LYP method. The lengths are as follows: Three NH bonds in the NH3
102 pm
N–H hydrogen bond in H3N–HOH
179 pm
Two OH bonds in H2O
98 and 100 pm
The hydrogen bond is substantially longer and therefore weaker than the NH bonds in NH3. So it is misleading to speak of it as part of an NH4 group as if the four bonds were similar. Moreover, the length of the two OH bonds are almost identical; the one that is hydrogen bonded to the N is only 2% longer than the other. Evidently the water is almost unaffected by the ammonia so there is little covalence in the bond, as is implied by the misnomer NH4OH. The nomenclature is misleading because it implies a greater integrity to the NH4 than actually exists and a relation between the OH group and the rest of the molecule that resembles the OH in alcohols or metal hydroxides. I hesitate to declare
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that there is no NH4 in ammonia hydrate because in the rare occasions when the hydrate dissociates, the NH4 group collapses to a symmetric NH4+ suggesting that there was some integrity to the original NH4 despite the anomaly in one of its bonds. However, NH4OH is a misleading nomenclature which should not be used. The name “ammonium hydroxide” is not merely misleading but is actually false since it asserts the existence of the ammonium ion in the hydrate for which there is no evidence whatever. When a formula for the hydrate is needed, the form HOHⴢNH3 describes the molecule as correctly as the symbolism allows. Literature Cited 1. 2. 3. 4. 5. 6.
Tuttle, T. R., Jr. J. Chem. Educ. 1991, 68, 533. Kauffman, G. B. J. Chem. Educ. 1991, 68, 534. Yoke, J. J. Chem. Educ. 1989, 66, 310. Laing, M.; Laing, M. Education in Chem. 1992, July, 116. Laing, M. S. African J. Sci. 1990, 86, 216 Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998. Stephen J. Hawkes Department of Chemistry Oregon State University Corvallis, Oregon, 97331-4003
[email protected] Vol. 81 No. 11 November 2004
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Journal of Chemical Education
1569
