Hydronium


General
Systematic name	Hydronium
Other names	Hydronium Ion
Molecular formula	H₃O⁺
Molar mass	19.02 g/mol
Properties
Acid dissociation constant pK_a	−1.7
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

In chemistry, hydronium is the common name for the cation H₃O⁺ derived from protonation of water.

Nomenclature

According to IUPAC ion nomenclature, the hydronium ion should be referred to as oxonium. Hydroxonium may also be used unambiguously to identify it. A draft IUPAC proposal also recommends the use of oxonium and oxidanium in organic and inorganic chemistry contexts, respectively.

An oxonium ion is any ion with a trivalent oxygen cation. For example, a protonated hydroxyl group is an oxonium ion, but not a hydronium.

Acids and Acidity

Hydronium is the cation that forms from water in the presence of hydrogen ions. These hydrons do not exist in a free state: they are extremely reactive and are solvated by water. An acid is generally the source of these hydrons; however, since water can behave as an acid, hydronium exists even in pure water. This special case of water reacting with water to produce hydronium (and hydroxide) ions is commonly known as the self-ionization of water. The resulting hydronium ions are few and short-lived. (Nevertheless, they form the basis for determining the pH of basic aqueous solutions, since the less there are these autoionized hydroniums, the more there is base.)

Hydronium is very acidic: at 25°C, its pKa=-1.7. It is also the most acidic species that can exist in water (assuming sufficient water for dissolution): any stronger acid will ionize and protonate a water molecule to form hydronium. The acidity of hydronium is the implicit standard used to judge the strength of an acid in water: strong acids must be better proton donors than hydronium, otherwise a significant portion of acid will exist in a non-ionized state. Unlike the hydronium that results from water's autodissociation, these hydronium ions are long-lasting and concentrated, in proportion to the strength of the dissolved acid.

The pH of a solution is a measure of its proton concentration. Since these protons react with water to form hydronium, the acidity of an aqueous solution is determined by its hydronium concentration.

Solvation

Researchers have yet to fully characterize the solvation of hydronium ion in water, in part because many different meanings of solvation exist. A freezing point depression study determined that the mean hydration ion in cold water is approximately H₃O⁺(H₂O)₆ Zavitsas, A. A. (2001) Properties of water solutions of electrolytes and nonelectrolytes. J. Phys. Chem. B 105 7805-7815. : on average, each hydronium ion is solvated by 6 water molecules which are unable to solvate other solute molecules.

Some hydration structures are quite large: the H₃O⁺(H₂O)₂₀ magic ion number structure (called magic because of its increased stability with respect to hydration structures involving a comparable number of water molecules) might place the hydronium inside a dodecahedral cage Hulthe, G.; Stenhagen, G.; Wennerström, O. & C-H. Ottosson, C-H. (1997) Water cluster studied by electrospray mass spectrometry. J. Chromatogr. A 512 155-165.. However, more recent ab intio molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the H₃O⁺(H₂O)₂₀ clusterIyengar, S. S. ;Petersen, M. K.; Burnham, C. J.; Day, T. J. F.; Voth, G. A. (2005) The Properties of Ion-Water Clusters. I. The Protonated 21-Water Cluster. J. Chem. Phys. 123 084309.. Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.

Two other well-known structures are the Zundel cations and Eigen cations. The Eigen solvation structure has the hydronium ion at the center of an H₉O₄⁺ complex in which the hydronium is strongly hydrogen-bonded to 3 neighbouring water molecules Zundel, G. & Metzger, H. (1968) Energiebänder der tunnelnden Übershuß-Protenon in flüssigen Säuren. Eine IR-spektroskopische Untersuchung der Natur der Gruppierungen H₅O₂⁺ Z. Phys. Chem. 58 225-245.. In the Zundel H₅O₂⁺ complex the proton is shared equally by two water molecules Wicke, E.; Eigen, M. & Ackermann, Th. (1954) Über den Zustand des Protons (Hydroniumions) in wäßriger Lösung. Z. Phys. Chem. 1 340-364.. Recent work indicates that both of these complexes represent ideal structures in a more general hydrogen bond network defect Marx, D.; Tuckerman, M. E.; Hutter, J. & Parrinello, M. (1999) The nature of the hydrated excess proton in water. Nature 397 601-604..

Solid hydronium salts

For many strong acids, it is possible to form crystals of their hydronium salt that are relatively stable. Sometimes these salts are called acid monohydrates. As a rule, any acid with an ionization constant of 10⁹ or higher may do this. Acids whose ionization constant is below 10⁹ generally cannot form stable H₃O⁺ salts. For example, hydrochloric acid has an ionization constant of 10⁷, and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid has an ionization constant of 10¹⁰, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, solid hydronium perchlorate forms.

The hydronium ion also forms stable compounds with the carborane superacid H(CB₁₁H(CH₃)₅Br₆) The Nature of the H3O+ Hydronium Ion in Benzene and Chlorinated Hydrocarbon Solvents. Conditions of Existence and Reinterpretation of Infrared Data Evgenii S. Stoyanov, Kee-Chan Kim, and Christopher A. Reed J. Am. Chem. Soc.; 2006; 128(6) pp 1948 - 1958; Abstract. X-ray crystallography shows a C_3v symmetry for the hydronium ion with each proton interacting with a bromine atoms from three different carborane anions at an average separation of 320 pm. The ^*^* salt is also soluble in benzene. In crystals grown from a benzene solution the solvent co-crystallizes and a H₃O.(benzene)₃ cation is completely separated from the anion. In the cation three benzene molecules surround hydronium forming pi-cation interactions with the hydrogen atoms. The closest (nonbonding) approach of the anion at chlorine to the cation at oxygen is 348 pm.

References

Acids | Cations

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Nomenclature

Acids and Acidity

Solvation

Solid hydronium salts

References