por What are hydrocids?
The hydrocids, or binary acids, are compounds dissolved in water that are made up of hydrogen and a non-metallic 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 group 16 elements excluding oxygen. Unlike oxoacids, hydracids lack oxygen. Since hydrocids are covalent or molecular compounds, the HX bond must be considered. This is of great importance and defines the characteristics of each hydrocid.
ANDHX link
What can be said about the HX link? As can be seen in the image above, there is a permanent dipole moment due to the different electronegativities between H and X. Because X is usually more electronegative than H, it attracts its electron cloud and ends up with a partial negative δ- charge.
Instead H, by giving up part of its electronic density to X, ends up with a partial positive charge δ+. The more negative δ- is, the more electron-rich X will be, and the greater the electron deficiency of H will be. Thus, depending on which element X is, a hydrocid can be more or less polar.
The image also reveals the structure of the hydrocids. HX is a linear molecule, which can interact with another by one of its ends. The more polar HX is, the more strongly or affinity its molecules will interact. As a result, their boiling or melting points will increase.
However, the HX — HX interactions are still weak enough to form a solid hydrocid. For this reason, under conditions of pressure and room temperature they are gaseous substances; with the exception of HF, which evaporates above 20ºC.
Because? Because HF is capable of forming strong hydrogen bonds. While the other hydrocids, whose non-metallic elements are less electronegative, can hardly be in the liquid phase below 0ºC. HCl, for example, boils at approximately -85ºC.
Are hydrocids acidic substances? The answer lies in the partial positive 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 is the case with all halogen hydracids, once their respective halides are dissolved in water.
Characteristics of hydrocids
physical
transparent solutions
Visibly all hydrocids are clear solutions, since HX are highly soluble in water. They may have yellowish tones according to the concentrations of dissolved HX.
they are smokers
This means that they give off dense, corrosive and irritating vapors (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.
They are conductors of electricity
Hydrocids 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.
Their boiling points are higher than those of their anhydrous forms.
That is, HX(aq), which denotes the hydracid, boils at temperatures higher than HX(g). For example, hydrogen chloride, HCl(g), boils at -85ºC, but hydrochloric acid, its hydrocid, around 48ºC.
Because? Because the gaseous molecules of HX are surrounded by those of water. Two types of interactions can occur between them at the same time: hydrogen bonds, HX — H2O — HX, or ion solvation, H3O+(aq) and X–(aq). This fact is directly related to the chemical characteristics of hydrocids.
chemical
Hydrocids are highly acidic solutions, so they have H3O+ acidic protons available to react with other substances.
Where does H3O+ come from? From the hydrogen atom with partial positive charge δ+, which dissociates in water and ends up being covalently incorporated into a water molecule:
HX(aq) + H2O(l) <=> X–(aq) + H3O+(aq)
Note that the equation corresponds to a reaction that establishes an equilibrium. When the formation of X–(aq) + H3O+(aq) is thermodynamically highly favored, HX will release its acidic proton to water; and then this, with H3O+ as its new “carrier”, can react with another compound, even if the latter is not a strong base.
This explains the acid characteristics of the hydrocids. 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 to the right, that is, towards X–(ac) + H3O+(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 rupture by the action of water.
Likewise, the HF link has much better overlap for reasons of atomic radii. On the other hand, the H-Cl, H-Br or HI bonds are weaker and tend to completely dissociate in water, to the point of breaking the equilibrium stated above.
This is because the other halogens or chalcogens (sulphur, for example) have larger atomic radii and therefore larger orbitals. Consequently, the HX bond presents poorer orbital overlap as X is larger, which in turn affects the acid strength when in contact with water.
Thus, the decreasing order of acidity for the halogen hydrocids is as follows: HF< HCl
Hydracid nomenclature
anhydrous form
In their anhydrous forms, HX(g), they should be listed as for hydrogen halides: adding the suffix -aurochs 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: iodaurochs of hydrogen. Because nonmetals are generally more electronegative than hydrogen, hydrogen 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(aq) and the hydracid is then obtained.
in aqueous solution
To name the hydracid, HX(ac), the suffix must be replaced -aurochs of their anhydrous forms by the suffix –hydric. And they should be mentioned as acids in the first place. Thus, for the example above, the HI(aq) is named as: iod acidhydric.
How are hydrocids formed?
Direct dissolution of hydrogen halides
Hydrocids 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 highly soluble in water, so there is no solubility equilibrium, 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) +H2SO4(aq) => HCl(aq) +NaHSO4(aq)
Sulfuric acid donates one of its acidic protons to the chloride anion Cl–, 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 hydrogen sulfate, NaHSO4.
Another way to produce it is to replace sulfuric acid with concentrated phosphoric acid:
NaCl(s) + H3PO4(aq) => HCl(aq) + NaH2PO4(aq)
H3PO4 reacts in the same way as H2SO4, producing hydrochloric acid and sodium dihydrogen phosphate. NaCl is the source of the Cl– anion, so to synthesize the other hydrocids, salts or minerals containing F–, Br–, I–, S2-, etc. are needed.
But the use of H2SO4 or H3PO4 will depend on its oxidative strength. H2SO4 is a very strong oxidizing agent, to such an extent that it oxidizes even Br– and I– to their molecular forms Br2 and I2; the first is a reddish liquid, and the second is a purple solid. Therefore, H3PO4 represents the preferred alternative in such syntheses.
Uses of hydrocids
Cleaners and solvents
Hydracids are essentially used to dissolve different types of matter. This is because they are strong acids, and in moderation they can clean any surface.
Its acidic protons are added to the compounds of the impurities or dirt, making them soluble in the aqueous medium and they are then washed 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 would dissolve it instantly. Hydrochloric acid is used to remove stains on swimming pool tiles.
Likewise, they are capable of dissolving rocks or solid samples, to later be 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 it takes. This is where the hydrocids come in.
An example of this is the use of hydroiodic acid in the synthesis of glacial acetic acid. The oil industry also needs hydrocids in refinery processes.
Reagents for the synthesis of organic and inorganic compounds
Hydrocids not only contribute acidic protons, but also their respective anions. These anions can react with an organic or inorganic compound to thus form a specific halide.
In this way, fluorides, chlorides, iodides, bromides, selenides, sulfides, and other compounds can be synthesized.
These halides can have very diverse applications. For example, they can be used to synthesize polymers, such as Teflon; or intermediates, from which the halogen atoms will be incorporated into the molecular structures of certain drugs.
Suppose the molecule CH3CH2OH, ethanol, reacts with HCl to form ethyl chloride:
CH3CH2OH + HCl => CH3CH2Cl + H2O
Each of these reactions hides a mechanism and many aspects that are considered in organic synthesis.
Examples of hydrocids
There are not many examples available for hydrocids, since the number of naturally possible compounds is limited. For this reason, they are listed below…