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Anoxyacid,oxoacid, orternary acid is anacid that containsoxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one otherelement, with at least onehydrogen atom bonded to oxygen that can dissociate to produce theH+cation and theanion of the acid.[1]
UnderLavoisier's original theory, all acids contained oxygen, which was named fromAncient Greek:ὀξύς + -γενής,romanized: oxys + -genes,lit. 'acid, sharp + creator'. It was later discovered that some acids, notablyhydrochloric acid, did not contain oxygen and so acids were divided into oxo-acids and these newhydroacids.
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, theelectronegativity 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 theoxyanions), 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 formoligomers (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 stabletautomeric 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]
An oxyacid molecule contains the structure X−O−H, where other atoms or atom groups can be connected to the central atom X. In asolution, such a molecule can be dissociated intoions in two distinct ways:
If the central atom X is stronglyelectronegative, then it strongly attracts theelectrons 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 twochemical equations above. In this case, the compound XOH is an acid, because it releases aproton, that is, a hydrogen ion. For example,nitrogen,sulfur andchlorine are strongly electronegative elements, and thereforenitric acid,sulfuric acid, andperchloric acid, arestrong acids. The acidity of oxoacids is also affected by the resonance stabilization of their conjugate bases. Double-bonded oxygen is electron withdrawing by resonance, so the negative charge of a deprotonated hydroxyl group can be distributed to other oxygen atoms. Both acetic acid and methanol contain C-O-H groups that can act as acids, but acetic acid is a far stronger acid because its conjugate base, acetate, can distribute its negative charge over two oxygen atoms. In contrast, the conjugate acid of methanol has the negative charge localized on oxygen, so it is a far stronger base than acetate, making acetic acid the stronger acid.
If, however, the electronegativity of X is low, then the compound is dissociated to ions according to the latter chemical equation, and XOH is analkalinehydroxide. Examples of such compounds aresodium hydroxide NaOH andcalcium hydroxide Ca(OH)2.[3] 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 ofsodium hydroxide,water, is 14.0, while that ofsodium amide,ammonia, is closer to 40, making sodium hydroxide a much weaker base than sodium amide.[4][3]
If the electronegativity of X is somewhere in between, the compound can beamphoteric, and in that case it can dissociate to ions in both ways, in the former case when reacting withbases, and in the latter case when reacting with acids. Examples of this include water, aliphaticalcohols, such asethanol, and aluminum hydroxide.[3]
Inorganic oxyacids typically have a chemical formula of type HmXOn, where X is an atom functioning as acentral atom, whereas parametersm andn depend on theoxidation state of the element X. In most cases, the elementX is anonmetal, but somemetals, for examplechromium andmanganese, can form oxyacids when occurring at their highestoxidation states.[3]
When oxyacids are heated, many of them dissociate to water and theanhydride of the acid. In most cases, such anhydrides areoxides of nonmetals. For example,carbon dioxide, CO2, is the anhydride ofcarbonic acid, H2CO3, andsulfur trioxide, SO3, is the anhydride ofsulfuric acid, H2SO4. These anhydrides react quickly with water and form those oxyacids again.[5]
Manyorganic acids, likecarboxylic acids andphenols, are oxyacids.[3] Their molecular structure, however, is much more complicated than that of inorganic oxyacids.
Most of the commonly encountered acids are oxyacids.[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 fromGreek and meaningacid-maker, which is still, in a more or less modified form, used in most languages.[6] Later, however,Humphry Davy showed that the so-calledmuriatic acid did not contain oxygen, despite its being astrong acid; instead, it is a solution ofhydrogen chloride, HCl.[7] Such acids which do not contain oxygen are nowadays known as hydroacids.
Many inorganic oxyacids are traditionally called with names ending with the wordacid 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 aresulfuric acid,nitric acid andphosphoric acid.
This practice is fully well-established, andIUPAC has accepted such names. In light of the currentchemical nomenclature, this practice is an exception, becausesystematic names of compounds are formed according to the elements they contain and their molecular structure, not according to other properties (for example,acidity) they have.[8]
IUPAC, however, recommends against calling future compounds not yet discovered with a name ending with the wordacid.[8] Indeed, acids can be called with names formed by adding the wordhydrogen in front of the correspondinganion; for example, sulfuric acid could just as well be calledhydrogen sulfate (ordihydrogen sulfate).[9] In fact, the fully systematic name of sulfuric acid, according to IUPAC's rules, would bedihydroxidodioxidosulfur and that of the sulfate ion,tetraoxidosulfate(2−),[10] 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, theEnglish 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, andsulfurous acid, H2SO3. Analogously,nitric acid is HNO3, andnitrous 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 prefixper- orhypo- to their names. The prefixper-, however, is used only when the central atom is ahalogen or agroup 7 element.[9] For example,chlorine has the four following oxyacids:
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 prefixhyper-. 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. Prefixeshypo- andper- occur in the name of anions and salts; for example the ionClO−
4 is calledperchlorate.[9]
In a few cases, the prefixesortho- andpara- occur in names of some oxyacids and their derivative anions. In such cases, thepara- acid is what can be thought as remaining of theortho- acid if awater molecule is separated from theortho- acid molecule. For example,phosphoric acid, H3PO4, has sometimes been calledorthophosphoric acid, in order to distinguish it frommetaphosphoric acid, HPO3.[9] However, according toIUPAC's current rules, the prefixortho- should only be used in names oforthotelluric acid andorthoperiodic acid, and their corresponding anions and salts.[11]
In the following table, the formula and the name of the anion refer to what remains of the acid when it losesall its hydrogen atoms as protons. Many of these acids, however, arepolyprotic, and in such cases, there also exists one or more intermediate anions. In name of such anions, the prefixhydrogen- (in older nomenclaturebi-) is added, withnumeral prefixes if needed. For example,SO2−
4 is thesulfate anion, andHSO−
4, thehydrogensulfate (or bisulfate) anion. Similarly,PO3−
4 isphosphate,HPO2−
4 is hydrogenphosphate, andH
2PO−
4 is dihydrogenphosphate.
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