- Characteristics and properties of an oxacid
- Hydroxyl groups
- Central atom
- Sulfur for sulfuric acid
- Acid strength
- How are oxacids formed?
- Training examples
- Metallic oxacids
- Nomenclature
- Calculation of valence
- Name the acid
- Examples
- Oxacids of the group of halogens
- VIA Group Oxacids
- Boron oxacids
- Carbon oxacids
- Chromium oxacids
- Silicon oxacids
- References
An oxacid or 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 therefore 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, H 2 CO 3. As its chemical formula alone demonstrates, it has three O, one C, and two H atoms.
Source: Pxhere
The two H atoms of H 2 CO 3 are released into the environment as H +, which explains its acidic characteristics. Heating an aqueous solution of carbonic acid will give off a gas.
This gas is carbon dioxide, CO 2, an inorganic molecule that originates from the combustion of hydrocarbons and cellular respiration. If CO 2 were returned to the water container, H 2 CO 3 would re-form; therefore, oxo acid is formed when a certain substance reacts with water.
This reaction is not only observed for CO 2, but for other inorganic covalent molecules called acid oxides.
Oxacids have a vast number of uses, which are difficult to describe in general. Its application will depend greatly on the central atom and the number of oxygens.
They can be used 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, H 3 PO 4, forming part of the composition of these drinks.
Characteristics and properties of an oxacid
Source: Gabriel Bolívar
Hydroxyl groups
A generic HEO 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.
Hydrogen in oxacids is usually attached to an oxygen atom and not to the central atom. Phosphorous acid, H 3 PO 3, represents a particular case where one of the hydrogens is linked to the phosphorous atom; therefore, its structural formula is best represented as (OH) 2 OPH.
While for nitrous acid, HNO 2, has a HON = 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 that it is also present as 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.
Central atom
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, the ionization of the acid occurs; that is, it acquires an electrical charge, which in its case is negative. An oxacid can release as many H + ions as there are OH groups in its structure; and the more there are, the greater the negative charge.
Sulfur for sulfuric acid
Sulfuric acid, polyprotic, has the molecular formula H 2 SO 4. This formula can also be written as follows: (OH) 2 SO 2, to emphasize that sulfuric acid has two hydroxyl groups attached to sulfur, its central atom.
The reactions of its ionization are:
H 2 SO 4 => H + + HSO 4 -
Then the second H + is released from the remaining OH group, more slowly until an equilibrium can be established:
HSO 4 - <=> H + + SO 4 2–
The second dissociation is more difficult than the first, since a positive charge (H +) must be separated from a doubly negative charge (SO 4 2-).
Acid strength
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:
H 2 SO 3 <H 2 SO 4
HNO 2 <HNO 3
HClO <HClO 2 <HClO 3 <HClO 4
In most oxacids that have different elements with the same oxidation state, but belonging to the same group in the periodic table, the acid strength increases directly with the electronegativity of the central atom:
H 2 SeO 3 <H 2 SO 3
H 3 PO 4 <HNO 3
HBrO 4 <HClO 4
How are oxacids formed?
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.
CO 2 + H 2 O <=> H 2 CO 3
Acidic oxide + water => oxacid
What happens is that the H 2 O molecule covalently binds with the CO 2 molecule. If the water is removed by heat, the equilibrium shifts to the regeneration of CO 2; 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 an oxide with a covalent character, the dissolution of which in water generates H + ions.
It has already been said that the H + ions are the product of the ionization of the resulting oxacid.
Training examples
Chloric oxide, Cl 2 O 5, reacts with water to give chloric acid:
Cl 2 O 5 + H 2 O => HClO 3
Sulfuric oxide, SO 3, reacts with water to form sulfuric acid:
SO 3 + H 2 O => H 2 SO 4
And periodic oxide, I 2 O 7, reacts with water to form periodic acid:
I 2 O 7 + H 2 O => HIO 4
In addition to these classical mechanisms for the formation of oxacids, there are other reactions with the same purpose.
For example, phosphorous trichloride, PCl 3, reacts with water to produce phosphorous acid, an oxacid, and hydrochloric acid, a hydrohalic acid.
PCl 3 + 3H 2 O => H 3 PO 3 + HCl
And phosphorous pentachloride, PCl 5, reacts with water to give phosphoric acid and hydrochloric acid.
PCl 5 + 4 H 2 O => H 3 PO 4 + HCl
Metallic oxacids
Some transition metals form acidic oxides, that is, they dissolve in water to give oxacids.
Manganese (VII) oxide (permanganic anhydrous) Mn 2 O 7 and chromium (VI) oxide are the most common examples.
Mn 2 O 7 + H 2 O => HMnO 4 (permanganic acid)
CrO 3 + H 2 O => H 2 CrO 4 (chromic acid)
Nomenclature
Calculation of valence
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:
-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 we must resort to the possible valences that E. can have. If the values +1, +3 and +4 are among its valences, E then "works" with its lowest valence.
Name the acid
To name HEO, you start by calling it acid, followed by the name of E with the suffixes –ico, if you work with the highest valence, or –oso, if you work 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:
Hypo acid (E name) bear
Since +1 is the smallest of its three valences. And if it was HEO 2, then E would have valence +3 and would be called:
Acid (E name) bear
And in the same way for HEO 3, with E working with the valence +5:
Acid (E name) ico
Examples
A series of oxacids with their respective nomenclatures are mentioned below.
Oxacids of the group of halogens
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 made from fluorine is hypofluoro 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. There are the acids chlorous (HClO 2), bromous (HBrO 2), and iodine (HIO 2).
With the valence +5 no prefix is used and only the suffix ico is used. There are chloric (HClO 3), bromic (HBrO 3) and iodic (HIO 3) acids.
While when working with the valence +7, the prefix per and the suffix ico are used. There are perchloric (HClO 4), perbromic (HBrO 4) and periodic (HIO 4) acids.
VIA Group Oxacids
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. There are the acids hyposulfurous (H 2 SO 2), hyposelenious (H 2 SeO 2) and hypotelurous (H 2 TeO 2).
With the valence +4 no prefix is used and the suffix bear is used. There are sulfurous acids (H 2 SO 3), selenious (H 2 SeO 3) and tellurous (H 2 TeO 3).
And when they work with the valence + 6, no prefix is used and the suffix ico is used. There are sulfuric acids (H 2 SO 4), selenic (H 2 SeO 4) and telluric (H 2 TeO 4).
Boron oxacids
Boron has a valence of +3. There are metabolic acids (HBO 2), pyroboric (H 4 B 2 O 5) and orthoboric (H 3 BO 3). The difference is in the number of water that reacts with boric oxide.
Carbon oxacids
Carbon has valences +2 and +4. Examples: with valence +2, carbonaceous acid (H 2 CO 2), and with valence +4, carbonic acid (H 2 CO 3).
Chromium oxacids
Chromium has valences +2, +4, and +6. Examples: with valence 2, hypochromic acid (H 2 CrO 2); with valence 4, chromous acid (H 2 CrO 3); and with valence 6, chromic acid (H 2 CrO 4).
Silicon oxacids
Silicon has valences -4, +2, and +4. You have the metasilicic acid (H 2 SiO 3), and the pyrosilicic acid (H 4 SiO 4). Note that in both, Si has a valence of +4, but the difference lies in the number of water molecules that reacted with its acid oxide.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Editor. (March 6, 2012). Formulation and nomenclature of oxacids. Recovered from: si-educa.net
- Wikipedia. (2018). Oxyacid. Recovered from: en.wikipedia.org
- Steven S. Zumdahl. (2019). Oxyacid. Encyclopædia Britannica. Recovered from: britannica.com
- Helmenstine, Anne Marie, Ph.D. (January 31, 2018). Common Oxoacid Compounds. Recovered from: thoughtco.com