Oxyacid

Not to be confused with keto acids, also known as oxocarboxylic acids, which are a type of oxoacid.

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(oxys: acid, sharp) and the root -γενής (-genes: creator). 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 (length) is not a factor, as it is with binary nonmetal hydrides. Rather, the electronegativity of the central atom (X) 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: (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.

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⁻

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 Ca(OH)₂. 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 H_mXO_n, 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 (for example, acidity) 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 (or dihydrogen 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(2−), 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:

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

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.

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- (in older nomenclature bi-) is added, with numeral prefixes if needed. For example, SO²⁻
₄ is the sulfate anion, and HSO⁻
₄, the hydrogensulfate (or bisulfate) anion. Similarly, PO³⁻
₄ is phosphate, HPO²⁻
₄ is hydrogenphosphate, and H
₂PO⁻
₄ is dihydrogenphosphate.

Oxyacids and their corresponding anions

Element group	Element (central atom)	Oxidation state	Acid formula	Acid name	Anion formula	Anion name
6	Chromium	+6	H 2CrO 4	Chromic acid	CrO2− 4	Chromate
			H 2Cr 2O 7	Dichromic acid	Cr 2O2− 7	Dichromate
7	Manganese	+7	HMnO 4	Permanganic acid	MnO− 4	Permanganate
		+6	H 2MnO 4	Manganic acid	MnO2− 4	Manganate
	Technetium	+7	HTcO 4	Pertechnetic acid	TcO− 4	Pertechnetate
		+6	H 2TcO 4	Technetic acid	TcO2− 4	Technetate
	Rhenium	+7	HReO 4	Perrhenic acid	ReO− 4	Perrhenate
		+6	H 2ReO 4	Tetraoxorhenic(VI) acid	ReO2− 4	Rhenate(VI)
		+5	HReO 3	Trioxorhenic(V) acid	ReO− 3	Trioxorhenate(V)
			H 3ReO 4	Tetraoxorhenic(V) acid	ReO3− 4	Tetraoxorhenate(V)
			H 4Re 2O 7	Heptaoxodirhenic(V) acid	Re 2O4− 7	Dirhenate(V)
8	Iron	+6	H2FeO4	Ferric acid	FeO42–	Ferrate
	Ruthenium	+6	H2RuO4	Ruthenic acid	RuO42–	Ruthenate
		+7	HRuO4	Perruthenic acid	RuO4–	Perruthenate (note difference in usage compared to osmium)
		+8	H2RuO5	Hyperruthenic acid	HRuO5–	Hyperruthenate
	Osmium	+6	H6OsO6	Osmic acid	H4OsO62–	Osmate
		+8	H4OsO6	Perosmic acid	H2OsO62–	Perosmate (note difference in usage compared to ruthenium)
13	Boron	+3	H 3BO 3	Boric acid (formerly orthoboric acid)	BO3− 3	Borate (formerly orthoborate)
			(HBO 2) n	Metaboric acid	BO− 2	Metaborate
14	Carbon	+4	H 2CO 3	Carbonic acid	CO2− 3	Carbonate
	Silicon	+4	H 4SiO 4	Silicic acid (formerly orthosilicic acid)	SiO4− 4	Silicate (formerly orthosilicate)
			H 2SiO 3	Metasilicic acid	SiO2− 3	Metasilicate
14, 15	Carbon, nitrogen	+4, −3	HOCN	Cyanic acid	OCN−	Cyanate
15	Nitrogen	+5	HNO 3	Nitric acid	NO− 3	Nitrate
			HNO 4	Peroxynitric acid	NO− 4	Peroxynitrate
			H 3NO 4	Orthonitric acid	NO3− 4	Orthonitrate
		+3	HNO 2	Nitrous acid	NO− 2	Nitrite
			HOONO	Peroxynitrous acid	OONO−	Peroxynitrite
		+2	H 2NO 2	Nitroxylic acid	NO2− 2	Nitroxylate
		+1	H 2N 2O 2	Hyponitrous acid	N 2O2− 2	Hyponitrite
	Phosphorus	+5	H 3PO 4	Phosphoric acid (formerly orthophosphoric acid)	PO3− 4	Phosphate (orthophosphate)
			HPO 3	Metaphosphoric acid	PO− 3	Metaphosphate
			H 4P 2O 7	Pyrophosphoric acid (diphosphoric acid)	P 2O4− 7	Pyrophosphate (diphosphate)
			H 3PO 5	Peroxomonophosphoric acid	PO3− 3	Peroxomonophosphate
		+5, +3	(HO) 2POPO(OH) 2	Diphosphoric(III,V) acid	O 2POPOO2− 2	Diphosphate(III,V)
		+4	(HO) 2OPPO(OH) 2	Hypophosphoric acid (diphosphoric(IV) acid)	O 2OPPOO4− 2	Hypophosphate (diphosphate(IV))
		+3	H 2PHO 3	Phosphonic acid	PHO2− 3	Phosphonate
			H 2P 2H 2O 5	Diphosphonic acid	P 2H 2O5− 3	Diphosphonate
		+1	HPH 2O 2	Phosphinic acid (hypophosphorous acid)	PH 2O− 2	Phosphinate (hypophosphite)
	Arsenic	+5	H 3AsO 4	Arsenic acid	AsO3− 4	Arsenate
		+3	H 3AsO 3	Arsenous acid	AsO3− 3	Arsenite
16	Sulfur	+6	H 2SO 4	Sulfuric acid	SO2− 4	Sulfate
			H 2S 2O 7	Disulfuric acid	S 2O2− 7	Disulfate
			H 2SO 5	Peroxomonosulfuric acid	SO2− 5	Peroxomonosulfate
			H 2S 2O 8	Peroxodisulfuric acid	S 2O2− 8	Peroxodisulfate
		+5	H 2S 2O 6	Dithionic acid	S 2O2− 6	Dithionate
		+5, 0	H 2S xO 6	Polythionic acids (x = 3, 4...)	S xO2− 6	Polythionates
		+4	H 2SO 3	Sulfurous acid	SO2− 3	Sulfite
			H 2S 2O 5	Disulfurous acid	S 2O2− 5	Disulfite
		+4, 0	H 2S 2O 3	Thiosulfuric acid	S 2O2− 3	Thiosulfate
		+3	H 2S 2O 4	Dithionous acid	S 2O2− 4	Dithionite
		+3, −1	H 2S 2O 2	Thiosulfurous acid	S 2O2− 2	Thiosulfite
		+2	H 2SO 2	Sulfoxylic acid (hyposulfurous acid)	SO2− 2	Sulfoxylate (hyposulfite)
		+1	H 2S 2O 2	Dihydroxydisulfane	S 2O2− 2
		0	HSOH	Sulfenic acid	HSO−	Sulfinite
	Selenium	+6	H 2SeO 4	Selenic acid	SeO2− 4	Selenate
		+4	H 2SeO 3	Selenous acid	SeO2− 3	Selenite
	Tellurium	+6	H 2TeO 4	Telluric acid	TeO2− 4	Tellurate
			H 6TeO 6	Orthotelluric acid	TeO6− 6	Orthotellurate
		+4	H 2TeO 3	Tellurous acid	TeO2− 3	Tellurite
17	Chlorine	+7	HClO 4	Perchloric acid	ClO− 4	Perchlorate
		+5	HClO 3	Chloric acid	ClO− 3	Chlorate
		+3	HClO 2	Chlorous acid	ClO− 2	Chlorite
		+1	HClO	Hypochlorous acid	ClO−	Hypochlorite
	Bromine	+7	HBrO 4	Perbromic acid	BrO− 4	Perbromate
		+5	HBrO 3	Bromic acid	BrO− 3	Bromate
		+3	HBrO 2	Bromous acid	BrO− 2	Bromite
		+1	HBrO	Hypobromous acid	BrO−	Hypobromite
	Iodine	+7	HIO 4	Periodic acid	IO− 4	Periodate
			H 5IO 6	Orthoperiodic acid	IO5− 6	Orthoperiodate
		+5	HIO 3	Iodic acid	IO− 3	Iodate
		+1	HIO	Hypoiodous acid	IO−	Hypoiodite
18	Xenon	+6	H2XeO4	Xenic acid	HXeO4–	Hydrogenxenate (dibasic xenate is unknown)
		+8	H4XeO6	Perxenic acid	XeO64–	Perxenate

Sources

• 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.
• Otavan suuri ensyklopedia, volume 2 (Cid-Harvey) (in Finnish). Helsinki, Finland: Otava. 1977. ISBN 951-1-04170-3.

See also

• Strong acid
• Weak acid
• Electronegativity
• Hypohalous acid
• Sulfur oxoacid

External links

• IUPAC definition of "oxoacid" (from the Gold Book)