Oxyacid

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An oxyacid, oxoacid, or ternary acid is an

All oxyacids have the acidic hydrogen bound to an oxygen atom, so bond strength (length) is not a factor, as it is with binary nonmetal hydrides. Rather, the electronegativity of the central atom and the number of oxygen atoms determine oxyacid acidity. For oxyacids with the same central atom, acid strength increases with the number of oxygen atoms attached to it. With the same number of oxygen atoms attached to it, acid strength increases with increasing electronegativity of the central atom.

Compared to the salts of their deprotonated forms (a class of compounds known as the oxyanions), oxyacids are generally less stable, and many of them only exist formally as hypothetical species, or only exist in solution and cannot be isolated in pure form. There are several general reasons for this: (1) they may condense to form oligomers (e.g., H₂CrO₄ to H₂Cr₂O₇), or dehydrate all the way to form the anhydride (e.g., H₂CO₃ to CO₂), (2) they may disproportionate to one compound of higher and another of lower oxidation state (e.g., HClO₂ to HClO and HClO₃), or (3) they might exist almost entirely as another, more stable tautomeric form (e.g., phosphorous acid P(OH)₃ exists almost entirely as phosphonic acid HP(=O)(OH)₂). Nevertheless, perchloric acid (HClO₄), sulfuric acid (H₂SO₄), and nitric acid (HNO₃) are a few common oxyacids that are relatively easily prepared as pure substances.

Imidic acids are created by replacing =O with =NR in an oxyacid.^[2]

Properties

An oxyacid molecule contains the structure X−O−H, where other atoms or atom groups can be connected to the central atom X. In a solution, such a molecule can be dissociated into ions in two distinct ways:

• X−O−H ⇄ (X−O)⁻ + H⁺
• X−O−H ⇄ X⁺ + OH^−[3]

If the central atom X is strongly

If, however, the electronegativity of X is low, then the compound is dissociated to ions according to the latter chemical equation, and XOH is an

Many organic acids, like carboxylic acids and phenols, are oxyacids.^[3] Their molecular structure, however, is much more complicated than that of inorganic oxyacids.

Most of the commonly encountered acids are oxyacids.

Many inorganic oxyacids are traditionally called with names ending with the word acid and which also contain, in a somewhat modified form, the name of the element they contain in addition to hydrogen and oxygen. Well-known examples of such acids are sulfuric acid, nitric acid and phosphoric acid.

This practice is fully well-established, and

However, the same element can form more than one acid when compounded with hydrogen and oxygen. In such cases, the English practice to distinguish such acids is to use the suffix -ic in the name of the element in the name of the acid containing more oxygen atoms, and the suffix -ous in the name of the element in the name of the acid containing fewer oxygen atoms. Thus, for example, sulfuric acid is H₂SO₄, and sulfurous acid, H₂SO₃. Analogously, nitric acid is HNO₃, and nitrous acid, HNO₂. If there are more than two oxyacids having the same element as the central atom, then, in some cases, acids are distinguished by adding the prefix per- or hypo- to their names. The prefix per-, however, is used only when the central atom is a halogen or a group 7 element.^[8] For example, chlorine has the four following oxyacids:

• hypochlorous acid HClO
• chlorous acid HClO₂
• chloric acid HClO₃
• perchloric acid HClO₄

Some elemental atoms can exist in a high enough oxidation state that they can hold one more double-bonded oxygen atom than the perhalic acids do. In that case, any acids regarding such element are given the prefix hyper-. Currently, the only known acid with this prefix is hyperruthenic acid, H₂RuO₅.

The suffix -ite occurs in names of anions and salts derived from acids whose names end to the suffix -ous. On the other hand, the suffix -ate occurs in names of anions and salts derived from acids whose names end to the suffix -ic. Prefixes hypo- and per- occur in the name of anions and salts; for example the ion ClO⁻
₄ is called perchlorate.^[8]

In a few cases, the prefixes ortho- and para- occur in names of some oxyacids and their derivative anions. In such cases, the para- acid is what can be thought as remaining of the ortho- acid if a

• Kivinen, Antti; Mäkitie, Osmo (1988). Kemia (in Finnish). Helsinki, Finland: Otava.
  ISBN 951-1-10136-6
  .
• Nomenclature of Inorganic Compounds, IUPAC Recommendations 2005 (Red Book 2005). International Union of Pure and Applied Chemistry. 2005.
  ISBN 0-85404-438-8.^{[dead link}
  ]
• Otavan suuri ensyklopedia, volume 2 (Cid-Harvey) (in Finnish). Helsinki, Finland: Otava. 1977.
  ISBN 951-1-04170-3
  .

• Weak acid
• Hypohalous acid
• Sulfur oxoacid

References