Hydrogen fluoride: characteristics and application

Among the compounds of halogens - elements of group 7 of the main subgroup of the periodic system of chemical elements of DI Mendeleev - hydrogen fluoride is of great practical importance. Along with other halogenated hydrogen, it is used in various branches of the national economy: for obtaining 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.

History of the discovery

In the 17th century K. Schwanquard conducted an experiment with a mineral of fluorspar and sulfate acid. The scientist discovered that during the reaction a gas was released, which began to destroy a plate of glass, covering the 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 material: fluorite and sulfuric acid. Ampere proved by his experiments that the structure of the molecule HF is similar to hydrogen chloride. This also applies to aqueous solutions of these halogens. The differences concern the strength of acids: the hydrofluoric is weak, and the chloride is strong.

Physical properties

Gas with chemical formula HF has a sharp characteristic smell, colorless, slightly lighter than air. In the series of HI-HBr-HCl halogenides, the boiling and melting points change smoothly, and on going to HF sharply increase. The explanation of this phenomenon is the following: 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 points increase. According to the gas density indices , in an interval 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 is the hydrofluoride

The methods of obtaining a substance not in laboratory conditions, which we have already mentioned, but in industry, have practically nothing to do with each other: reagents are all the same fluorspar (fluorite) and sulfate acid.

The mineral, whose deposits are located in Primorye, Transbaikalia, Mexico, USA, are first enriched by flotation, and then used in the manufacturing process for the production of HF, which is carried out in special steel furnaces. They load ore and mix with sulfate acid. The enriched ore contains 55-60% fluorite _. The walls of the furnace are lined with lead sheets that capture hydrogen fluoride. It is purified in a washing column, cooled, and then condensed. To produce hydrogen fluoride, rotary kilns are used, indirectly heated by electricity. The mass fraction of HF at the outlet is approximately 0.98, but the process has its drawbacks. It is quite long and requires a large consumption of sulphate acid.

The polarity of HF molecules

Anhydrous hydrogen fluoride consists of particles having the ability to bind to each other 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 the polar covalent bond. It is represented by a common electronic pair displaced to a more electronegative fluorine atom. As a result, the fluoride hydride molecules become polar and have the form of dipoles.

Between them there are forces of electrostatic attraction, which leads to the appearance of associates. The length of the chemical bond between the hydrogen and fluorine atoms is 92 nm, and its energy is 42 kJ / mol. Both in gaseous and in liquid state the substance consists of a polymer mixture of the type 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. With oxidative properties, HF reduces the above compounds to carbon dioxide, silicon tetrafluoride, hydrogen sulphide and nitrogen oxides. Aqueous 40% hydrogen fluoride solution destroys concrete, glass, leather, rubber, and also interacts with some oxides, for example with 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.

An 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 its chemical properties depends on its concentration, unlimitedly dissolves in water. In this case, heat is released, which characterizes this process as exothermic. Being an acid of medium strength, the aqueous solution of HF interacts with metals (a substitution reaction). Salts are formed - fluorides - and hydrogen is released. Passive metals - platinum and gold, as well as lead - do not react with hydrofluoric acid. The acid passes through it, that is, forms a protective film on the surface of the metal, consisting of insoluble lead fluoride. An aqueous solution of HF can contain impurities of iron, arsenic, sulfur dioxide, in which case it is called a technical acid. A concentrated 60% solution of HF is important in the chemistry of organic synthesis. It is stored in polyethylene or Teflon containers, and a large-tonnage hydrofluoric acid is transported in steel cisterns.

The role of hydrofluoric acid in the national economy

A solution of hydrogen fluoride is used for the production of ammonium borafluoride, 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 resistant to the action of mineral acids.

Floats give waterproof properties to building materials. Care should be taken in the process of their application, since all silicofluorides are toxic. An aqueous solution of HF is also used in the production of synthetic lubricating oils. Unlike mineral ones, they retain viscosity and form a protective film on the surface of working parts: compressors, reducers, bearings both at high and low temperatures. Of great importance is aqueous hydrogen fluoride in the etching (matting) of glass, as well as in the semiconductor industry, where it is used for etching silicon.

Fluorinated plastics

The most popular of them is Teflon (fluoroplastic - 4). It was discovered by accident. The organic chemist Roy Plunkett, who was engaged in the synthesis of freons, found gas in bottles with gaseous chlorine ethylene stored at an abnormally low temperature, not gas, but a white powder that was 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 food, chemical industry, in the production of dishes with non-stick properties. Even at 70 ^o C, fluoroplastic products - 4 do not lose their properties. Exceptional is the high chemical inertness of Teflon. It does not decompose on contact with corrosive substances - alkalis and acids. This is very important for equipment used in the technological processes for the production of nitrate and sulfate acids, ammonium hydroxide, sodium hydroxide. Fluoroplastic can contain additional components - modifiers such as glass fibers or metals, so that they change their properties, for example, increase heat resistance and wear resistance.

Dissociation of hydrogen fluoride

Earlier, we mentioned that a strong covalent bond is formed in 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 decomposed into ions in aqueous solution. Hydrofluoric acid is weaker than chloride or bromide. These features of its dissociation explain the existence of persistent, acidic salts, whereas neither chloride nor iodine forms them. The dissociation constant of the aqueous solution of hydrogen fluoride is 7x10 ^-4 , which is confirmed by the fact that in its solution there is a large number of undissociated molecules and a low content of hydrogen and fluoride ions is noted.

Than fluorhydride is dangerous

It should be noted that both gaseous and liquid hydrogen fluoride are toxic. The substance code is 0342. Hydrofluoric acid also possesses narcotic properties. On its impact on the human body we will stop a little later. In the classifier, this substance, as well as anhydrous hydrofluoride, is in the second class of danger. This is explained, first of all, by the ability of fluorine compounds to ignite easily. In particular, this property is especially evident in a compound such as gaseous hydrogen fluoride, whose fire and explosion hazard is particularly high.

Why determine the level of hydrogen fluoride in the air

In industrial production of HF, obtained from fluorspar and sulfuric acid, losses of gaseous product, whose vapors enter the atmosphere, are possible. Recall that hydrogen fluoride (whose class of danger is the second) - the substance is highly toxic and requires constant measurement of its concentration. Industrial emissions contain a large number of harmful and potentially hazardous chemicals, primarily nitrogen and sulfur oxides, heavy metal sulfides, and gaseous hydrogen halides. Among them, a large fraction falls to hydrogen fluoride, whose MPC in the ambient air is 0.005 mg / m ³ in terms of fluorine per day. For the plant sites where the 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 how many have got into the atmosphere, there are special measuring instruments. To detect HF vapors, photocolorimetric gas analyzers are used, in which both incandescent and semiconductor light-emitting diodes are used as radiation sources, and photodiodes and phototransistors play the role of photodetectors. Determination of hydrogen fluoride in atmospheric air is also carried out by infrared gas analyzers. They are quite sensitive. HF molecules absorb long-wave 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 permissible norm and in isolated extreme cases (man-made disasters, technological cycle failures due to power supply failures, etc.). These functions are performed by thermo-conductometric gas analyzers of hydrogen fluoride. Prom. They differentiate emissions on the basis of the dependence of the thermal conductivity of HF on the composition of the gaseous mixture.

The harmful effect of hydrofluoride on the human body

Both anhydrous hydrogen fluoride and hydrofluoric acid, which is its solution in water, belong to the second class of danger. Especially negatively, these compounds affect vital systems: cardiovascular, excretory, respiratory, as well as skin and mucous membranes. Penetration of the substance through the skin passes unnoticed and asymptomatic. The phenomena of toxicosis can be manifested the next day, and they are diagnosed as avalanche, namely: the skin is ulcerated, on the surface of the mucous eyes, burn areas are formed. The tissues of the lungs are destroyed due to necrotic lesions of the alveoli. Fluorine ions that get into the intercellular fluid, then penetrate into the cells and bind in them the particles of magnesium and calcium, which are part of the nervous tissue, blood, and kidney tubules - structures of nephrons. Therefore, it is especially important to carefully monitor the content of gaseous hydrogen fluoride and hydrofluoric acid vapors in the atmosphere.