A oxacid u oxoacid is a ternary acid composed of hydrogen, oxygen and a non-metallic element that constitutes the so-called central atom. Depending on the number of oxygen atoms, and consequently, the oxidation states of the non-metallic element, various oxacids can be formed.
These substances are purely inorganic; However, carbon can form one of the best known oxacids: carbonic acid, HtwoCO3. As its chemical formula alone shows, it has three atoms of O, one of C and two of H.
The two H atoms of HtwoCO3 are released to the medium as H+, which explains its acidic characteristics. Heating an aqueous solution of carbonic acid will give off a gas.
This gas is carbon dioxide, COtwo, an inorganic molecule that originates from the combustion of hydrocarbons and cellular respiration. If CO was returnedtwo to the water container, the HtwoCO3 it would be formed again; therefore, oxo acid is formed when a certain substance reacts with water.
This reaction is not only observed for COtwo, but for other inorganic covalent molecules called acid oxides.
Oxacids have a vast number of uses, which are difficult to describe in a general way. Its application will depend greatly on the central atom and the number of oxygens.
They can serve from compounds for the synthesis of materials, fertilizers and explosives, to analytical purposes or production of soft drinks; as with carbonic acid and phosphoric acid, H3PO4, forming part of the composition of these drinks.
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A generic H.E.O formula for oxacids is shown in the image above. As can be seen, it has hydrogen (H), oxygen (O) and a central atom (E); which in the case of carbonic acid, is carbon, C.
The hydrogen in oxacids is usually attached to an oxygen atom and not to the central atom. Phosphorous acid, H3PO3, represents a particular case where one of the hydrogens is bound to the phosphorous atom; therefore its structural formula is best represented as (OH)twoOPH.
While for nitrous acid, HNOtwo, has an H-O-N = O backbone, so it has a hydroxyl group (OH) that dissociates to release hydrogen.
So one of the main characteristics of an oxacid is not only that it has oxygen, but also that it has an OH group.
On the other hand, some oxacids have what is called an oxo group, E = O. In the case of phosphorous acid, it has an oxo group, P = O. They lack H atoms, so they are "not responsible" for acidity.
The central atom (E) may or may not be an electronegative element, depending on its location in the p block of the periodic table. On the other hand, oxygen, an element slightly more electronegative than nitrogen, attracts electrons from the OH bond; thus allowing the release of the H ion+.
E is therefore linked to OH groups. When an H ion is released+ acid ionization occurs; that is, it acquires an electrical charge, which in its case is negative. An oxacid can release as many H ions+ as OH groups it has in its structure; and the more there is, the greater the negative charge.
Sulfuric acid, polyprotic, has the molecular formula HtwoSW4. This formula can also be written as follows: (OH)twoSWtwo, to emphasize that sulfuric acid has two hydroxyl groups attached to sulfur, its central atom.
The reactions of its ionization are:
HtwoSW4 => H+ + HSO4-
Then the second H is released+ of the remaining OH group, more slowly until an equilibrium can be established:
HSO4- <=> H+ + SW4two-
The second dissociation is more difficult than the first, since a positive charge (H+) of a doubly negative charge (SO4two-).
The strength of almost all oxacids that have the same central atom (not metal) increases with the increase in the oxidation state of the central element; which in turn is directly related to the increase in the number of oxygen atoms.
For example, three series of oxacids are shown whose acidity forces are ordered from least to greatest:
HtwoSW3 < HtwoSW4
HNOtwo < HNO3
HClO < HClOtwo < HClO3 < HClO4
In most oxacids that have different elements with the same oxidation state, but belonging to the same group on the periodic table, the acidity strength increases directly with the electronegativity of the central atom:
HtwoSeO3 < HtwoSW3
H3PO4 < HNO3
HBrO4 < HClO4
As mentioned at the beginning, oxacids are generated when certain substances, called acid oxides, react with water. This will be explained using the same example for carbonic acid.
COtwo + HtwoOR <=> HtwoCO3
Acid oxide + water => oxacid
What happens is that the H moleculetwoOr is covalently bound with that of COtwo. If the water is removed by heat, the equilibrium shifts to the regeneration of COtwo; that is, a hot soda will lose its effervescent sensation sooner than a cold one.
On the other hand, acidic oxides are formed when a non-metallic element reacts with water; although, more precisely, when the reacting element forms a covalent oxide, whose dissolution in water generates H ions+.
It has already been said that H ions+ are the product of the ionization of the resulting oxacid.
Chloric oxide, CltwoOR5, reacts with water to give chloric acid:
CltwoOR5 + HtwoO => HClO3
Sulfuric oxide, SO3, reacts with water to form sulfuric acid:
SW3 + HtwoO => HtwoSW4
And periodic rust, ItwoOR7, reacts with water to form periodic acid:
ItwoOR7 + HtwoO => HIO4
In addition to these classic mechanisms for the formation of oxacids, there are other reactions with the same purpose.
For example, phosphorous trichloride, PCl3, reacts with water to produce phosphorous acid, an oxacid, and hydrochloric acid, a hydrohalic acid.
PCl3 + 3HtwoO => H3PO3 + HCl
And phosphorus pentachloride, PCl5, reacts with water to give phosphoric acid and hydrochloric acid.
PCl5 + 4 htwoO => H3PO4 + HCl
Some transition metals form acidic oxides, that is, they dissolve in water to give oxacids.
Manganese (VII) oxide (permanganic anhydrous) MntwoOR7 and chromium (VI) oxide are the most common examples.
MntwoOR7 + HtwoO => HMnO4 (permanganic acid)
CrO3 + HtwoO => HtwoCrO4 (chromic acid)
To correctly name an oxacid, we must start by determining the valence or oxidation number of the central atom E. Starting from the generic formula HEO, the following is considered:
-The O has valence -2
-The valence of the H is +1
With this in mind, the oxacid HEO is neutral, so the sum of the charges of the valences must equal zero. Thus, we have the following algebraic sum:
-2 + 1 + E = 0
E = 1
Therefore, the valence of E is +1.
Then one must resort to the possible valences that E. may have. If the values +1, +3 and +4 are among its valences, then E "works" with its lowest valence.
To name HEO, you start by calling it acid, followed by the name of E with the suffixes -ico, if it works with the highest valence, or -oso, if it works with the lowest valence. When there are three or more, the prefixes hypo- and per- are used to refer to the smallest and largest valences.
Thus, HEO would be called:
Acid hiccup(E's name)bear
Since +1 is the smallest of its three valences. And if it were HEOtwo, then E would have valence +3 and would be called:
Acid (E name)bear
And likewise for HEO3, with E working with the valence +5:
Acid (E name)ico
A series of oxacids with their respective nomenclatures are mentioned below..
Halogens intervene by forming oxacids with the valences +1, +3, +5 and +7. Chlorine, bromine and iodine can form 4 types of oxacids corresponding to these valences. But the only oxacid that has been prepared from fluorine is hypofluorous acid (HOF), which is unstable..
When an oxacid of the group uses the valence +1, it is named as follows: hypochlorous acid (HClO); hypobromous acid (HBrO); hypoiodine acid (HIO); hypofluoro acid (HOF).
With the valence +3 no prefix is used and only the suffix bear is used. Chlorous acids (HClOtwo), bromous (HBrOtwo), and iodine (HIOtwo).
With the valence +5 no prefix is used and only the suffix ico is used. Chloric acids (HClO3), bromic (HBrO3) and iodine (HIO3).
Whereas when working with the valence +7, the prefix per and the suffix ico are used. Perchloric acids (HClO4), perbromic (HBrO4) and periodic (HIO4).
The nonmetal elements of this group have the most common valences -2, +2, +4, and +6, forming three oxacids in the most well-known reactions.
With the valence +2 the prefix hiccup and the suffix bear are used. Hyposulfurous acids (HtwoSWtwo), hyposelenious (HtwoSeOtwo) and hypotelous (HtwoTeOtwo).
With the valence +4 no prefix is used and the suffix bear is used. Sulfurous acids (HtwoSW3), selenious (HtwoSeO3) and tellurium (HtwoTeO3).
And when they work with the valence + 6, no prefix is used and the suffix ico is used. Sulfuric acids (HtwoSW4), selenic (HtwoSeO4) and telluric (HtwoTeO4).
Boron has valence +3. They have metabolic acids (HBOtwo), pyroboric (H4BtwoOR5) and orthoboric (H3BO3). The difference is in the number of water that reacts with boric oxide.
Carbon has valences +2 and +4. Examples: with valence +2, carbonaceous acid (HtwoCOtwo), and with valence +4, carbonic acid (HtwoCO3).
Chromium has valences +2, +4, and +6. Examples: with valence 2, hypochromic acid (HtwoCrOtwo); with valence 4, chromous acid (HtwoCrO3); and with valence 6, chromic acid (HtwoCrO4).
Silicon has valences -4, +2, and +4. It has the metasilicic acid (HtwoYes3), and pyrosilicic acid (H4Yes4). Note that in both the Si has valence +4, but the difference lies in the number of water molecules that reacted with its acid oxide.
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