From Wikipedia, the free encyclopedia - View original article

Jump to: navigation, search
Not to be confused with keto acids or oxo carboxylic acids, which are sometimes called oxo acids.

An oxoacid (or oxyacid) is an acid that contains oxygen. To be more specific, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bound to oxygen that can dissociate to produce the H+ cation, and the anion of the acid.[1]


Generally, oxoacids are simply polyatomic ions with a positively polarized hydrogen, which can be split off as a cation(ion).

Under Lavoisier's original theory, all acids contained oxygen, which was named from the Greek ὀξύς (oxys) (acid, sharp) and the root –γενής (–genes) (engender). It was later discovered that some acids, notably hydrochloric acid, did not contain oxygen and so acids were divided into oxoacids and these new hydracids.

All oxoacids 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 (E) and the number of O atoms determine oxoacid acidity. With the same central atom E, acid strength increases as the number of oxygen attached to E increases. With the same number of oxygens around E, acid strength increases with the electronegativity of E.

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


Inorganic oxoacids typically have a chemical formula of type HmXOn, where X is some element functioning as a central atom, whereas parameters m and d depend on the oxidation state of the element X. In most cases, the element X is a nonmetal, but even some metals, for example chromium and manganese, can form oxoacids when occurring at their highest oxidation state.[3]

In a very simplified manner, the structure of an oxoacid molecule can be described as M-O-H, where, however, even other atoms or atom groups can be connected to the central atom M. In a solution, such a molecule can be dissociated to ions in two distinct ways:

If the central atom M is strongly electro­negative, then it attracts strongly 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 MOH is thus an acid, because it releases a proton, it 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 M is weak, then the compound is dissociated to ions according to the latter chemical equation, and MOH is an alkaline hydroxide. Examples of such compounds are sodium hydroxide NaOH and calcium hydroxide Ca(OH)2.[3] If the electronegativity of M is somewhere in between, the compound can even be amphoteric, and in that case, it can dissociate to ions in both ways, in the former case when reacting with acids, and in the latter case when reacting with bases.[3]

When oxoacids 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 oxoacids again.[4]

Even many organic acids, like carboxylic acids and phenols, are oxoacids.[3] Their molecular structure, however, is much more cumbersome than that of inorganic oxyacids.

Most acids belong to the group of oxoacids.[3] 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.[5] 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.[6] Such acids which do not contain oxygen are nowadays known as hydracids.

Names of inorganic oxoacids[edit]

Many inorganic oxoacids 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 of hydrogen and oxygen. Well-known examples of such acids are sulfuric acid, nitric acid and phosphoric acid.

This practice is fully well-established, and even 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 (for example, acidity) they have.[7]

IUPAC, however, does not recommend to call future compounds not yet discovered with a name ending with the word acid.[7] Indeed, acids can even 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 (or dihydrogen sulfate).[8] In fact, the fully systematic name of sulfuric acid, according to IUPAC's rules, would be dihydroxido­dioxido­sulfur and that of the sulfate ion, tetra­oxido­sulfate(2-),[9] 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 less 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 oxoacids 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 oxoacids:

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 even in name of anions and salts; for example the ion ClO4- is called perchlorate.[8]

In a few cases, even prefixes ortho- and para- occur in names of some oxoacids 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 even be called as orthophosphoric acid, in order to distinguish it from metaphosphoric acid, HPO3.[8] 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.[10]


In the following table, the formula and the name of the anion refer to what remains of the acid when it cedes all hydrogen atoms as protons. Many of these acids, however, are polyprotic, and in such cases, there exists also one or more intermediate anions. In name of such anions, the prefix hydro-, is added if needed, with some numeral prefixes. For example, SO42- is the sulfate anion, and HSO4-, the hydrosulfate anion. In a similar way, PO43- is the phosphate, H2PO42-, the dihydrophosphate, and HPO4-, the hydrophosphate ion.

Element groupElement (central atom)Formula of the acidName of the acid[8][9]Formula of the corresponding anionName of the anion
6ChromiumH2CrO4Chromic acidCrO42-Chromate
H2Cr2O7Dichromic acidCr2O72-Dichromate
7ManganeseHMnO4Permanganic acidMnO4Permanganate
HMnO3Manganic acidMnO3Manganate
TechnetiumHTcO4Pertechnetic acidTcO4Pertechnetate
HTcO3Technetic acidTcO3Technetate
RheniumHReO4Perrhenic acidReO4Perrhenate
HReO3Rhenic acidReO3Rhenate
13BoronH3BO3Boric acid
(formerly orthoboric acid)[10]
(formerly orthoborate)
(HBO2)nMetaboric acidBO2-Metaborate
14CarbonH2CO3Carbonic acidCO32-Carbonate
SiliconH4SiO4Silicic acid
(formerly. orthosilicic acid)[10]
SiO44-Silicate (formerly orthosilicate)
H2SiO3Metasilicic acidSiO32-Metasilicate
14, 15Carbon, nitrogenHOCNCyanic acidOCN-Cyanate
HNCOIsocyanic acidNCO-Isocyanate
HONCFulminic acidONC- Fulminate
15NitrogenHNO3Nitric acidNO3-Nitrate
HNO2Nitrous acidNO2-Nitrite
HNO4Peroxynitric acidNO4-Peroxynitrate
HOONOPeroxynitrous acidOONO-Peroxynitrite
H2NO2Nitroxylic acidNO22-Nitroxylate
H2N2O2Hyponitrous acidN2O22-Hyponitrite
PhosphorusH3PO4Phosphoric acid
(formerly even orthophosphoric acid)[10]
HPO3Metaphosphoric acidPO3-Metaphosphate
H4P2O7Pyrophosphoric acid
(diphosphoric acid)
H3PO5Peroxomonophosphoric acidPO33-Peroxomonophosphate
(HO)2OP-PO(OH)2Hypophosphoric acid
(diphosphoric(IV) acid)
(HO)2P-O-PO(OH)2Diphosphoric(III,V) acidO2-P-O-POO22-Diphosphate(III,V)
H2PHO3Phosphonic acidPHO33-Phosphonate
H2P2H2O5Diphosphonic acidP2H2O35-Diphosphonate
HPH2O2Phosphinic acidPH2O2-Phosphinate
ArsenicH3AsO4Arsenic acidAsO43-Arsenate
H3AsO3Arsenous acidAsO33-Arsenite
16SulfurH2SO4Sulfuric acidSO42-Sulfate
H2SO3Sulfurous acidSO32-Sulfite
H2S2O7Disulfuric acidS2O72-Disulfate
H2SO5Peroxomonosulfuric acidSO52-Peroxomonosulfate
H2S2O8Peroxodisulfuric acidS2O72-Peroxodisulfate
H2S2O3Thiosulfuric acidS2O32-Thiosulfate
H2S2O6Dithionic acidS2O62-Dithionate
H2S2O5Disulfurous acidS2O52-Disulfite
H2S2O2Thiosulfurous acidS2O22-Thiosulfite
H2S2O4Dithionous acidS2O42-Dithionite
H2SO2Sulfoxylic acidSO22-Sulfoxylate
H2SxO5Polythionic acids
(x = 3, 4...)
SeleniumH2SeO4Selenic acidSeO42-Selenate
H2SeO3Selenious acidSeO32-Selenite
TelluriumH2TeO4Telluric acidTeO42-Tellurate
H6TeO6Orthotelluric acidTeO66-Orthotellurate
H2TeO3Tellurous acidTeO32-Tellurite
17ChlorineHClO4Perchloric acidClO4-Perchlorate
HClO3Chloric acidClO3-Chlorate
HClO2Chlorous acidClO2-Chlorite
HClOHypochlorous acidClO-Hypochlorite
BromineHBrO4Perbromic acidBrO4-Perbromate
HBrO3Bromic acidBrO3-Bromate
HBrO2Bromous acidBrO2-Bromite
HBrOHypobromous acidBrO-Hypobromite
IodineHIO4Periodic acidIO4-Periodate
H5IO6Orthoperiodic acidIO65-Orthoperiodate
HIO3Iodic acidIO3-Iodate
HIOHypoiodous acidIO-Hypoiodite


See also[edit]


  1. ^ http://goldbook.iupac.org/O04374.html
  2. ^ http://goldbook.iupac.org/I02949.html
  3. ^ a b c d e f Kivinen, Mäkitie: Kemia, p. 202-203, chapter=Happihapot
  4. ^ "Hapot". Otavan iso Fokus, Part 2 (El-Io). Otava. 1973. p. 990. ISBN 951-1-00272-4. 
  5. ^ Otavan suuri Ensyklopedia, s. 1606, art. Happi
  6. ^ Otavan suuri Ensyklopedia, s. 1605, art. Hapot ja emäxet
  7. ^ a b Red Book 2005, s. 124, chapter IR-8: Inorganic Acids and Derivatives
  8. ^ a b c d e Kivinen, Mäkitie: Kemia, p. 459-461, chapter Kemian nimistö: Hapot
  9. ^ a b Red Book 2005, p. 129-132, table IR-8-1
  10. ^ a b c d Red Book 2005, p. 132, note a

External links[edit]