Lesson of the elective course "Chromium and its compounds". See what "chrome" is in other dictionaries

The article is devoted to element No. 24 of the periodic table - chromium, the history of its discovery and distribution in nature, the structure of its atom, chemical properties and compounds, how it is obtained and why we need it. The average content of chromium in the earth's crust is not high 0.0083%. This element is probably more characteristic of the Earth's mantle.

Chromium forms massive and disseminated ores in ultramafic rocks; the formation of the largest deposits of Chromium is associated with them. In basic rocks, the content of Chromium reaches only 2 10-2%, in acidic rocks - 2.5 10-3%, in sedimentary rocks (sandstones) - 3.5 10-3%, shale - 9 10-3 %. Chromium is a relatively weak water migrant: Chromium content in sea ​​water 0.00005 mg/l, in surface water -0.0015 mg/l.
In general, chromium is a metal deep zones Earth.

Today, the total consumption of pure chromium (at least 99% Cr) is about 15 thousand tons, of which about a third is electrolytic chromium. The world leader in the production of high-purity chromium is the British company Bell Metals. The first place in terms of consumption is occupied by the USA (50%), the second - by the countries of Europe (25%), the third - by Japan. The market for chromium metal is quite volatile and prices for the metal fluctuate widely.

1. CHROMIUM AS A CHEMICAL ELEMENT

Chromium– (Chromium) Cr, chemical element 6(VIb) of group of the Periodic system. Atomic number 24, atomic mass 51,996. There are 24 known isotopes of chromium from 42 Cr to 66 Cr. Isotopes 52 Cr, 53 Cr, 54 Cr are stable. The isotopic composition of natural chromium: 50 Cr (half-life 1.8 10 17 years) - 4.345%, 52 Cr - 83.489%, 53 Cr - 9.501%, 54 Cr - 2.365%. The main oxidation states are +3 and +6.

In 1761, professor of chemistry at St. Petersburg University, Johann Gottlob Lehmann, at the eastern foot of Ural mountains at the Berezovsky mine, he discovered a wonderful red mineral, which, when crushed into powder, gave a bright yellow color. In 1766 Leman brought samples of the mineral to St. Petersburg. After treating the crystals with hydrochloric acid, he obtained a white precipitate, in which he found lead. Leman called the mineral Siberian red lead (plomb rouge de Sibérie), now it is known that it was crocoite (from the Greek "krokos" - saffron) - natural lead chromate PbCrO 4.

The German traveler and naturalist Peter Simon Pallas (1741-1811) led the expedition of the St. Petersburg Academy of Sciences to the central regions of Russia and in 1770 visited the Southern and Middle Urals, including the Berezovsky mine and, like Lehman, became interested in crocoite. Pallas wrote: “This amazing red lead mineral is not found in any other deposit. Turns yellow when ground into powder and can be used in miniature art. Despite the rarity and difficulty of delivering crocoite from the Berezovsky mine to Europe (it took almost two years), the use of the mineral as a coloring matter was appreciated. In London and Paris at the end of the 17th century. all noble persons rode in carriages painted with finely ground crocoite, in addition, the best samples of Siberian red lead were added to the collections of many mineralogical cabinets in Europe.

In 1796, a sample of crocoite came to Nicolas-Louis Vauquelin (1763–1829), professor of chemistry at the Paris Mineralogical School, who analyzed the mineral, but found nothing in it except oxides of lead, iron, and aluminum. Continuing the study of Siberian red lead, Vauquelin boiled the mineral with a solution of potash and, after separating the white precipitate of lead carbonate, obtained a yellow solution of an unknown salt. When it was treated with a lead salt, a yellow precipitate formed, with a mercury salt, a red one, and when tin chloride was added, the solution turned green. Decomposing crocoite with mineral acids, he obtained a solution of "red lead acid", the evaporation of which gave ruby-red crystals (it is now clear that this was chromic anhydride). Having calcined them with coal in a graphite crucible, after the reaction, he discovered a lot of intergrown gray needle-shaped crystals of a metal unknown until that time. Vauquelin stated the high refractoriness of the metal and its resistance to acids.

Vauquelin called the new element chromium (from the Greek  - color, color) in view of the many multi-colored compounds it forms. Based on his research, Vauquelin stated for the first time that the emerald color of some precious stones due to the admixture of chromium compounds in them. For example, natural emerald is a deep green colored beryl in which aluminum is partially replaced by chromium.

Most likely, Vauquelin obtained not pure metal, but its carbides, as evidenced by the needle-like shape of the crystals obtained, but the Paris Academy of Sciences nevertheless registered the discovery of a new element, and now Vauquelin is rightly considered the discoverer of element No. 24.

In 1798, Lovitz and Klaproth, independently of Vauquelen, discovered chromium in a sample of a heavy black mineral (it was FeCr 2 O 4 chromite) found in the Urals, but much to the north of the Berezovsky deposit. In 1799, F. Tassert discovered a new element in the same mineral found in the southeast of France. It is believed that it was Tassert who first managed to obtain relatively pure metallic chromium.

2. CHROMIUM IN NATURE AND ITS INDUSTRIAL EXTRACTION

Chromium is a fairly common element on Earth. Its clarke (average content in the earth's crust) of the crust is 8.3 10 -3%. Chromium is never found in the free state. In chrome ores practical value has only chromite FeCr 2 O 4, which belongs to spinels - isomorphic minerals of the cubic system with the general formula MO·Me 2 O 3, where M is a divalent metal ion, and Me is a trivalent metal ion. Spinels can form solid solutions with each other, therefore, magnochromite (Mg,Fe)Cr 2 O 4 , aluminum chromite Fe(Cr,Al) 2 O 4 , chromopicotite (Mg,Fe) ( Cr, Al) 2 O 4 - they all belong to the class of chrome spinels. In addition to spinels, chromium is found in many much less common minerals, for example, melanochroite 3PbO 2Cr 2 O 3, wokelenite 2(Pb,Cu)CrO 4 (Pb,Cu) 3 (PO 4) 2, tarapakaite K 2 CrO 4 , ditzeite CaIO 3 CaCrO 4 and others.

Chromites are dark or almost black in color, have a metallic luster, and usually lie in the form of continuous arrays. Chromite deposits are of igneous origin. Its identified resources are estimated in 47 countries of the world and amount to 15 billion tons. The first place in terms of chromite reserves is occupied by South Africa (76% of the proven world reserves), where highest value has a group of Bushveld deposits, the content of chromium ore in which is 1 billion tons. The second place in the world in terms of chromite resources is occupied by Kazakhstan (9% of the world reserves), chrome ores there are of very high quality. All chromite resources in Kazakhstan are concentrated in the Aktobe region (Kempirsai massif with reserves of 300 million tons); deposits have been developed since the late 1930s. The third place is occupied by Zimbabwe (6% of world reserves). In addition, the United States, India, the Philippines, Turkey, Madagascar, and Brazil have significant chromite resources. In Russia, rather large deposits of chromite are found in the Urals (Saranovskoye, Verblyuzhyegorskoye, Alapaevskoye, Monetnaya Dacha, Khalilovskoye and other deposits).

At the beginning of the 19th century the main source of chromite was the Ural deposits, but in 1827 the American Isaac Tyson (Isaac Tyson) discovered a large deposit of chromite ore on the border of Maryland and Pennsylvania, becoming a monopolist in the field of mining on long years. In 1848 deposits of high quality chromite were found in Turkey, not far from Bursa. After the depletion of reserves in Maryland, Turkey was the leader in the extraction of chromites, until India and South Africa intercepted the baton in 1906.

Now 11-14 million tons of chromites are mined annually in the world. The leading place in the extraction of chromium ore is occupied by South Africa (about 6 million tons annually), followed by Kazakhstan, providing 20% ​​of world demand. Due to the great depth of occurrence of chromium ore, it is usually mined by mines (85%), but sometimes open-pit (quarry) mining is also practiced, for example, in Finland and Madagascar. Usually mined ores are of sufficient quality and need only mechanical sorting. It is often impractical to enrich chromites, since in this case only the content of Cr 2 O 3 can be increased, and the ratio of Fe : Cr remains unchanged. The price of chromite on the world market fluctuates between 40-120 US dollars per ton.

Chrome is a silvery metal with a density of 7200 kg / m 3. Determining the melting point of pure chromium is an extremely difficult task, since the slightest impurities of oxygen or nitrogen significantly affect the value of this temperature. According to the results of modern measurements, it is equal to 1907 ° C. The boiling point of chromium is 2671 ° C. Completely pure (without gas impurities and carbon) chromium is quite viscous, forging and malleable. At the slightest pollution with carbon, hydrogen, nitrogen, etc. becomes brittle, brittle and hard. At ordinary temperatures, it exists in the form of an a-modification and has a cubic body-centered lattice. Chemically, chromium is rather inert due to the formation of a strong thin oxide film on its surface. It does not oxidize in air even in the presence of moisture, and when heated, oxidation occurs only on the surface. Chromium is passivated by dilute and concentrated nitric acid, aqua regia, and even when the metal is boiled with these reagents, it dissolves only slightly. Chromium passivated with nitric acid, unlike a metal without a protective layer, does not dissolve in dilute sulfuric and hydrochloric acids, even with prolonged boiling in solutions of these acids, however, at a certain point, rapid dissolution begins, accompanied by foaming from the released hydrogen - from the passive form chromium becomes activated, not protected by an oxide film:

Cr + 2HCl \u003d CrCl 2 + H 2

If nitric acid is added during the dissolution process, the reaction immediately stops - chromium is again passivated.

When heated, metallic chromium combines with halogens, sulfur, silicon, boron, carbon and some other elements:

Cr + 2F 2 = CrF 4 (with admixture of CrF 5)

2Cr + 3Cl 2 = 2CrCl 3

Cr + C = Cr 23 C 6 + Cr 7 C 3 mixture.

When chromium is heated with molten soda in air, nitrates or chlorates of alkali metals, the corresponding chromates (VI) are obtained:

2Cr + 2Na 2 CO 3 + 3O 2 \u003d 2Na 2 CrO 4 + 2CO 2.

Depending on the required degree of purity of the metal, there are several industrial methods for obtaining chromium.

Possibility aluminothermic The reduction of chromium(III) oxide was demonstrated by Friedrich Wöhler in 1859, however, this method became available on an industrial scale as soon as it became possible to obtain cheap aluminum. Industrial aluminothermic production of chromium began with the work of Goldschmidt, who was the first to develop a reliable method for controlling a highly exothermic (and therefore explosive) reduction process:

Cr 2 O 3 + 2Al \u003d 2Cr + 2Al 2 O 3.

The mixture is preheated uniformly to 500–600°C. Reduction can be initiated either by a mixture of barium peroxide and aluminum powder, or by igniting a small portion of the mixture, followed by the addition of the rest of the mixture. It is important that the heat released during the reaction is enough to melt the resulting chromium and separate it from the slag. Chromium obtained by the aluminothermic method usually contains 0.015–0.02% C, 0.02% S and 0.25–0.40% Fe, and the mass fraction of the main substance in it is 99.1–99.4% Cr. It is very brittle and easily ground into powder.

When obtaining high-purity chromium, electrolytic methods are used, the possibility of this was shown in 1854 by Bunsen, who subjected water solution chromium chloride. Now, mixtures of chromic anhydride or chromoammonium alum with dilute sulfuric acid are subjected to electrolysis. Chromium released during electrolysis contains dissolved gases as impurities. Modern technologies make it possible to obtain on an industrial scale a metal with a purity of 99.90–99.995% using high-temperature purification in a hydrogen flow and vacuum degassing. Unique techniques for refining electrolytic chromium allow you to get rid of oxygen, sulfur, nitrogen and hydrogen contained in the "raw" product.

There are several other less significant ways to obtain chromium metal. Silicothermal reduction is based on the reaction:

2Cr 2 O 3 + 3Si + 3CaO = 4Cr + 3CaSiO 3 .

Silicon reduction, although exothermic, requires the process to be carried out in an arc furnace. The addition of quicklime makes it possible to convert refractory silicon dioxide into a low-melting calcium silicate slag.

The reduction of chromium(III) oxide with coal is used to obtain high-carbon chromium, intended for the production of special alloys. The process is also carried out in an electric arc furnace.

The Van Arkel-Kuchman-De Boer process uses the decomposition of chromium(III) iodide on a wire heated to 1100°C with the deposition of pure metal on it.

Chromium can also be obtained by the reduction of Cr 2 O 3 with hydrogen at 1500 ° C, the reduction of anhydrous CrCl 3 with hydrogen, alkali or alkaline earth metals, magnesium and zinc.

3. APPLICATIONS OF CHROMIUM IN THE INDUSTRY

For many decades since the discovery of metallic chromium, only crocoite and some of its other compounds have been used as pigments in the manufacture of paints. In 1820 Cochlin proposed the use of potassium dichromate as a mordant in textile dyeing. In 1884, the active use of soluble chromium compounds as tannins in the leather industry began. Chromite was first used in France in 1879 as a refractory substance, but its main use began in the 1880s in England and Sweden, when the industrial smelting of ferrochrome began to increase. In small quantities, ferrochrome was already obtained at the beginning of the 19th century, so Berthier, as early as 1821, proposed to restore a mixture of iron and chromium oxides charcoal in the crucible. The first patent for the manufacture of chromium steel was issued in 1865. Industrial production of high-carbon ferrochromium began with the use of blast furnaces to reduce chromite with coke. Ferrochrome late 19th century was of very low quality, as it usually contained 7-8% chromium, and was known as "Tasmanian pig iron" due to the fact that the original iron-chromium ore was imported from Tasmania. The turning point in the production of ferrochrome came in 1893, when Henri Moissan first smelted high-carbon ferrochrome containing 60% Cr. The main achievement in this industry was the replacement of the blast furnace with an electric arc furnace created by Moissan, which made it possible to increase the process temperature, reduce energy consumption and significantly improve the quality of the smelted ferrochromium, which began to contain 67–71% Cr and 4–6% C. Moissan's method is still is at the heart of modern industrial production ferrochrome. Chromite recovery is usually carried out in open electric arc furnaces, and the charge is loaded from above. The arc is formed between the electrodes immersed in the mixture.

Chromium occurs in nature mainly in the form of chromium iron ore Fe (CrO 2) 2 (iron chromite). Ferrochromium is obtained from it by reduction in electric furnaces with coke (carbon):

FeO Cr 2 O 3 + 4C → Fe + 2Cr + 4CO

6) using electrolysis, electrolytic chromium is obtained from a solution of chromic anhydride in water containing the addition of sulfuric acid. At the same time, 3 processes take place on the cathodes:

– reduction of hexavalent chromium to trivalent chromium with its transition into solution;

– discharge of hydrogen ions with evolution of gaseous hydrogen;

– discharge of ions containing hexavalent chromium with deposition of metallic chromium;

Cr 2 O 7 2− + 14Н + + 12е − = 2Сr + 7H 2 O

In its free form, it is a bluish-white metal with a cubic body-centered lattice, a = 0.28845 nm. At a temperature of 39 °C, it changes from a paramagnetic state to an antiferromagnetic one (the Neel point).

Air resistant. At 300 °C, it burns out with the formation of green chromium (III) oxide Cr 2 O 3, which has amphoteric properties. Fusing Cr 2 O 3 with alkalis, chromites are obtained

Despite the great importance of high-carbon ferrochromium for the production of many grades of stainless steels, it is not suitable for the smelting of some high-chromium steels, since the presence of carbon (in the form of Cr 23 C 6 carbide, which crystallizes along the grain boundaries) makes them brittle and easily corroded. The production of low-carbon ferrochromium began to develop with the beginning of the use of industrial aluminothermic reduction of chromites. Now the aluminothermic process has been supplanted by the silicothermal process (Perrin process) and the simplex process, which consists in mixing high-carbon ferrochrome with partially oxidized ferrochrome powder, followed by briquetting and heating to 1360 ° C in vacuum. Ferrochrome prepared by the simplex process usually contains only 0.008% carbon, and briquettes from it easily dissolve in the steel melt.

The ferrochromium market is cyclical. World production of ferrochromium in 2000 was 4.8 million tons, and in 2001, due to low demand, 3.4 million tons. In 2002 the demand for ferrochromium became active again. The first place in the world in the smelting of ferrochromium is occupied by the South African "Big Two" (The "Big Two") - the company Xstrata South Africa (Pty) Ltd. (a subsidiary of Xstrata AG) and Samancor Chrome Division (a subsidiary of Samancor Ltd.). They account for up to 40% of the world ferrochromium smelting. In South Africa and Finland, mainly charge-chrome (from the English charge - to load coal) containing 52-55% Cr is produced, and in China, Russia, Zimbabwe, Kazakhstan, ferrochrome containing more than 60% Cr. Ferrochromium is used as an alloying addition to low alloy steels. With a content of more than 12% chromium, steel almost does not rust.

The corrosion resistance of iron alloys can be significantly increased by applying a thin layer of chromium to their surface. This procedure is called chromium plating. Chrome-plated layers resist damp atmospheres well, sea ​​air, tap water, nitric and many organic acids. All methods of chromium plating can be divided into two types - diffusion and electrolytic. The Becker-Davis-Steinberg diffusion method consists in heating the chrome-plated product to 1050–1100 ° C in a hydrogen atmosphere, covered with a mixture of ferrochrome and refractory, pre-treated with hydrogen chloride at 1050 ° C. CrCl 2 located in the pores of the refractory volatilizes and chromizes the product. In the process of electrolytic chromium plating, the metal is deposited on the surface of the workpiece, which acts as a cathode. The electrolyte is often a hexavalent chromium compound (usually CrO 3 ) dissolved in aqueous H 2 SO 4 . Chrome coatings are protective and decorative. Thickness protective coatings reaches 0.1 mm, they are applied directly to the product and give it increased wear resistance. Decorative coatings are of aesthetic value, and are applied to a sublayer of another metal (nickel or copper), which performs the actual protective function. The thickness of such a coating is only 0.0002–0.0005 mm.

4. BIOLOGICAL ROLE OF CHROMIUM

Chromium is a microelement necessary for the normal development and functioning of the human body. It has been established that only trivalent chromium takes part in biochemical processes. Its most important biological role is in the regulation of carbohydrate metabolism and blood glucose levels. Chromium is an integral part of a low molecular weight complex - glucose tolerance factor (GTF), which facilitates the interaction of cellular receptors with insulin, thereby reducing the body's need for it. The tolerance factor enhances the action of insulin in all metabolic processes with its participation. In addition, chromium is involved in the regulation of cholesterol metabolism and is an activator of certain enzymes.

The content of chromium in the human body is 6–12 mg. There is no exact information about the physiological need of a person for this element, in addition, it strongly depends on the nature of the diet (for example, it greatly increases with an excess of sugar in the diet). According to various estimates, the daily intake of chromium in the body is 20–300 mcg. An indicator of the availability of chromium in the body is its content in the hair (the norm is 0.15–0.5 μg / g). Unlike many trace elements, the content of chromium in body tissues (with the exception of the lung), as a person ages, decreases.

The concentration of the element in plant foods is an order of magnitude lower than its concentration in the tissues of mammals. The content of chromium in brewer's yeast is especially high, in addition, it is found in significant amounts in meat, liver, legumes, and whole grains. Chromium deficiency in the body can cause a diabetes-like condition, contribute to the development of atherosclerosis and disruption of higher nervous activity.

Already in relatively low concentrations (fractions of a milligram per m 3 for the atmosphere), all chromium compounds have a toxic effect on the body. Particularly dangerous in this regard are soluble compounds of hexavalent chromium, which have allergic, mutagenic and carcinogenic effects.

Chromium poisoning and its compounds occur during their production; in mechanical engineering (electroplated coatings); metallurgy (alloying additives, alloys, refractories); in the manufacture of leather, paints, etc. The toxicity of chromium compounds depends on their chemical structure: dichromates are more toxic than chromates, Cr (VI) compounds are more toxic than Cr (II), Cr (III) compounds. The initial forms of the disease are manifested by a feeling of dryness and pain in the nose, sore throat, difficulty breathing, coughing, etc.; they may disappear when contact with Chrome is discontinued. With prolonged contact with Chromium compounds, signs of chronic poisoning develop: headache, weakness, dyspepsia, weight loss and others. Functions of a stomach, a liver and a pancreas are broken. Bronchitis, bronchial asthma, diffuse pneumosclerosis are possible. When exposed to Chromium, dermatitis and eczema may develop on the skin. According to some reports, Chromium compounds, mainly Cr(III), have a carcinogenic effect.
chrome plating. A decrease in the content of chromium in food and blood leads to a decrease in the growth rate, an increase

Ripan R., Chetyanu I. Inorganic chemistry, v.2. – M.: Mir, 1972.

It has the necessary properties for successful use in the metallurgical industry. This metal is distinguished by a steel shade and high density. AT vivo it is mined from fossil chromium iron ore.

Raw materials are subjected to recovery (aluminothermic or silicone-thermal method) at metallurgical enterprises by using coke.

For the production of this metal, the method of metallothermic smelting can also be used, in which it is possible to achieve a reduction in aluminum consumption. The extraction of chromium increases up to 92%.

The smelting temperature of chromium is 2300 degrees Celsius, in the composition of this metal it is possible to distinguish: 98.9-99.2% Chromium (Cr), 0.01-0.2% Carbon (C), 0.07-0.12 % Silicon (Si), 0.25-0.4% Iron and Aluminum (Al, Fe), 0.005% Phosphorus (P).

This metal is indispensable when it is required to give steel products high heat resistance and corrosion resistance. It is used to alloy alloys, increase the strength of steel. replaces ferrochromium, and with its help it is possible to obtain steel of special grades in which the percentage of iron is strictly limited.

For the production of steel, chromium is taken without impurities and foreign inclusions, only traces of an oxidizing film are allowed. Pieces of metal weighing less than 10 kg are used, chrome is transported to the place of application in special containers - metal drums and wooden boxes.

The production of metallic chromium is carried out in accordance with the requirements of GOST 5905-79, it may contain a small amount of impurity lead, carbon, sulfur, cobalt, phosphorus, silicon, etc.

By adding chromium, they achieve a reduction in the size of the steel grain, an increase in strength, ductility and an increase in its hardenability. At high temperatures, chromium does not affect oxidizability.

The scope of this material is the aircraft industry, the creation spacecraft, chemical production and production of jet engines, gas turbines, etc.

Nichrome, ball bearings, heat-resistant and stainless alloys - all this is created through the skillful use of the remarkable properties of metallic chromium. Products made of chrome steel have a much longer service life and high resistance to chemical and other influences.

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Chemical properties of chromium compounds.

Cr2+. The charge concentration of the divalent chromium cation corresponds to the charge concentration of the magnesium cation and the divalent iron cation, so a number of properties, especially the acid-base behavior of these cations, are close. At the same time, as already mentioned, Cr 2+ is a strong reducing agent, therefore the following reactions take place in the solution: but even water oxidation occurs: 2CrSO 4 + 2H 2 O \u003d 2Cr (OH) SO 4 + H 2. The oxidation of divalent chromium occurs even more easily than the oxidation of ferrous iron, salts are also hydrolyzed by the cation to a moderate degree (i.e., the first step is dominant).

CrO - basic oxide, black, pyrophoric. At 700 ° C, it disproportionates: 3CrO \u003d Cr 2 O 3 + Cr. It can be obtained by thermal decomposition of the corresponding hydroxide in the absence of oxygen.

Cr(OH) 2 is an insoluble yellow base. It reacts with acids, while oxidizing acids simultaneously with acid-base interaction oxidize divalent chromium, under certain conditions this also happens with non-oxidizing acids (oxidizing agent - H +). When obtained by an exchange reaction, chromium (II) hydroxide quickly turns green due to oxidation:

4Cr(OH) 2 + O 2 = 4CrO(OH) + 2H 2 O.

Oxidation is also accompanied by the decomposition of chromium (II) hydroxide in the presence of oxygen: 4Cr(OH) 2 = 2Cr 2 O 3 + 4H 2 O.

Cr3+. Chromium(III) compounds are chemically similar to aluminum and iron(III) compounds. Oxide and hydroxide are amphoteric. Salts of weak unstable and insoluble acids (H 2 CO 3, H 2 SO 3, H 2 S, H 2 SiO 3) undergo irreversible hydrolysis:

2CrCl 3 + 3K 2 S + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3H 2 S + 6KCl; Cr 2 S 3 + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3H 2 S.

But the chromium (III) cation is not a very strong oxidizing agent, therefore chromium (III) sulfide exists and can be obtained under anhydrous conditions, however, not from simple substances, since it decomposes when heated, but by the reaction: 2CrCl 3 (cr) + 2H 2 S (gas) \u003d Cr 2 S 3 (cr) + 6HCl. The oxidizing properties of trivalent chromium are not enough for solutions of its salts to interact with copper, but such a reaction takes place with zinc: 2CrCl 3 + Zn = 2CrCl 2 + ZnCl 2.

Cr2O3 - amphoteric oxide of green color, has a very strong crystal lattice, therefore it exhibits chemical activity only in the amorphous state. Reacts mainly when fused with acids and basic oxides, with acids and alkalis, as well as with compounds having acidic or basic functions:

Cr 2 O 3 + 3K 2 S 2 O 7 \u003d Cr 2 (SO 4) 3 + 3K 2 SO 4; Cr 2 O 3 + K 2 CO 3 \u003d 2KCrO 2 + CO 2.

Cr(OH) 3 (CrO(OH), Cr 2 O 3 *nH 2 O) - amphoteric hydroxide of gray-blue color. It dissolves in both acids and alkalis. When dissolved in alkalis, hydroxocomplexes are formed, in which the chromium cation has a coordination number of 4 or 6:

Cr(OH) 3 + NaOH = Na; Cr(OH) 3 + 3NaOH \u003d Na 3.

Hydroxocomplexes are easily decomposed by acids, while the processes are different with strong and weak acids:

Na + 4HCl \u003d NaCl + CrCl 3 + 4H 2 O; Na + CO 2 \u003d Cr (OH) 3 ↓ + NaHCO 3.

Cr(III) compounds are not only oxidizing agents, but also reducing agents with respect to transformation into Cr(VI) compounds. The reaction proceeds especially easily in an alkaline medium:

2Na 3 + 3Cl 2 + 4NaOH \u003d 2Na 2 CrO 4 + 6NaCl + 8H 2 O E 0 \u003d - 0.72 V.

In an acidic environment: 2Cr 3+ → Cr 2 O 7 2- E 0 = +1.38 V.

cr +6 . All Cr(VI) compounds are strong oxidizers. The acid-base behavior of these compounds is similar to that of sulfur compounds in the same oxidation state. Such similarity of the properties of the compounds of the elements of the main and secondary subgroups in the maximum positive oxidation state is typical for most groups periodic system.

CrO3 - a dark red compound, a typical acidic oxide. At the melting point, it decomposes: 4CrO 3 \u003d 2Cr 2 O 3 + 3O 2.

An example of an oxidizing action: CrO 3 + NH 3 = Cr 2 O 3 + N 2 + H 2 O (When heated).

Chromium(VI) oxide easily dissolves in water, attaching it and turning into hydroxide:

H2CrO4 - chromic acid, is a strong dibasic acid. It does not stand out in a free form, because. at a concentration above 75%, a condensation reaction occurs with the formation of dichromic acid: 2H 2 CrO 4 (yellow) \u003d H 2 Cr 2 O 7 (orange) + H 2 O.

Further concentration leads to the formation of trichromic (H 2 Cr 3 O 10) and even tetrachromic (H 2 Cr 4 O 13) acids.

Dimerization of the chromate anion also occurs upon acidification. As a result, salts of chromic acid at pH > 6 exist as yellow chromates (K 2 CrO 4), and at pH< 6 как бихроматы(K 2 Cr 2 O 7) оранжевого цвета. Большинство бихроматов растворимы, а растворимость хроматов чётко соответствует растворимости сульфатов соответствующих металлов. В растворах возможно взаимопревращения соответствующих солей:

2K 2 CrO 4 + H 2 SO 4 = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O; K 2 Cr 2 O 7 + 2KOH \u003d 2K 2 CrO 4 + H 2 O.

The interaction of potassium dichromate with concentrated sulfuric acid leads to the formation of chromic anhydride, which is insoluble in it:

K 2 Cr 2 O 7 (crystal) + + H 2 SO 4 (conc.) = 2CrO 3 ↓ + K 2 SO 4 + H 2 O;

When heated, ammonium bichromate undergoes an intramolecular redox reaction: (NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O.

HALOGENS ("giving birth to salts")

Halogens are called elements of the main subgroup of group VII of the periodic system. These are fluorine, chlorine, bromine, iodine, astatine. The structure of the outer electronic layer of their atoms: ns 2 np 5. Thus, there are 7 electrons in the outer electronic level, and only one electron is missing from them to the stable noble gas shell. Being the penultimate elements in the period, halogens have the smallest radius in the period. All this leads to the fact that halogens exhibit the properties of non-metals, have a high electronegativity and a high ionization potential. Halogens are strong oxidizing agents, they are able to accept an electron to become an anion with a charge of "1-" or exhibit an oxidation state of "-1" when covalently bonded to less electronegative elements. At the same time, when moving down the group from top to bottom, the radius of the atom increases and the oxidizing ability of halogens decreases. If fluorine is the strongest oxidizing agent, then iodine, when interacting with some complex substances, as well as with oxygen and other halogens, exhibits reducing properties.

The fluorine atom is different from the other members of the group. First, it only shows negative degree oxidation, as it is the most electronegative element, and secondly, like any element of the II period, it has only 4 atomic orbitals on the outer electronic level, three of which are occupied by unshared electron pairs, the fourth is an unpaired electron, which in most cases and is the only valence electron. In the atoms of other elements, there is an unfilled d-electron sublevel on the outer level, where an excited electron can go. Each lone pair gives two electrons when steamed, so the main oxidation states of chlorine, bromine and iodine, except for "-1", are "+1", "+3", "+5", "+7". Less stable, but fundamentally achievable are the oxidation states "+2", "+4" and "+6".

How simple substances all halogens are diatomic molecules with a single bond between the atoms. The bond dissociation energies in the series of molecules F 2 , Cl 2 , Br 2 , J 2 are as follows: 151 kJ/mol, 239 kJ/mol, 192 kJ/mol, 149 kJ/mol. The monotonic decrease in the binding energy upon passing from chlorine to iodine is easily explained by the increase in the bond length due to the increase in the atomic radius. The anomalously low binding energy in the fluorine molecule has two explanations. The first concerns the fluorine molecule itself. As already mentioned, fluorine has a very small atomic radius and as many as seven electrons at the outer level, therefore, when atoms approach each other during the formation of a molecule, interelectronic repulsion occurs, as a result of which the orbitals overlap incompletely, and the bond order in the fluorine molecule is slightly less than unity. According to the second explanation, in the molecules of the remaining halogens there is an additional donor-acceptor overlap of the lone electron pair of one atom and the free d-orbital of the other atom, two such opposite interactions per molecule. Thus, the bond in the molecules of chlorine, bromine and iodine is defined as almost triple in terms of the presence of interactions. But donor-acceptor overlaps occur only partially, and the bond has an order (for a chlorine molecule) of 1.12.

Physical properties: At normal conditions fluorine is a gas that is difficult to liquefy (boiling point of which is -187 0 C) of a light yellow color, chlorine is an easily liquefied gas (boiling point is -34.2 0 C) of a yellow-green color, bromine is a brown, easily evaporating liquid, iodine is solid gray color with a metallic sheen. In the solid state, all halogens form a molecular crystal lattice characterized by weak intermolecular interactions. In this connection, iodine has a tendency to sublimate - when heated at atmospheric pressure passes into a gaseous state (forms violet vapors), bypassing the liquid state. When moving through the group from top to bottom, the melting and boiling points increase both due to an increase in molecular weight s substances, and due to the strengthening of the van der Waals forces acting between the molecules. The magnitude of these forces is the greater, the greater the polarizability of the molecule, which, in turn, increases with increasing atomic radius.

All halogens are poorly soluble in water, but well - in non-polar organic solvents, for example, in carbon tetrachloride. Poor solubility in water is due to the fact that when a cavity is formed for the dissolution of the halogen molecule, water loses sufficiently strong hydrogen bonds, instead of which no strong interactions occur between its polar molecule and the nonpolar halogen molecule. The dissolution of halogens in non-polar solvents corresponds to the situation: “like dissolves in like”, when the nature of the breaking and forming bonds is the same.

Chromium is an important component in many alloyed steels (in particular, stainless steels), as well as in a number of other alloys. It is used as wear-resistant and beautiful galvanic coatings (chrome plating). Chromium is used for the production of alloys: chromium-30 and chromium-90, indispensable for the production of high-power plasma torch nozzles and in the aerospace industry.

Chromium is used to obtain various types of special steels in the manufacture of firearms barrels (from rifles to cannons), armor plates, fireproof cabinets, etc. Steels containing more than 13% chromium almost do not rust and are used for the manufacture of underwater parts of ships, in in particular, for the construction of submarine hulls.

Chromium is widely used for chrome plating products. Chrome plating is carried out electrolytically. Despite the fact that the thickness of the applied films often does not exceed 0.005 mm, chrome-plated products become resistant to external influences (moisture, air) and do not rust.

Chromium compounds are used to make chromium bricks - chromomagnesite, used in the working space of metallurgical furnaces and other metallurgical devices and structures.

"Stainless steel"-steel, excellent resistance to corrosion and oxidation, contains approximately 17-19% chromium and 8-13% nickel. But carbon is harmful to this steel: the carbide-forming "inclinations" of chromium lead to the fact that large amounts of this element bind into carbides that precipitate at the grain boundaries of the steel, and the grains themselves turn out to be poor in chromium and cannot staunchly defend themselves against the onslaught of acids and oxygen. Therefore, the carbon content in stainless steel should be minimal (no more than 0.1%).

At high temperatures, steel can become covered with "scales" of scale. In some machines, parts heat up to hundreds of degrees. So that the steel from which these parts are made does not “suffer” from scale formation, 25-30% chromium is introduced into it. Such steel can withstand temperatures up to 1000°C!

As heating elements, alloys of chromium with nickel - nichrome successfully serve. The addition of cobalt and molybdenum to chromium-nickel alloys gives the metal the ability to withstand heavy loads at 650-900 ° C. These alloys are used, for example, to make gas turbine blades. An alloy of cobalt, molybdenum and chromium (“comochrome”) is harmless to the human body and is therefore used in reconstructive surgery.

An American firm has recently created new materials whose magnetic properties change with temperature. These materials, which are based on compounds of manganese, chromium and antimony, according to scientists, will find application in various automatic devices that are sensitive to temperature fluctuations, and will be able to replace more expensive thermoelements.

Chromites are also widely used in the refractory industry. Magnesite-chromite brick is an excellent refractory material for lining open-hearth furnaces and other metallurgical units. This material has high heat resistance, it is not afraid of repeated sudden changes in temperature.

Chemists use chromites to produce potassium and sodium dichromates, as well as chromium alum, which are used to tan leather, giving it a beautiful sheen and strength. Such skin is called "chrome", and boots from it are called "chrome".

As if justifying its name, chromium takes an active part in the production of dyes for the glass, ceramic, and textile industries.

Chromium oxide allowed tractor builders to significantly reduce engine break-in time. Usually this operation, during which all the rubbing parts must "get used" to each other, lasted quite a long time and this, of course, did not suit the workers of the tractor factories. A way out was found when it was possible to develop a new fuel additive, which included chromium oxide. The secret of the additive's action is simple: when the fuel is burned, the smallest abrasive particles of chromium oxide are formed, which, settling on the inner walls of the cylinders and other surfaces subjected to friction, quickly eliminate roughness, polish and tightly fit the parts. This additive, combined with a new type of oil, made it possible to reduce the break-in time by 30 times.

Application of protective chrome coatings

It has long been noted that chromium is not only very hard (in this respect it has no competitors among metals), but also resists air oxidation well and does not interact with acids. They tried to electrolytically deposit a thin layer of this metal on the surface of products made of other materials in order to protect them from corrosion, scratches and other “injuries”. However, the chrome coatings turned out to be porous, peeled off easily and did not justify the hopes placed on them.

For almost three quarters of a century, scientists struggled with the problem of chromium plating, and only in the 20s of our century the problem was solved. The reason for the failures was that the electrolyte used in this case contained trivalent chromium, which could not create the desired coating. But his hexavalent "brother" such a task was up to the task. Since that time, chromic acid has been used as an electrolyte - in it the valence of chromium is 6. The thickness of protective coatings (for example, on some external parts of cars, motorcycles, bicycles) is up to 0.1 mm. But sometimes chrome plating is used for decorative purposes - for finishing watches, door handles and other items that are not in serious danger. In such cases, the thinnest layer of chromium (0.0002-0.0005 mm) is applied to the product.

Chrome car

There is another way of chromium plating - diffusion, which takes place not in galvanic baths, but in furnaces. Initially, a steel part was placed in chromium powder and heated in a reducing atmosphere to high temperatures. At the same time, a layer enriched with chromium appeared on the surface of the part, which, in terms of hardness and corrosion resistance, significantly exceeded the steel from which the part was made. But (and here there were some “buts”) at a temperature of about 1000 ° C, chromium powder sinters and, in addition, carbides are formed on the surface of the coated metal, which prevent the diffusion of chromium into steel. I had to look for another chromium carrier; instead of powder, volatile halogen salts of chromium - chloride or iodide - began to be used for this purpose, which made it possible to lower the temperature of the process.

Chromium chloride (or iodide) is obtained directly in a chromium plating plant by passing vapors of the corresponding hydrohalic acid through powdered chromium or ferrochromium. The resulting gaseous chloride envelops the chrome-plated product, and the surface layer is saturated with chromium. Such a coating is much stronger associated with the base material than galvanized.

Lithuanian chemists have developed a way to create a multi-layer "chain mail" for especially critical parts. The thinnest top layer of this coating (under the microscope, its surface actually resembles chain mail) consists of chromium: in the process of service, it is the first to “take over the fire”, but many years pass while the chromium oxidizes. In the meantime, the detail quietly carries out its responsible service.

DEFINITION

Chromium- twenty-fourth element Periodic table. Designation - Cr from the Latin "chromium". Located in the fourth period, VIB group. Refers to metals. The core charge is 24.

Chromium is contained in the earth's crust in an amount of 0.02% (wt.). In nature, it occurs mainly in the form of iron chromium FeO×Cr 2 O 3 .

Chromium is a solid shiny metal (Fig. 1), melting at 1890 o C; its density is 7.19 g / cm 3. At room temperature, chromium is resistant to both water and air. Dilute sulfuric and hydrochloric acids dissolve chromium, releasing hydrogen. In cold concentrated nitric acid, chromium is insoluble and becomes passive after treatment with it.

Rice. 1. Chrome. Appearance.

Atomic and molecular weight of chromium

DEFINITION

Relative molecular weight of a substance(M r) is a number showing how many times the mass of a given molecule is greater than 1/12 of the mass of a carbon atom, and relative atomic mass of an element(A r) - how many times the average mass of atoms chemical element more than 1/12 of the mass of a carbon atom.

Since chromium exists in the free state in the form of monatomic Cr molecules, the values ​​of its atomic and molecular masses are the same. They are equal to 51.9962.

Isotopes of chromium

It is known that chromium can occur in nature in the form of four stable isotopes 50Cr, 52Cr, 53Cr, and 54Cr. Their mass numbers are 50, 52, 53, and 54, respectively. The nucleus of the atom of the chromium isotope 50 Cr contains twenty-four protons and twenty-six neutrons, and the remaining isotopes differ from it only in the number of neutrons.

There are artificial isotopes of chromium with mass numbers from 42 to 67, among which the most stable is 59 Cr with a half-life of 42.3 minutes, as well as one nuclear isotope.

Chromium ions

On the outer energy level of the chromium atom, there are six electrons that are valence:

1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 .

As a result of chemical interaction, chromium gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:

Cr 0 -2e → Cr 2+;

Cr 0 -3e → Cr 3+;

Cr 0 -6e → Cr 6+.

Molecule and atom of chromium

In the free state, chromium exists in the form of monatomic Cr molecules. Here are some properties that characterize the atom and molecule of chromium:

Chromium alloys

Chromium metal is used for chromium plating, and also as one of the most important components of alloy steels. The introduction of chromium into steel increases its resistance to corrosion both in aqueous media at ordinary temperatures and in gases at elevated temperatures. In addition, chromium steels have increased hardness. Chromium is a part of stainless acid-resistant, heat-resistant steels.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2