Fluorine (F) is the most reactive chemical element and the lightest halogen of group 17 (VIIa) of the periodic table. This characteristic of fluorine is explained by its ability to attract electrons (this is the most electronegative element) and the small size of its atoms.
Discovery story
Fluorine-containing mineral fluorspar was described in 1529 by the German physician and mineralogist George Agricola. It is likely that hydrofluoric acid was first obtained by an unknown English glassblower in 1720. In 1771, the Swedish chemist Karl Wilhelm Scheele obtained crude hydrofluoric acid by heating fluorspar with concentrated sulfuric acid in a glass retort, which corroded to a large extent under the action of the obtained product. . Therefore, in further experiments, metal vessels were used. Almost anhydrous acid was obtained in 1809, and two years later, the French physicist Andre-Marie Ampère suggested that it was a hydrogen compound with an unknown element similar to chlorine, for which he proposed the name fluorine from the Greek φθόριος, “destructive”. Fluorspar turned out to be calcium fluoride.
The release of fluorine remained one of the main unsolved problems of inorganic chemistry until 1886, when the French chemist Henri Moissant obtained an element by electrolysis of a solution of potassium hydrofluoride in hydrogen fluoride. For this in 1906 he received the Nobel Prize. The difficulty in handling this element and the toxic properties of fluorine have contributed to the slow progress in the chemistry of this element. Until World War II, he was a laboratory curiosity. Then, however, the use of uranium hexafluoride in the separation of uranium isotopes, along with the growing industrial importance of organic compounds of this element, made it a chemical that brings significant benefits.
Prevalence
Fluorine-containing fluorspar (fluorite, CaF 2 ) has been used for many centuries as a flux (cleaning agent) in metallurgical processes. The mineral subsequently turned out to be the source of the element, which was also called fluor. Colorless transparent crystals of fluorite under light have a bluish tint. This property is known as fluorescence.
Fluorine is an element that is found in nature only in the form of its chemical compounds, with the exception of extremely small amounts of the free element in fluorspar, exposed to radiation of radium. The content of the element in the earth's crust is about 0.065%. The main fluorine-containing minerals are fluorspar, cryolite (Na 3 AlF 6 ), fluorapatite (Ca 5 [PO 4 ] 3 [F, Cl]), topaz (Al 2 SiO 4 [F, OH] 2 ) and lepidolite.
Physical and chemical properties of fluoride
At room temperature, fluorine is a pale yellow gas with an irritating odor. Inhaling it is dangerous. When cooled, it becomes a yellow liquid. There is only one stable isotope of this chemical element - fluorine-19.
The first ionization energy of this halogen is very high (402 kcal / mol), which is the standard heat of formation of the F + cation 420 kcal / mol.
The small size of the atom of the element allows you to place a relatively large number of them around the central atom with the formation of many stable complexes, for example, hexafluorosilicate (SiF 6 ) 2- and hexafluoroaluminate (AlF 6 ) 3- . Fluorine is an element that has the most powerful oxidizing properties. No other substance is capable of oxidizing a fluoride anion so that it turns into a free element, and for this reason the element is not in a free state in nature. For more than 150 years, this characteristic of fluorine has not made it possible to obtain it by any chemical method. This was only possible through the use of electrolysis. Nevertheless, in 1986, the American chemist Karl Christ announced the first "chemical" production of fluoride. He used K 2 MnF 6 and antimony pentafluoride (SbF 5 ), which can be obtained from HF solutions.
Fluorine: valency and oxidation state
The outer halogen shell contains an unpaired electron. This is why fluorine valency in compounds is unity. However, the atoms of the elements of group VIIa can increase the number of such electrons to 7. The maximum valence of fluorine and its oxidation state are -1. An element is not able to expand its valence shell, since its atom lacks a d-orbital. Other halogens, due to its presence, are capable of exhibiting valency up to 7.
High oxidizing ability of the element allows to achieve the highest possible degree of oxidation in other elements. Fluorine (valency I) can form compounds that no other halides exist: silver difluoride (AgF 2 ), cobalt trifluoride (CoF 3 ), rhenium heptafluoride (ReF 7 ), bromine pentafluoride (BrF 5 ) and iodine heptafluoride (IF 7 ).
Connections
The fluorine formula (F 2 ) is composed of two element atoms. It can join compounds with all other elements except helium and neon, forming ionic or covalent fluorides. Some metals, such as nickel, quickly become coated with a layer of this halogen, which prevents further metal bonding to the element. Some dry metals, such as mild steel, copper, aluminum or monel (66% nickel and 31.5% copper alloy) do not react with fluorine at ordinary temperatures. Monel is suitable for working with the element at temperatures up to 600 ° C; sintered alumina is stable up to 700 ° C.
Fluorocarbon oils are the most suitable lubricants. The element reacts violently with organic substances (for example, rubber, wood and fabrics), therefore, controlled fluorination of organic compounds with elemental fluorine is possible only with special precautions.
Production
Fluorspar is the main source of fluoride. In the production of hydrogen fluoride (HF), powdered fluorite is distilled with concentrated sulfuric acid in a lead or cast iron apparatus. During the distillation, calcium sulfate (CaSO 4 ) is formed, insoluble in HF. Hydrogen fluoride is obtained in a fairly anhydrous state by fractional distillation in copper or steel vessels and stored in steel cylinders. Sulfuric and sulfuric acids, as well as hydrofluoric acid (H 2 SiF 6 ), which are formed due to the presence of silica in fluorspar, are common impurities in industrial hydrogen fluoride. Traces of moisture can be removed by electrolysis using platinum electrodes, treatment with elemental fluorine or storage over stronger Lewis acid (MF 5 , where M is metal), which can form salts (H 3 O) + (MF 6 ) - : H 2 O + SbF 5 + HF → (H 3 O) + (SbF 6 ) - .
Hydrogen fluoride is used in the preparation of many industrial inorganic and organic fluorine compounds, for example, aluminum fluoride aluminum (Na 3 AlF 6 ), used as an electrolyte in the smelting of aluminum metal. A solution of gaseous hydrogen fluoride in water is called hydrofluoric acid, a large amount of which is used in the cleaning of metals and for polishing, making the glass dull or etching.
Obtaining a free cell is carried out using electrolytic procedures in the absence of water. As a rule, they take the form of electrolysis of molten potassium fluoride with hydrogen fluoride (in the ratio of 1 to 2.5-5) at temperatures of 30–70, 80–120, or 250 ° C. During the process, the hydrogen fluoride content in the electrolyte decreases, and the melting temperature rises. Therefore, it is necessary that its addition occurs continuously. In a high-temperature chamber, the electrolyte is replaced when the temperature exceeds 300 ° C. Fluoride can be safely stored under pressure in stainless steel cylinders if the cylinder valves are free of traces of organic matter.
Using
The element is used to produce various fluorides, such as chlorine trifluoride (ClF 3 ), sulfur hexafluoride (SF 6 ) or cobalt trifluoride (CoF 3 ). Chlorine and cobalt compounds are important fluorinating agents of organic compounds. (With appropriate precautions, fluoride can be used directly for this). Sulfur hexafluoride is used as a gaseous dielectric.
Elementary fluorine, often diluted with nitrogen, reacts with hydrocarbons to form the corresponding fluorocarbons, in which part or all of the hydrogen is replaced by halogen. The resulting compounds are usually characterized by high stability, chemical inertness, high electrical resistance, as well as other valuable physical and chemical properties.
Fluorination can also be done by treating organic compounds with cobalt trifluoride (CoF 3 ) or by electrolysis of their solutions in anhydrous hydrogen fluoride. Useful non-stick plastics such as polytetrafluoroethylene [(CF 2 CF 2 ) x ], known under the commercial name Teflon, are derived from unsaturated fluorinated hydrocarbons.
Organic compounds containing chlorine, bromine or iodine are fluorinated to produce substances such as dichlorodifluoromethane (Cl 2 CF 2 ), a refrigerant that has been widely used in domestic refrigerators and air conditioners. Since chlorofluorocarbons, such as dichlorodifluoromethane, play an active role in the depletion of the ozone layer, their production and use has been limited, and now refrigerants containing hydrofluorocarbons are preferred.
The element is also used to produce uranium hexafluoride (UF 6 ) used in the gas diffusion process of separating uranium-235 from uranium-238 in the production of nuclear fuel. Hydrogen fluoride and boron trifluoride (BF 3 ) are produced on an industrial scale, as they are good catalysts for the alkylation reactions used to produce many organic compounds. Sodium fluoride is usually added to drinking water in order to reduce the incidence of dental caries in children. In recent years, the use of fluorine compounds in the pharmaceutical and agricultural fields has become most important. Selective fluorine substitution dramatically changes the biological properties of substances.
Analysis
It is difficult to accurately determine the amount of a given halogen in the compounds. Free fluorine, whose valency is 1, can be detected by its oxidation of mercury Hg + F 2 → HgF 2 , as well as by measuring the increase in mercury weight and the change in gas volume. The main qualitative tests for the presence of element ions are:
- the release of hydrogen fluoride under the action of sulfuric acid,
- the formation of a precipitate of calcium fluoride when adding a solution of calcium chloride,
- discoloration of a yellow solution of titanium tetraoxide (TiO 4 ) and hydrogen peroxide in sulfuric acid.
Quantitative analysis methods:
- precipitating calcium fluoride in the presence of sodium carbonate and treating the precipitate with acetic acid,
- precipitation of lead chlorofluoride by adding sodium chloride and lead nitrate,
- titration (determination of solute concentration) with a solution of thorium nitrate (Th [NO 3 ] 4 ) using sodium alizarin sulfonate as an indicator: Th (NO 3 ) 4 + 4KF ↔ ThF 4 + 4KNO 3 .
Covalently bound fluorine (valency I), as, for example, in fluorocarbons, is more difficult to analyze. This requires a compound with metallic sodium followed by analysis of F - ions, as described above.
Item Properties
Finally, we present some properties of fluorine:
- Atomic number: 9.
- Atomic weight: 18,9984.
- Possible fluorine valencies: 1.
- Melting point: -219.62 ° C.
- Boiling point: -188 ° C.
- Density (1 atm, 0 ° C): 1,696 g / l.
- Fluorine electronic formula: 1s 2 2s 2 2p 5 .