Among the compounds of halogens - elements of group 7the main subgroup of the periodic system of chemical elements of DI Mendeleyev - hydrogen fluoride is of great practical importance. Along with other halogenated hydrogen, it is used in various branches 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.
History of discovery
In the 17th century, K.Schwankward 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 centuryGay-Lussac of the same raw materials: fluorite and sulfuric acid. Ampere with his experiments proved that the structure of the molecule HF is similar to hydrogen chloride. This also applies to aqueous solutions of these halogens. Differences concern the strength of acids: the hydrofluoric is weak, and the chloride is strong.
Physical properties
The gas with the chemical formula HF has a sharpcharacteristic odor, colorless, slightly lighter than air. In the series of hydrogen halides, HI-HBr-HCl-the boiling and melting points change smoothly, and in the transition 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 are formed). 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 HF2. When heated above 25 aboutWith these complexes gradually decompose, and at a temperature of about 90 aboutWith hydrogen, the fluoride consists of HF molecules.
How is the hydrofluoride
Methods of obtaining a substance not in laboratoryconditions, which we have already mentioned, but in industry, there is practically nothing to differ: reagents are all the same fluorspar (fluorite) and sulfate acid.
The mineral, whose deposits are located in Primorye,Transbaikalia, Mexico, USA, 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. Enriched ore contains 55-60% fluorite. The walls of the furnace are lined with lead sheets,capturing 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 sulfate acid.
The polarity of HF molecules
Anhydrous hydrogen fluoride consists of particles,They have 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. Because of this, the fluoride hydride molecules become polar and have the form of dipoles.
Between them arise the forces of electrostaticattraction, 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 the gaseous and in the liquid state, the substance consists of a polymer mixture of the form H2F2, H4F4.
Chemical properties
Anhydrous hydrogen fluoride has the ability tointeraction 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. The aqueous 40% hydrogen fluoride solution destroys concrete, glass, leather, rubber, and also interacts with some oxides, for example Cu2O. 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 andis 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, an 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 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 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 productionammonium boron fluoride, 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 Na2SiF6. It is used to produce cements and enamels resistant to the action of mineral acids.
Fluxes are attached to building materialswaterproof properties. 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 these 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 newplastic 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 aboutWith products made of fluoroplastic - 4 do not lose theirproperties. 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, caustic soda. Fluoroplastic can contain additional components - modifiers, such as glass fibers or metals, as a result of which they change their properties, for example, increase heat resistance and wear resistance.
Dissociation of hydrogen fluoride
Earlier, we mentioned that HF molecules forma strong covalent bond, in addition, they themselves are able to combine in 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, acid 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 it is toxicgaseous, and liquid hydrogen fluoride. 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 the industrial production of HF, obtained fromfluorspar and sulfuric acid, losses of gaseous product, whose vapors enter the atmosphere, are possible. Recall that hydrogen fluoride (the class of danger of which is the second one) - 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 on hydrogen fluoride, whose maximum concentration in the ambient air is 0.005 mg / m3 in terms of fluoride per day. For factory sites where the drum furnaces are located, the maximum permissible concentration (MPC) should be 0, 1 mg / m3.
Hydrogen fluoride gas analyzers
To find out which harmful gases and in whatthe quantity hit 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. The 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 thermal conductivity metering gas analyzer 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 a hydrofluoricthe acid, which is its solution in water, belongs 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 becomes ulcerated, burns are formed on the surface of the mucous eyes. Lung tissue is 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.
