Oxyacid


An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bond to oxygen that can dissociate to produce the H+ cation and the anion of the acid.

Description

Under Lavoisier's original theory, all acids contained oxygen, which wa and the root -γενής. It was later discovered that some acids, notably hydrochloric acid, did not contain oxygen and so acids were divided into oxyacids and these new hydroacids.
All oxyacids have the acidic hydrogen bound to an oxygen atom, so bond strength is not a factor, as it is with binary nonmetal hydrides. Rather, the electronegativity of the central atom and the number of O atoms determine oxyacid acidity. With the same central atom X, acid strength increases as the number of oxygens attached to X increases. With the same number of oxygens around E, acid strength increases with the electronegativity of X.
Compared to salt of their deprotonated forms, the oxyanions, oxyacids are generally less stable, and many of them only exist formally as hypothetical species, or exist only in solution and cannot be isolated in pure form. There are several general reasons for this: they may condense to form oligomers, or dehydrate all the way to form the anhydride, they may disproportionate to one compound of higher and another of lower oxidation state, or they might exist almost entirely as another, more stable tautomeric form 3 exists almost entirely as phosphonic acid HP. Nevertheless, perchloric acid, sulfuric acid, and nitric acid 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.

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:
If the central atom X is strongly electronegative, then it strongly attracts the electrons of the oxygen atom. In that case, the bond between the oxygen and hydrogen atom is weak, and the compound ionizes easily in the way of the former of the two chemical equations above. In this case, the compound XOH is an acid, because it releases a proton, that is, a hydrogen ion. For example, nitrogen, sulfur and chlorine are strongly electronegative elements, and therefore nitric acid, sulfuric acid, and perchloric acid, are strong acids.
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 alkaline hydroxide. Examples of such compounds are sodium hydroxide NaOH and calcium hydroxide Ca2. Owing to the high electronegativity of oxygen, however, most of the common oxobases, such as sodium hydroxide, while strongly basic in water, are only moderately basic in comparison to other bases. For example, the pKa of the conjugate acid of sodium hydroxide, water, is 15.7, while that of sodium amide, ammonia, is closer to 40, making sodium hydroxide a much weaker base than sodium amide.
If the electronegativity of X is somewhere in between, the compound can be amphoteric, and in that case it can dissociate to ions in both ways, in the former case when reacting with bases, and in the latter case when reacting with acids. Examples of this include aliphatic alcohols, such as ethanol.
Inorganic oxyacids typically have a chemical formula of type HmXOn, where X is an atom functioning as a central atom, whereas parameters m and n depend on the oxidation state of the element X. In most cases, the element X is a nonmetal, but some metals, for example chromium and manganese, can form oxyacids when occurring at their highest oxidation states.
When oxyacids are heated, many of them dissociate to water and the anhydride of the acid. In most cases, such anhydrides are oxides of nonmetals. For example, carbon dioxide, CO2, is the anhydride of carbonic acid, H2CO3, and sulfur trioxide, SO3, is the anhydride of sulfuric acid, H2SO4. These anhydrides react quickly with water and form those oxyacids again.
Many organic acids, like carboxylic acids and phenols, are oxyacids. Their molecular structure, however, is much more complicated than that of inorganic oxyacids.
Most of the commonly encountered acids are oxyacids. Indeed, in the 18th century, Lavoisier assumed that all acids contain oxygen and that oxygen causes their acidity. Because of this, he gave to this element its name, oxygenium, derived from Greek and meaning acid-maker, which is still, in a more or less modified form, used in most languages. Later, however, Humphry Davy showed that the so-called muriatic acid did not contain oxygen, despite its being a strong acid; instead, it is a solution of hydrogen chloride, HCl. Such acids which do not contain oxygen are nowadays known as hydroacids.

Names of inorganic 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 IUPAC has accepted such names. In light of the current chemical nomenclature, this practice is, however, very exceptional, because systematic names of all other compounds are formed only according to what elements they contain and what is their molecular structure, not according to what other properties they have.
IUPAC, however, recommends against calling future compounds not yet discovered with a name ending with the word acid. Indeed, acids can be called with names formed by adding the word hydrogen in front of the corresponding anion; for example, sulfuric acid could just as well be called hydrogen sulfate. In fact, the fully systematic name of sulfuric acid, according to IUPAC's rules, would be dihydroxidodioxidosulfur and that of the sulfate ion, tetraoxidosulfate, Such names, however, are almost never used.
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. For example, chlorine has the four following oxyacids:
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 is called perchlorate.
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 water molecule is separated from the ortho- acid molecule. For example, phosphoric acid, H3PO4, has sometimes been called orthophosphoric acid, in order to distinguish it from metaphosphoric acid, HPO3. However, according to IUPAC's current rules, the prefix ortho- should only be used in names of orthotelluric acid and orthoperiodic acid, and their corresponding anions and salts.

Examples

In the following table, the formula and the name of the anion refer to what remains of the acid when it loses all its hydrogen atoms as protons. Many of these acids, however, are polyprotic, and in such cases, there also exists one or more intermediate anions. In name of such anions, the prefix hydrogen- is added, with numeral prefixes if needed. For example, is the sulfate anion, and, the hydrogensulfate anion. Similarly, is phosphate, is hydrogenphosphate, and is dihydrogenphosphate.
Element groupElement Oxidation stateAcid formulaAcid nameAnion formulaAnion name
6Chromium+6Chromic acidChromate
6Chromium+6Dichromic acidDichromate
7Manganese+7Permanganic acidPermanganate
7Manganese+6Manganic acidManganate
7Technetium+7Pertechnetic acidPertechnetate
7Technetium+6Technetic acidTechnetate
7Rhenium+7Perrhenic acidPerrhenate
7Rhenium+6Tetraoxorhenic acidRhenate
7Rhenium+5Trioxorhenic acidTrioxorhenate
7Rhenium+5Tetraoxorhenic acidTetraoxorhenate
7Rhenium+5Heptaoxodirhenic acidDirhenate
8Iron+6H2FeO4Ferric acidFeO42–Ferrate
8Ruthenium+6H2RuO4Ruthenic acidRuO42–Ruthenate
8Ruthenium+7HRuO4Perruthenic acidRuO4–Perruthenate
8Ruthenium+8H2RuO5Hyperruthenic acidHRuO5–Hyperruthenate
8Osmium+6H6OsO6Osmic acidH4OsO62–Osmate
8Osmium+8H4OsO6Perosmic acidH2OsO62–Perosmate
13Boron+3Boric acid
Borate
13Boron+3Metaboric acidMetaborate
14Carbon+4Carbonic acidCarbonate
14Silicon+4Silicic acid
Silicate
14Silicon+4Metasilicic acidMetasilicate
14, 15Carbon, nitrogen+4, −3HOCNCyanic acidCyanate
15Nitrogen+5Nitric acidNitrate
15Nitrogen+5Peroxynitric acidPeroxynitrate
15Nitrogen+5Orthonitric acidOrthonitrate
15Nitrogen+3Nitrous acidNitrite
15Nitrogen+3HOONOPeroxynitrous acidPeroxynitrite
15Nitrogen+2Nitroxylic acidNitroxylate
15Nitrogen+1Hyponitrous acidHyponitrite
15Phosphorus+5Phosphoric acid
Phosphate
15Phosphorus+5Metaphosphoric acidMetaphosphate
15Phosphorus+5Pyrophosphoric acid
Pyrophosphate
15Phosphorus+5Peroxomonophosphoric acidPeroxomonophosphate
15Phosphorus+5, +3Diphosphoric acidDiphosphate
15Phosphorus+4Hypophosphoric acid
Hypophosphate
15Phosphorus+3Phosphonic acidPhosphonate
15Phosphorus+3Diphosphonic acidDiphosphonate
15Phosphorus+1Phosphinic acid Phosphinate
15Arsenic+5Arsenic acidArsenate
15Arsenic+3Arsenous acidArsenite
16Sulfur+6Sulfuric acidSulfate
16Sulfur+6Disulfuric acidDisulfate
16Sulfur+6Peroxomonosulfuric acidPeroxomonosulfate
16Sulfur+6Peroxodisulfuric acidPeroxodisulfate
16Sulfur+5Dithionic acidDithionate
16Sulfur+5, 0Polythionic acids
Polythionates
16Sulfur+4Sulfurous acidSulfite
16Sulfur+4Disulfurous acidDisulfite
16Sulfur+4, 0Thiosulfuric acidThiosulfate
16Sulfur+3Dithionous acidDithionite
16Sulfur+3, −1Thiosulfurous acidThiosulfite
16Sulfur+2Sulfoxylic acid Sulfoxylate
16Sulfur+1Dihydroxydisulfane
16Sulfur0Sulfenic acidSulfinite
16Selenium+6Selenic acidSelenate
16Selenium+4Selenous acidSelenite
16Tellurium+6Telluric acidTellurate
16Tellurium+6Orthotelluric acidOrthotellurate
16Tellurium+4Tellurous acidTellurite
17Chlorine+7Perchloric acidPerchlorate
17Chlorine+5Chloric acidChlorate
17Chlorine+3Chlorous acidChlorite
17Chlorine+1HClOHypochlorous acidHypochlorite
17Bromine+7Perbromic acidPerbromate
17Bromine+5Bromic acidBromate
17Bromine+3Bromous acidBromite
17Bromine+1HBrOHypobromous acidHypobromite
17Iodine+7Periodic acidPeriodate
17Iodine+7Orthoperiodic acidOrthoperiodate
17Iodine+5Iodic acidIodate
17Iodine+1HIOHypoiodous acidHypoiodite
18Xenon+6H2XeO4Xenic acidHXeO4–Hydrogenxenate
18Xenon+8H4XeO6Perxenic acidXeO64–Perxenate