Abstract
The enthalpies of formation of simple nonmetal or metalloid oxyanions in aqueous solution are discussed. Archival values are cited and estimates are made. Trends prove evasive.
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REFERENCES
Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th edn.; Wiley: New York, 1999. All generalizations of descriptive inorganic chemistry in this paper are taken from this source.
Unless otherwise said, all thermochemical data in the current study are taken from Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data, 1982, 11, Suppl. 2. It is important to note that the enthalpy of formation scales and reference states are different for ions as found in the gas phase and as solids or in aqueous solution. For the former, it entails a thermochemical cycle and assumes all elements, as found in their standard state, have enthalpies of formation set precisely to 0. (There is some ambiguity in the enthalpy of formation of the electron, however.) Ionization energies and electron affinities are explicitly considered. For the latter, all of this is assumed except that aqueous H+ is also assumed to have an enthalpy of formation of precisely 0.
Bartmess, John E., unpublished results and personal communication to the authors.
Bartberger, D.; Fukuto, J. M.; Houk, K. N. Proc. Natl. Acad. Sci. U.S., 2001, 98, 2194.
All gas-phase thermochemical data (electron affinities, deprotonation energies, gas phase acidities, and enthalpies of formation) cited in the current study are taken from Bartmess, J. E. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Linstrom P. J.; and Mallard, W. G. Eds.; National Institute of Standards and Technology, Gaithersburg, Maryland, 2001 (http://webbook.nist.gov).
From thermochemical data in our archival references [2] and [5], we deduce the related gas phase reaction NO− + NO → N2O2 − is exothermic by the comparable 290 kJ/mol. No gas-phase data is seemingly known for energetics of the above-cited dimerization reaction of NO−.
For a discussion of the nonexistence of solid phase [O3]2− salts and their reformulation as mixed salts of [O2]− and [O2]2 −, see Pauling, L. The Nature of the Chemical Bond, 2nd edn.; Cornell University Press: Ithaca, 1945.
This is based on other pairs of valence isoelectronically related sulfur and selenium compounds. The enthalpy-Gibbs energy differences for aqueous [SH]− and [SeH]− is nearly the same, 29.7 and 28.1; those for aqueous H2S and H2Se are somewhat larger, 11.9 and 3.0 kJ/mol. The differences for aqueous [SO3]2 − and [SeO3]2 − are 149.0 and 139.4 and for [SO4]2 − and [SeO4]2 − are 164.7 and 157.3 kJ/mol. Other comparable differences are for the elements as found as solids, 0.0 and 0.0 (by definition); monomeric gases, 40.6 and 40.0; diatomic elemental gases, 49.1 and 49.8; gaseous diatomic oxides 26.1 and 26.8; and gaseous hexafluorides, 103.7 and 100 kJ/mol.
Fieser, M.; Fieser, L. F. Reagents for Organic Synthesis; Wiley: New York, 1967; Vol. 1. SeO2 oxidizes ketones to α-diketones and 1,4-diketones to enediones. It also effects allylic hydroxylations of olefins.
The gas phase deprotonation energies of HOX for X = F, Cl and I are 1517 ± 14, 1488 ± 5 and 1480 ± 8 kJ/mol. Admittedly, this runs against intuition based on inductive effects invoked by organic chemists but, then again, in the gas phase the deprotonation energies for the haloacetic acids, XCH2COOH perversely decrease in the same order.
Tomazkiewicz, I.; Hope, G. A.; O'Hare, P. A. G. J. Chem. Thermodyn. 1997, 29, 1031
Deakyne, C. A.; Li, L.; Zheng, S.; Xu, D.; and Liebman, J. F. J. Chem. Thermodyn., 2002, 34, 185.
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Toscano, J.P., Deakyne, C.A. & Liebman, J.F. Paradigms and Paradoxes: Energetics of Aqueous Oxyanions of Nonmetals and Metalloids. Structural Chemistry 14, 315–320 (2003). https://doi.org/10.1023/A:1023872015174
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DOI: https://doi.org/10.1023/A:1023872015174