Among the halogen compounds — elements of group 7 of the main subgroup of the periodic system of chemical elements of D. I. Mendeleev — hydrogen fluoride is of great practical importance. Along with other hydrogen halides, it is used in various sectors of the national economy: for the production of fluorine-containing plastics, hydrofluoric acid and its salts. In this paper, we will study the structure of the molecule, the physical and chemical properties of this substance, and consider the areas of its application.
Discovery story
In the 17th century, C. Schwankward conducted an experiment with minerals, fluorspar and sulfate acid. The scientist discovered that a gas is released during the reaction, which began to destroy a glass plate covering the test tube with a mixture of reagents. This gaseous compound is called hydrogen fluoride.
Hydrofluoric acid was obtained in the 19th century by Gay-Lussac from the same raw materials: fluorite and sulfuric acid. Ampere, through his experiments, proved that the structure of the HF molecule is similar to hydrogen chloride. This also applies to aqueous solutions of these hydrogen halides. The differences relate to the strength of acids: hydrofluoric - weak, and chloride - strong.
Physical properties
Gas with the chemical formula HF has a sharp characteristic odor, colorless, slightly lighter than air. In a series of HI-HBr-HCl-hydrogen halides, the boiling and melting temperatures change smoothly, and upon transition to HF they increase sharply. The explanation of this phenomenon is as follows: molecular hydrogen fluoride forms associates (groups of neutral particles between which hydrogen bonds arise). To break them, additional energy is required, so the boiling and melting temperatures increase. According to the gas density indicators , in the range close to the boiling point (+19.5), hydrogen fluoride consists of aggregates with an average composition of HF 2. When heated above 25 ° C, these complexes gradually decompose, and at a temperature of about 90 ° C, hydrogen fluoride consists from HF molecules.
How fluorohydride is produced
The methods for obtaining the substance are not in laboratory conditions, which we have already mentioned, but in industry, practically do not differ from each other: the reagents are the same fluorspar (fluorite) and sulfate acid.
The mineral, whose deposits are located in Primorye, Transbaikalia, Mexico, and the USA, is first enriched by flotation and then used in the technological process of producing HF, which is carried out in special steel furnaces. Ore is loaded into them and mixed with sulfate acid. Enriched ore contains 55-60% fluorite . The walls of the furnace are lined with lead sheets that trap hydrogen fluoride. It is purified in a washing column, cooled, and then condensed. To obtain hydrogen fluoride, rotary kilns are used, indirectly heated by electricity. The mass fraction of HF in the output is approximately 0.98, but the process has its drawbacks. It is quite long and requires a large consumption of sulfate acid.
HF molecule polarity
Anhydrous hydrogen fluoride is composed of particles having the ability to bind together and form aggregates. This is explained by the internal structure of the molecule. Between the hydrogen and fluorine atoms there is a strong chemical bond called polar covalent. It is represented by a common electron pair shifted to a more electronegative fluorine atom. As a result, fluoride molecules become polar and have the form of dipoles.
Between them, electrostatic attraction forces arise, which leads to the appearance of associates. The chemical bond between the hydrogen and fluorine atoms is 92 nm, and its energy is 42 kJ / mol. Both in the gaseous and in the liquid state, the substance consists of a polymer mixture of the form H 2 F 2 , H 4 F 4 .
Chemical properties
Anhydrous hydrogen fluoride has the ability to interact with salts of carbonate, silicate, nitrite and sulfide acids. By exhibiting oxidizing properties, HF reduces the above compounds to carbon dioxide, silicon tetrafluoride, hydrogen sulfide and nitrogen oxides. An aqueous 40% hydrogen fluoride solution destroys concrete, glass, leather, rubber, and also interacts with certain oxides, for example, Cu 2 O. Free copper, copper fluoride, and water are found in the products. There is a group of substances with which HF does not react, for example, heavy metals, as well as magnesium, iron, aluminum, nickel.
Aqueous solution of hydrogen fluoride
It is called hydrofluoric acid and is used in the form of 40% and 72% solutions. Hydrogen fluoride, the characteristic of the chemical properties of which depends on its concentration, is unlimitedly dissolved in water. In this case, heat is released, which characterizes this process as exothermic. Being an acid of medium strength, an aqueous solution of HF interacts with metals (substitution reaction). Salts — fluorides — are formed and hydrogen is released. Passive metals such as platinum and gold, as well as lead, do not react with hydrofluoric acid. The acid passivates it, that is, forms a protective film on the metal surface, consisting of insoluble lead fluoride. An aqueous solution of HF may contain impurities of iron, arsenic, sulfur dioxide, in which case it is called technical acid. A concentrated 60% HF solution is important in organic chemistry. It is stored in polyethylene or Teflon containers, and large tonnage of hydrofluoric acid is transported in steel tanks.
The role of hydrofluoric acid in the national economy
Hydrogen fluoride solution is used to produce ammonium borofluoride, which is a component of fluxes in ferrous and non-ferrous metallurgy. It is also used in the electrolysis process to produce pure boron. Hydrofluoric acid is used in the production of silicofluorides, for example, such as Na 2 SiF 6 . It is used to produce cements and enamels that are resistant to the action of mineral acids.
Fluates give waterproof materials to building materials. In the process of their application, care must be taken, since all cremnefluorides are toxic. An aqueous HF solution is also used in the manufacture of synthetic lubricants. Unlike mineral ones, they retain viscosity and form a protective film on the surface of working parts: compressors, gearboxes, bearings both at high and low temperatures. Of great importance is hydrogen fluoride in glass etching (matting), as well as in the semiconductor industry, where it is used for silicon etching.
Fluorinated plastics
The most popular of them is Teflon (fluoroplast - 4). It was opened completely by accident. The organic chemist Roy Plunkett, who was involved in the synthesis of freons, found in cylinders with gaseous tetrachloride ethylene stored at an abnormally low temperature, not gas, but a white powder, greasy to the touch. It turned out that at high pressure and low temperature, tetrafluoroethylene polymerized.
This reaction led to the formation of a new plastic mass. Subsequently, it was called Teflon. It has exceptional heat and frost resistance. Teflon coatings are successfully used in the food and chemical industries, in the manufacture of cookware with non-stick properties. Even at 70 ° C, fluoroplastic - 4 products do not lose their properties. Exceptional is the high chemical inertness of Teflon. It does not collapse upon contact with aggressive substances - alkalis and acids. This is very important for equipment used in technological processes for the production of nitrate and sulfate acids, ammonium hydroxide, sodium hydroxide. Fluoroplastics may contain additional components - modifiers, such as fiberglass or metals, as a result of which they change their properties, for example, increase heat resistance and wear resistance.
Hydrogen fluoride dissociation
We mentioned earlier that a strong covalent bond forms in the HF molecules, in addition, they themselves are able to combine into aggregates, forming hydrogen bonds. That is why hydrogen fluoride has a low degree of dissociation and is poorly split into ions in an aqueous solution. Hydrofluoric acid is weaker than chloride or bromide. These features of its dissociation explain the existence of persistent, acidic salts, while neither chloride nor iodine form them. The dissociation constant of an aqueous solution of hydrogen fluoride is 7x10 -4 , which is confirmed by the fact that a large number of undissociated molecules are present in its solution and a low content of hydrogen and fluorine ions is noted.
What is the danger of fluoride
It should be noted that both gaseous and liquid hydrogen fluoride are toxic. The code of the substance is 0342. Hydrofluoric acid also has narcotic properties. We will dwell on its effect on the human body a little later. In the classifier, this substance, as well as anhydrous fluorohydride, is in the second hazard class. This is due primarily to the ability of fluorine compounds to easily ignite. In particular, this property is especially manifested in such a compound as gaseous hydrogen fluoride, the fire and explosion hazard of which is especially great.
Why determine the level of hydrogen fluoride in the air
In the industrial production of HF, obtained from fluorspar and sulfuric acid, there may be a loss of gaseous product, the vapors of which enter the atmosphere. Recall that hydrogen fluoride (whose hazard class is second) is a highly toxic substance and requires constant measurement of its concentration. Industrial emissions contain a large number of harmful and potentially hazardous chemicals, primarily oxides of nitrogen and sulfur, sulfides of heavy metals, as well as gaseous hydrogen halides. Among them, a large proportion is accounted for by hydrogen fluoride, whose MPC in atmospheric air is 0.005 mg / m 3 in terms of fluorine per day. For factory sites where drum furnaces are located, the maximum permissible concentration (MPC) should be 0.1 mg / m 3 .
Hydrogen fluoride gas analyzers
To find out which harmful gases and in what quantity have entered the atmosphere, there are special measuring instruments. To detect HF vapors, photocolorimetric gas analyzers are used, in which both incandescent lamps and semiconductor LEDs are used as radiation sources, and photodiodes and phototransistors play the role of photodetectors. The determination of hydrogen fluoride in atmospheric air is also carried out by infrared gas analyzers. They are quite sensitive. HF molecules absorb long-wavelength radiation in the range of 1-15 microns. Devices used to determine toxic waste in the air and in the working area of industrial enterprises record fluctuations in HF concentration both within the acceptable limits and in isolated extreme cases (industrial disasters, disruption of technological cycles due to damage to power supply, etc.). These functions are performed by thermoconductometric hydrogen fluoride gas analyzers. Prom. they differentiate emissions based on the dependence of the thermal conductivity HF on the composition of the gaseous mixture.
The harmful effects of fluoride on the human body
Both anhydrous hydrogen fluoride and hydrofluoric acid, which is its solution in water, belong to the second hazard class. Especially negatively these compounds affect vital systems: cardiovascular, excretory, respiratory, as well as skin and mucous membranes. The penetration of the substance through the skin passes imperceptibly and asymptomatically. The phenomena of toxicosis can occur on the next day, and they are diagnosed in an avalanche-like manner, namely: the skin ulcerates, burns form on the surface of the eye mucosa. Lung tissue is destroyed due to necrotic damage to the alveoli. Fluorine ions that enter the intercellular fluid then penetrate into the cells and bind to them particles of magnesium and calcium, which are part of the nervous tissue, blood, and also renal tubules - nephron structures. Therefore, careful monitoring of the content of gaseous hydrogen fluoride and hydrofluoric acid vapor in the atmosphere is especially relevant.