In chemistry, hydronium or the hydronium ion refers to the chemical species H3O+. Hydronium is the simplest oxonium ion, where an oxonium ion is any oxygen cation having three chemical bonds. The International Union of Pure and Applied Chemistry (IUPAC) recommends the term “oxonium” or “hydroxonium” be used in place of the word “hydronium.” However, hydronium remains the common name for the ion.
No Free Hydrogen in Water
Hydrogen ions (H+) appear in chemical equations in aqueous solution and pH calculations, but free hydrogen ions don’t actually float around in water. Instead, the incredibly high positive charge density of the proton (hydrogen ion) makes it immediately associate with one or more water molecules, forming hydronium.
So, one equation for the auto-dissociation of water is this:
H2O → H+(aq) + OH–(aq)
But, a more accurate chemical equation shows hydronium:
2 H2O(l) ↔ H3O+(aq) + OH–(aq)
Even this equation is an over-simplification. When water auto-dissociates, the hydrogen ion does not always associate with a single water molecule and form hydronium. Often, the proton associates with multiple molecules, hopping from one to another. Similarly, the hydroxide ion interacts with multiple water molecules.
In cold water, a hydronium ion interacts with an average of six water molecules. However, other structures occur. For example, H3O+(H2O)20 is a highly stable or “magic number” structure. At the other end of the spectrum, the Zundel cation (H5O2+) involves two water molecules equally sharing the hydrogen via a symmetric hydrogen bond. The Eigen cation (H9O4+) has the hydronium ion acting as a center for three water molecules, also connected with hydrogen bonding.
The hydronium ion itself takes the shape of a trigonal pyramid, with oxygen at the apex of the pyramid. The center of mass of the ion is close to the oxygen ion. The bond angle H-O-H is about 113°.
Importance of Hydronium
The concept of the hydronium ion is important in pH calculations, acid-base chemistry, and interstellar chemistry.
The pH formula thus takes two forms:
pH = -log[H+]
pH = -log[H3O+]
The hydrogen ion or hydronium ion forms the basis for the Arrhenius acid definition. So, the dissociation of hydrochloric acid becomes:
HCl(aq) + H2O → H3O+(aq) + Cl–(aq)
For most calculations, it doesn’t matter whether you use H+ or H3O+, but it’s worth knowing some interesting chemistry involves hydronium acting as the cation in compounds. For example, some strong acids form crystals of hydronium salts. Mixing liquid anhydrous perchloric acid and water in a 1:1 ratio forms solid hydronium perchlorate (H3O+·ClO4−).
In interstellar chemistry, hydronium occurs in diffuse and dense molecular clouds, the interstellar medium, and comet plasma tails. Typically, this starts with the ionization of H2 (molecular hydrogen) into H2+ by cosmic radiation. However, several recombination reactions are possible, forming water, the hydronium ion, and the hydroxide ion.
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