Hydronium

Hydronium (also called oxonium) is an ion (a chemical compound with electric charge) with the chemical formula H3O+. It is the result of adding a hydrogen ion to a water molecule (making it the conjugate acid of water). It is the simplest oxonium ion.

Hydronium ions are important in the chemistry of acids and bases. Because of the leveling effect, hydronium is the most acidic chemical species that can exist in water. Stronger acids will react with water to make hydronium. The pH of an acid in water is related to the concentration of hydronium in the solution.

Structure

Hydronium is often written as H3O+, but this is a very simplified formula. The full behavior of the hydrogen ion in water is not completely understood by scientists. Hydronium participates in hydrogen bonding like water, and tends to form clusters of surrounding water molecules. Because of the extra charge, these bonds are even stronger than the bonds between water molecules.

Two reasonably well-defined hydronium cations are the Zundel cation H5O+2 and Eigen cation H9O+4.[1]

In the Zundel cation, a symmetric hydrogen bond shares the hydrogen ion equally between two water molecules. The Zundel cation forms four more hydrogen bonds from the other four hydrogen atoms, so it is sometimes written H13O+6 or (H2O)4H5O+2 when dissolved in water.[2]

In the Eigen cation, a central H3O+ forms a hydrogen bond through each of its hydrogen atoms, each attaching to a different water molecule.

Compounds

Solid salts containing hydronium are rare, but do exist for some very strong acids. They are sometimes called acid monohydrates, as they are the result of adding one equivalent of water to one equivalent of acid.[3] Hydronium salts of perchloric acid,[3] carborane acids,[4] and fluoroantimonic acid [5] have been studied in crystal form.

The oxygen atom in a hydronium ion still has one lone pair, so it is in theory able to be a Lewis base and react with extremely strong acids. No stable compounds or solutions of protonated hydronium H4O2+ have been made, but it has been shown to exist as a reaction intermediate in extremely acidic conditions.[6]

References

  1. ↑ Tian, Ye; Hong, Jiani; Cao, Duanyun; You, Sifan; Song, Yizhi; Cheng, Bowei; Wang, Zhichang; Guan, Dong; Liu, Xinmeng; Zhao, Zhengpu; Li, Xin-Zheng; Xu, Li-Mei; Guo, Jing; Chen, Ji; Wang, En-Ge; Jiang, Ying (2022). "Visualizing Eigen/Zundel cations and their interconversion in monolayer water on metal surfaces". Science. 377 (6603): 315–319. Bibcode:2022Sci...377..315T. doi:10.1126/science.abo0823. PMID 35857595.
  2. ↑ Bell, Robert A.; Christoph, Gary G.; Fronczek, Frank R.; Marsh, Richard E. (1975). "The Cation H 13 O 6 + : A Short, Symmetric Hydrogen Bond". Science. 190 (4210): 151–152. Bibcode:1975Sci...190..151B. doi:10.1126/science.190.4210.151.
  3. ↑ 3.0 3.1 Nordman, C. E. (1962). "The crystal structure of hydronium perchlorate at –80°C". Acta Crystallographica. 15 (1): 18–23. Bibcode:1962AcCry..15...18N. doi:10.1107/S0365110X62000055.
  4. ↑ Stoyanov, Evgenii S.; Kim, Kee-Chan; Reed, Christopher A. (2006). "The Nature of the H3O+ Hydronium Ion in Benzene and Chlorinated Hydrocarbon Solvents. Conditions of Existence and Reinterpretation of Infrared Data". Journal of the American Chemical Society. 128 (6): 1948–1958. Bibcode:2006JAChS.128.1948S. doi:10.1021/ja0551335. PMID 16464096.
  5. ↑ Zhang, Dingliang; Rettig, Steven J.; Trotter, James; Aubke, Friedhelm (1996). "Superacid Anions: Crystal and Molecular Structures of Oxonium Undecafluorodiantimonate(V), [H3O][Sb2F11], Cesium Fluorosulfate, CsSO3F, Cesium Hydrogen Bis(fluorosulfate), Cs[H(SO3F)2], Cesium Tetrakis(fluorosulfato)aurate(III), Cs[Au(SO3F)4], Cesium Hexakis(fluorosulfato)platinate(IV), Cs2[Pt(SO3F)6], and Cesium Hexakis(fluorosulfato)antimonate(V), Cs[Sb(SO3F)6]". Inorganic Chemistry. 35 (21): 6113–6130. doi:10.1021/ic960525l.
  6. ↑ Olah, George A.; Prakash, G. K. Surya; Barzaghi, Mario; Lammertsma, Koop; Schleyer, Paul von R.; Pople, John A. (2003). "Protonated Hydronium Dication, H4O2+: Hydrogen–Deuterium Exchange of D2H17O+ in HF:SBF5 and DH217O+ in DF:SBF5and Theoretical Calculations". Across Conventional Lines. World Scientific Series in 20th Century Chemistry. Vol. 11. pp. 837–840. doi:10.1142/9789812791405_0154. ISBN 978-981-238-991-6.
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