HYDRACIDS: CHARACTERISTICS, NOMENCLATURE, USES AND EXAMPLES - CHEMISTRY - 2025

The hydrohalic or binary acids are dissolved in water compounds which consist of hydrogen and a nonmetallic element: hydrogen halides. Its general chemical formula can be expressed as HX, where H is the hydrogen atom, and X is the non-metallic element.

X can belong to group 17, the halogens, or to the elements of group 16 without including oxygen. Unlike oxoacids, hydracids lack oxygen. Since hydracids are covalent or molecular compounds, the HX bond must be considered. This is of great importance and defines the characteristics of each hydracid.

Source: Gabriel Bolívar

What about the HX link? As can be seen in the image above, there is a permanent dipole moment product of the different electronegativities between H and X. Since X is usually more electronegative than H, it attracts its electron cloud and ends up with a negative partial charge δ-.

On the other hand, H, yielding part of its electron density to X, ends up with a positive partial charge δ +. The more negative δ- is, the richer in electrons X will be and the greater the electron deficiency of H. Therefore, depending on which element X is, a hydracid can be more or less polar.

The image also reveals the structure of the hydracids. HX is a linear molecule, which can interact with another at one of its ends. The more polar HX is, the more strongly or affinity its molecules will interact with. As a result, its boiling or melting points will increase.

However, the HX-HX interactions are still weak enough to give rise to a solid hydracid. For this reason, under conditions of pressure and ambient temperature they are gaseous substances; With the exception of HF, which evaporates above 20ºC.

Why? Because HF is capable of forming strong hydrogen bonds. While the other hydracids, whose non-metallic elements are less electronegative, they can hardly be in the liquid phase below 0ºC. HCl, for example, boils at about -85 ° C.

Are hydracids acidic substances? The answer lies in the positive partial charge δ + on the hydrogen atom. If δ + is very large or the HX bond is very weak, then HX will be a strong acid; As with all the hydroacids of the halogens, once their respective halides are dissolved in water.

characteristics

Physical

-Visibly all hydracids are transparent solutions, since HX are very soluble in water. They may have yellowish tones according to the concentrations of dissolved HX.

-They are smokers, which means that they give off dense, corrosive and irritating fumes (some of them are even nauseating). This is because the HX molecules are very volatile and interact with the water vapor in the medium surrounding the solutions. Furthermore, HX in its anhydrous forms are gaseous compounds.

-Hydracids are good conductors of electricity. Although HX are gaseous species at atmospheric conditions, when they dissolve in water they release ions (H ⁺ X ^-), which allow the passage of electric current.

-Its boiling points are higher than those of its anhydrous forms. That is, HX (ac), which denotes the hydracid, boils at temperatures above HX (g). For example, hydrogen chloride, HCl (g), boils at -85ºC, but hydrochloric acid, its hydracid, is around 48ºC.

Why? Because the gaseous HX molecules are surrounded by those of water. Two types of interactions can occur at the same time: hydrogen bonds, HX - H ₂ O - HX, or ion solvation, H ₃ O ⁺ (aq) and X ^- (aq). This fact is directly related to the chemical characteristics of hydracids.

Chemical

Hydracids are very acidic solutions, so they have acidic protons H ₃ O ⁺ available to react with other substances. Where does H ₃ O ^{+ come from} ? From the hydrogen atom with a positive partial charge δ +, which dissociates in water and ends up covalently incorporating into a water molecule:

HX (aq) + H ₂ O (l) <=> X ^- (aq) + H ₃ O ⁺ (aq)

Note that the equation corresponds to a reaction that establishes an equilibrium. When the formation of X ^- (aq) + H ₃ O ⁺ (aq) is thermodynamically highly favored, HX will release its acidic proton into water; and then it, with H ₃ O ⁺ as its new "carrier", can react with another compound, even if the latter is not a strong base.

The above explains the acidic characteristics of hydracids. This is the case for all HX dissolved in water; but some generate more acidic solutions than others. For what is this? The reasons can be very complicated. Not all HX (ac) favor the above equilibrium towards the right, that is, towards X ^- (ac) + H ₃ O ⁺ (ac).

Acidity

And the exception is observed in hydrofluoric acid, HF (aq). Fluorine is very electronegative, therefore, it shortens the distance of the HX bond, strengthening it against its breakdown by the action of water.

Likewise, the HF bond has much better overlap for atomic radius reasons. On the other hand, the H-Cl, H-Br or HI bonds are weaker and tend to dissociate completely in water, to the point of breaking the balance raised above.

This is because the other halogens or chalcogens (sulfur, for example), have larger atomic radii and, therefore, larger orbitals. Consequently, the HX bond exhibits poorer orbital overlap as X is larger, which in turn affects the acid force when in contact with water.

Thus, the decreasing order of acidity for the hydro acids of the halogens is the following: HF <HCl

Nomenclature

Anhydrous form

How are hydracids named? In their anhydrous forms, HX (g), they must be mentioned as dictated for hydrogen halides: by adding the suffix –uro at the end of their names.

For example, HI (g) consists of a halide (or hydride) formed by hydrogen and iodine, therefore its name is: hydrogen iodide. Since nonmetals are generally more electronegative than hydrogen, it has an oxidation number of +1. In NaH, on the other hand, hydrogen has an oxidation number of -1.

This is another indirect way of differentiating molecular hydrides from halogens or hydrogen halides from other compounds.

Once HX (g) comes into contact with water, it is represented as HX (ac) and the hydracid is then obtained.

In aqueous solution

To name the hydracid, HX (ac), the suffix –uro of its anhydrous forms must be replaced by the suffix –hydric. And they should be mentioned as acids in the first place. Thus, for the above example, HI (aq) is named as: yod acid water.

How are they formed?

Direct dissolution of hydrogen halides

Hydracids can be formed by simply dissolving their corresponding hydrogen halides in water. This can be represented by the following chemical equation:

HX (g) => HX (ac)

HX (g) is very soluble in water, so there is no balance of solubility, unlike its ionic dissociation to release acidic protons.

However, there is a synthetic method that is preferred because it uses salts or minerals as raw material, dissolving them at low temperatures with strong acids.

Dissolution of salts of non-metals with acids

If table salt, NaCl, is dissolved with concentrated sulfuric acid, the following reaction occurs:

NaCl (s) + H ₂ SO ₄ (aq) => HCl (aq) + NaHSO ₄ (aq)

Sulfuric acid donates one of its acidic protons to the Cl ^- chloride anion, thus converting it to hydrochloric acid. Hydrogen chloride, HCl (g), can escape from this mixture because it is very volatile, especially if its concentration in water is very high. The other salt produced is sodium acid sulfate, NaHSO ₄.

Another way to produce it is to replace sulfuric acid with concentrated phosphoric acid:

NaCl (s) + H ₃ PO ₄ (aq) => HCl (aq) + NaH ₂ PO ₄ (aq)

H ₃ PO ₄ reacts in the same way as H ₂ SO ₄, producing hydrochloric acid and sodium diacid phosphate. NaCl is the source of the Cl ^- anion, so that to synthesize the other hydracids, salts or minerals containing F ^-, Br ^-, I ^-, S ^2-, etc. are needed.

But, the use of H ₂ SO ₄ or H ₃ PO ₄ will depend on its oxidative strength. H ₂ SO ₄ is a very strong oxidizing agent, to the point that it oxidizes even Br ^- and I ^- to their Br ₂ and I ₂ molecular forms; the first is a reddish liquid, and the second a purple solid. Therefore, H ₃ PO ₄ represents the preferred alternative in such syntheses.

Applications

Cleaners and solvents

Hydracids are essentially used to dissolve different types of matter. This is because they are strong acids, and can clean any surface in moderation.

Its acidic protons are added to the compounds of impurities or dirt, making them soluble in the aqueous medium and are then carried away by the water.

Depending on the chemical nature of said surface, one hydracid or another can be used. For example, hydrofluoric acid cannot be used to clean glass as it will dissolve it on the spot. Hydrochloric acid is used to remove stains from swimming pool tiles.

They are also capable of dissolving rocks or solid samples, and then used for analytical or production purposes on small or large scales. In ion exchange chromatography, dilute hydrochloric acid is used to clean the column of remaining ions.

Acid catalysts

Some reactions require highly acidic solutions to speed them up and reduce the time they take place. This is where the hydracids come in.

An example of this is the use of hydroiodic acid in the synthesis of glacial acetic acid. The oil industry also needs hydracids in refinery processes.

Reagents for the synthesis of organic and inorganic compounds

Hydracids not only provide acidic protons, but also their respective anions. These anions can react with an organic or inorganic compound to form a specific halide. In this way, they can be synthesized: fluorides, chlorides, iodides, bromides, selenides, sulfides, and other compounds.

These halides can have very diverse applications. For example, they can be used to synthesize polymers, such as Teflon; or intermediaries, from which the halogen atoms will be incorporated into the molecular structures of certain drugs.

Suppose the molecule CH ₃ CH ₂ OH, ethanol, reacts with HCl to form ethyl chloride:

CH ₃ CH ₂ OH + HCl => CH ₃ CH ₂ Cl + H ₂ O

Each of these reactions hides a mechanism and many aspects that are considered in organic syntheses.

Examples

There are not many examples available for hydracids, since the number of possible compounds is naturally limited. For this reason, some additional hydracids with their respective nomenclature are listed below (the abbreviation (ac) is ignored):

HF, hydrofluoric acid

Binary hydracid whose HF molecules form strong hydrogen bonds, to the point that in water it is a weak acid.

H

Unlike the hydracids considered until then, it is polyatomic, that is, it has more than two atoms, however, it continues to be binary since it is two elements: sulfur and hydrogen.

Its angular MSM molecules do not form appreciable hydrogen bonds and can be detected by their characteristic rotten egg odor.

HCl, hydrochloric acid

One of the best known acids in popular culture. It is even part of the composition of gastric juice, present in the stomach, and together with digestive enzymes they degrade food.

HBr, hydrobromic acid

Like hydroiodic acid, in the gas phase it consists of linear H-Br molecules, which dissociate into H ⁺ (H ₃ O ⁺) and Br ^- ions when they enter water.

H

Although tellurium has a certain metallic character, its hydracid gives off unpleasant and highly poisonous vapors, such as hydrogen selenide.

Like the other hydracids of the chalcogenides (from group 16 of the periodic table), in solution it produces the anion Te ^2-, so its valence is -2.

References

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