DEFINITION
Chromium- the twenty-fourth element of the Periodic Table. Designation - Cr from the Latin "chromium". Located in the fourth period, VIB group. Refers to metals. The core has a charge of 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 chromium iron ore FeO × Cr 2 O 3.
Chromium is a hard shiny metal (Fig. 1), melting at 1890 o C; its density is 7.19 g / cm 3. At room temperature, chrome is resistant to both water and air. Diluted sulfuric and hydrochloric acids dissolve chromium to release hydrogen. In cold concentrated nitric acid, chromium is insoluble and after processing it becomes passive.
Rice. 1. Chrome. Appearance.
Atomic and molecular weight of chromium
DEFINITION
Relative molecular weight of the 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 of a chemical element is more than 1/12 of the mass of a carbon atom.
Since in the free state chromium exists in the form of monatomic Cr molecules, the values of its atomic and molecular masses coincide. They are equal to 51.9962.
Chromium isotopes
It is known that in nature, chromium can be in the form of four stable isotopes 50 Cr, 52 Cr, 53 Cr and 54 Cr. Their mass numbers are 50, 52, 53 and 54, respectively. The nucleus of the chromium isotope 50 Cr contains twenty-four protons and twenty-six neutrons, and the rest of the isotopes differ from it only in the number of neutrons.
There are artificial chromium isotopes 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
At the outer energy level of the chromium atom, there are six electrons, which 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+.
Chromium molecule and atom
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
Metallic chromium is used for chrome 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 normal 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
| Exercise | Chromium oxide (VI) weighing 2 g was dissolved in water weighing 500 g. Calculate the mass fraction of chromic acid H 2 CrO 4 in the resulting solution. |
| Solution | Let us write down the reaction equation for producing chromic acid from chromium (VI) oxide: CrO 3 + H 2 O = H 2 CrO 4. Find the mass of the solution: m solution = m (CrO 3) + m (H 2 O) = 2 + 500 = 502 g. n (CrO 3) = m (CrO 3) / M (CrO 3); n (CrO 3) = 2/100 = 0.02 mol. According to the reaction equation n (CrO 3): n (H 2 CrO 4) = 1: 1, which means n (CrO 3) = n (H 2 CrO 4) = 0.02 mol. Then the mass of chromic acid will be equal (molar mass - 118 g / mol): m (H 2 CrO 4) = n (H 2 CrO 4) × M (H 2 CrO 4); m (H 2 CrO 4) = 0.02 × 118 = 2.36 g. The mass fraction of chromic acid in the solution is: ω = m solute / m solution × 100%; ω (H 2 CrO 4) = m solute (H 2 CrO 4) / m solution × 100%; ω (H 2 CrO 4) = 2.36 / 502 × 100% = 0.47%. |
| Answer | The mass fraction of chromic acid is 0.47%. |
Chromium
Element number 24. One of the hardest metals. Possesses high chemical resistance. One of the most important metals used in the manufacture of alloy steels. Most chromium compounds are brightly colored in a wide variety of colors. For this feature, the element was named chrome, which in Greek means "paint".
How was he found
A mineral containing chromium was discovered near Yekaterinburg in 1766 by I.G. Lehmann and named "Siberian red lead". This mineral is now called crocoite. Its composition is also known - PbCrO 4. And at one time "Siberian red lead" caused a lot of controversy among scientists. For thirty years they argued about its composition, until, finally, in 1797, the French chemist Louis Nicolas Vauquelin isolated a metal from it, which (also, by the way, after some controversy) was called chromium.
Vauquelin treated crocoite with potash K 2 CO 3: lead chromate turned into potassium chromate. Then, using hydrochloric acid, potassium chromate was converted into chromium oxide and water (chromic acid exists only in dilute solutions). By heating green chromium oxide powder in a graphite crucible with coal, Vauquelin obtained a new refractory metal.
The Paris Academy of Sciences witnessed the discovery in all its form. But, most likely, Vauquelin isolated not elemental chromium, but its carbides. This is evidenced by the needle-like shape of the light gray crystals obtained by Vauquelin.
The name "chrome" was suggested by Vauquelin's friends, but he did not like it - the metal did not differ in a special color. However, friends managed to persuade the chemist, referring to the fact that good paints can be obtained from brightly colored chromium compounds. (By the way, it was in Vauquelin's works that the emerald color of some natural silicates of beryllium and aluminum was first explained; they, as Vauquelin found out, were colored by impurities of chromium compounds.) This name was confirmed for the new element.
Incidentally, the syllable "chrome", precisely in the sense of "colored", is included in many scientific, technical and even musical terms. Isopanchrome, panchrome and orthochrome films are widely known. The word "chromosome" in translation from Greek means "a body that is colored." There is a "chromatic" scale (in music) and there is a "chrome" harmonic.
Where is he located
There is quite a lot of chromium in the earth's crust - 0.02%. The main mineral from which the industry obtains chromium is chromium spinel of variable composition with the general formula (Mg, Fe) O · (Cr, Al, Fe) 2 O 3. Chromium ore is called chromite or chromium iron ore (because it almost always contains iron). Chromium ore deposits are found in many places. Our country has huge reserves of chromite. One of the largest deposits is located in Kazakhstan, in the Aktyubinsk region; it was discovered in 1936. There are also significant reserves of chrome ores in the Urals.
Chromites are mainly used for the smelting of ferrochrome. It is one of the most important ferroalloys, absolutely essential for the mass production of alloy steels.
Ferroalloys are alloys of iron with other elements used in the main rite for alloying and deoxidizing steel. Ferrochrome contains at least 60% Cr.
Tsarist Russia almost did not produce ferroalloys. In several blast furnaces of the southern factories, low-percentage (in terms of alloying metal) ferrosilicon and ferromanganese were smelted. Moreover, on the Satka River, which flows in the Southern Urals, in 1910 a tiny factory was built that smelted scanty amounts of ferromanganese and ferrochrome.
In the early years of development, the young Soviet country had to import ferroalloys from abroad. Such dependence on capitalist countries was unacceptable. Already in 1927 ... 1928. construction of Soviet ferroalloy plants began. At the end of 1930, the first large ferroalloy furnace was built in Chelyabinsk, and in 1931 the Chelyabinsk plant, the firstborn of the USSR ferroalloy industry, was commissioned. In 1933, two more factories were launched - in Zaporozhye and Zestafoni. This made it possible to stop the import of ferroalloys. In just a few years, the Soviet Union organized the production of many types of special steels - ball bearing, heat-resistant, stainless, automotive, high-speed ... All these steels include chromium.
At the 17th Party Congress, the People's Commissar of Heavy Industry Sergo Ordzhonikidze said: “... if we did not have high-quality steels, we would not have an automobile and tractor industry. The cost of high-quality steels we are currently consuming is estimated at over 400 million rubles. If it was necessary to import it, it would be 400 million rubles. every year, you would, damn it, fall into bondage to the capitalists ... "
The plant on the basis of the Aktobe field was built later, during the Great Patriotic War. He gave the first smelting of ferrochrome on January 20, 1943. The workers of the city of Aktyubinsk took part in the construction of the plant. The building was declared a national building. Ferrochrome of the new plant was used to manufacture metal for tanks and guns, for the needs of the front.
Years have passed. Now the Aktobe Ferroalloy Plant is the largest enterprise producing ferrochromium of all grades. Highly qualified national cadres of metallurgists have grown at the plant. From year to year, the plant and chromite mines are increasing their capacity, providing our ferrous metallurgy with high-quality ferrochrome.
In our country there is a unique deposit of naturally alloyed iron ores, rich in chromium and nickel. It is located in the Orenburg steppes. On the basis of this deposit, the Orsk-Khalilovsky metallurgical plant was built and operates. In the blast furnaces of the plant, naturally-alloyed cast iron with high heat resistance is smelted. Part of it is used in the form of casting, but most of it is sent for processing into nickel steel; chrome burns out when steel is smelted from cast iron.
Cuba, Yugoslavia, many countries of Asia and Africa have large reserves of chromites.
How is it received
Chromite is mainly used in three industries: metallurgy, chemistry and refractory production, and metallurgy consumes about two-thirds of all chromite.
Steel alloyed with chromium has increased strength, corrosion resistance in aggressive and oxidizing environments.
Obtaining pure chromium is an expensive and laborious process. Therefore, for alloying steel, ferrochrome is mainly used, which is obtained in electric arc furnaces directly from chromite. Coke serves as a reducing agent. The chromium oxide content in chromite must be at least 48%, and the Cr: Fe ratio must be at least 3: 1.
Ferrochrome obtained in an electric furnace usually contains up to 80% chromium and 4 ... 7% carbon (the rest is iron).
But for alloying many high-quality steels, ferrochrome is needed, which contains little carbon (the reasons for this - below, in the chapter "Chromium in alloys"). Therefore, part of the high-carbon ferrochrome is subjected to a special treatment in order to reduce the carbon content in it to tenths and hundredths of a percent.
Elemental metallic chromium is also obtained from chromite. The production of technically pure chromium (97 ... 99%) is based on the aluminothermy method, discovered back in 1865 by the famous Russian chemist N.N. Beketov. The essence of the method is in the reduction of oxides with aluminum; the reaction is accompanied by a significant release of heat.
But first you need to get pure chromium oxide Cr 2 O 3. For this, finely ground chromite is mixed with soda and limestone or iron oxide is added to this mixture. The entire mass is fired, and sodium chromate is formed:
2Сr 2 О 3 + 4Na 2 CO 3 + 3О 2 → 4Na 2 CrO 4 + 4CO 2.
Then sodium chromate is leached from the fired mass with water; the liquor is filtered, evaporated and treated with acid. The result is sodium dichromate Na 2 Cr 2 O 7. By reducing it with sulfur or carbon when heated, green chromium oxide is obtained.
Metallic chromium can be obtained by mixing pure chromium oxide with aluminum powder, heating this mixture in a crucible to 500 ... 600 ° C and igniting it with barium peroxide.Aluminum removes oxygen from chromium oxide. This reaction Cr 2 O 3 + 2Al → Al 2 O 3 + 2Cr is the basis of the industrial (aluminothermic) method for producing chromium, although, of course, the factory technology is much more complicated. Chromium, obtained aluminothermically, contains tenths of a percent of aluminum and iron, and hundredths of a percent of silicon, carbon and sulfur.
The silicothermal method for producing commercially pure chromium is also used. In this case, chromium is reduced from oxide by silicon according to the reaction
2Сr 2 О 3 + 3Si → 3SiO 2 + 4Сr.
This reaction takes place in arc furnaces. To bind silica, limestone is added to the charge. The purity of silicothermic chromium is approximately the same as that of aluminothermic, although, of course, the content of silicon in it is slightly higher, and aluminum is somewhat lower. To obtain chromium, they tried to use other reducing agents - carbon, hydrogen, magnesium. However, these methods are not widely used.
Chromium of high purity (about 99.8%) is obtained electrolytically.
Technically pure and electrolytic chromium is mainly used for the production of complex chromium alloys.
Chromium constants and properties
The atomic mass of chromium is 51.996. In the Mendeleev table, he ranks in the sixth group. Its closest neighbors and analogues are molybdenum and tungsten. It is characteristic that the neighbors of chromium, as well as itself, are widely used for alloying steels.
The melting point of chromium depends on its purity. Many researchers tried to determine it and obtained values from 1513 to 1920 ° C. Such a large "spread" is primarily due to the amount and composition of impurities contained in chromium. Chromium is now believed to melt at around 1875 ° C. Boiling point 2199 ° C. The density of chromium is less than that of iron; it is 7.19.
In terms of chemical properties, chromium is close to molybdenum and tungsten. Its highest oxide CrO 3 is acidic, it is chromic acid anhydride H 2 CrO 4. The mineral crocoite, with which we began our acquaintance with element 24, is the salt of this acid. In addition to chromic acid, bichromic acid H 2 Cr 2 O 7 is known; its salts, bichromates, are widely used in chemistry. The most common chromium oxide Cr 2 O 3 is amphoteric. In general, under different conditions, chromium can exhibit valences from 2 to 6. Only compounds of tri- and hexavalent chromium are widely used.
Chromium (Cr) is an element with atomic number 24 and atomic mass 51.996 of a secondary subgroup of the sixth group of the fourth period of the periodic system of chemical elements of D.I.Mendeleev. Chromium is a bluish-white hard metal. Possesses high chemical resistance. At room temperature, Cr is resistant to water and air. This element is one of the most important metals used in the industrial alloying of steels. Chromium compounds are brightly colored in various colors, for which, in fact, it got its name. Indeed, in translation from Greek "chrome" means "paint".
There are 24 known chromium isotopes from 42Cr to 66Cr. Stable natural isotopes 50Cr (4.31%), 52Cr (87.76%), 53Cr (9.55%) and 54Cr (2.38%). Of the six artificial radioactive isotopes, 51Cr is the most important with a half-life of 27.8 days. It is used as an isotope indicator.
Unlike the metals of antiquity (gold, silver, copper, iron, tin and lead), chromium has its own "discoverer". In 1766, a mineral was found in the vicinity of Yekaterinburg, which was named "Siberian red lead" - PbCrO4. In 1797, L. N. Vauquelin discovered element No. 24 in the mineral crocoite, a natural lead chromate. Around the same time (1798), independently of Vauquelin, chromium was discovered by German scientists M. G. Klaproth and Lovitz in a sample of heavy black mineral (it was chromite FeCr2O4) found in the Urals. Later, in 1799, F. Tassert discovered a new metal in the same mineral found in the south-east of France. It is believed that it was Tassert who first managed to obtain relatively pure metallic chromium.
Metallic chromium is used for chromium plating, as well as one of the most important components of alloy steels (in particular stainless steels). In addition, chromium has found application in a number of other alloys (acid-resistant and heat-resistant steels). Indeed, the introduction of this metal into steel increases its resistance to corrosion both in aqueous media at normal temperatures and in gases at elevated temperatures. Chromium steels are characterized by increased hardness. Chromium is used in thermochromizing - a process in which the protective effect of Cr is due to the formation of a thin but strong oxide film on the steel surface, which prevents the metal from interacting with the environment.
Chromium compounds are also widely used, so chromites are successfully used in the refractory industry: magnesite-chromite bricks are lined with open-hearth furnaces and other metallurgical equipment.
Chromium is one of the biogenic elements that are constantly included in the tissues of plants and animals. Plants contain chromium in the leaves, where it is present as a low molecular weight complex not associated with subcellular structures. Until now, scientists have not been able to prove the need for this element for plants. However, in animals, Cr is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), carbohydrates (a structural component of the glucose-resistant factor). It is known that exclusively trivalent chromium is involved in biochemical processes. Like most other important nutrients, chromium enters the body of an animal or human through food. A decrease in this trace element in the body leads to a slowdown in growth, a sharp increase in blood cholesterol levels and a decrease in the sensitivity of peripheral tissues to insulin.
At the same time, in its pure form, chromium is very toxic - metal dust of Cr irritates the tissues of the lungs, chromium (III) compounds cause dermatitis. Chromium (VI) compounds lead to various human diseases, including cancer.
Biological properties
Chromium is an important biogenic element, which is certainly a part of the tissues of plants, animals and humans. The average content of this element in plants is 0.0005%, and almost all of it accumulates in the roots (92-95%), the rest is contained in the leaves. Higher plants do not tolerate concentrations of this metal above 3 ∙ 10-4 mol / l. In animals, the chromium content ranges from ten-thousandths to ten-millionths of a percent. But in plankton the coefficient of chromium accumulation is striking - 10,000-26,000. In an adult human body, the content of Cr ranges from 6 to 12 mg. Moreover, the physiological need for chromium for a person has not been established quite accurately. It largely depends on the diet - when eating food with a high sugar content, the body's need for chromium increases. It is generally accepted that a person needs about 20-300 mcg of this element per day. Like other nutrients, chromium is able to accumulate in body tissues, especially hair. It is in them that the chromium content indicates the degree of the body's supply with this metal. Unfortunately, with age, the "reserves" of chromium in tissues are depleted, with the exception of the lungs.
Chromium is involved in the metabolism of lipids, proteins (present in the enzyme trypsin), carbohydrates (is a structural component of the glucose-resistant factor). This factor ensures the interaction of cellular receptors with insulin, thereby reducing the body's need for it. Glucose Tolerance Factor (GTF) enhances the action of insulin in all metabolic processes with its participation. In addition, chromium takes part in the regulation of cholesterol metabolism and is an activator of some enzymes.
The main source of chromium entering the body of animals and humans is food. Scientists have found that the concentration of chromium in plant foods is significantly lower than in animals. The richest in chromium are brewer's yeast, meat, liver, legumes and whole unprocessed grains. A decrease in the content of this metal in food and blood leads to a decrease in the growth rate, an increase in blood cholesterol, and a decrease in the sensitivity of peripheral tissues to insulin (a diabetes-like state). In addition, the risk of developing atherosclerosis and disorders of higher nervous activity increases.
However, even at concentrations of a fraction of a milligram per cubic meter in the atmosphere, all chromium compounds have a toxic effect on the body. Poisoning with chromium and its compounds is frequent during their production, in mechanical engineering, metallurgy, and in the textile industry. The degree of toxicity of chromium depends on the chemical structure of its compounds - dichromates are more toxic than chromates, compounds Cr + 6 are more toxic than compounds Cr + 2 and Cr + 3. Signs of poisoning are manifested by a feeling of dryness and pain in the nasal cavity, acute sore throat, difficulty breathing, coughing and similar symptoms. With a slight excess of chromium vapors or dust, signs of poisoning disappear shortly after stopping work in the workshop. With prolonged constant contact with chromium compounds, signs of chronic poisoning appear - weakness, persistent headaches, weight loss, dyspepsia. Disorders begin in the work of the gastrointestinal tract, pancreas, and liver. Bronchitis, bronchial asthma, pneumosclerosis develop. Skin diseases appear - dermatitis, eczema. In addition, chromium compounds are dangerous carcinogens that can accumulate in body tissues, causing cancer.
Prevention of poisoning is periodic medical examinations of personnel working with chromium and its compounds; installation of ventilation, dust suppression and dust collection facilities; use of personal protective equipment by workers (respirators, gloves).
The root "chrome" in its concept of "color", "paint" is part of many words used in a wide variety of fields: science, technology and even music. So many names of photographic films contain this root: "orthochrome", "panchrome", "isopanchrome" and others. The word chromosome is made up of two Greek words: chromo and soma. Literally, this can be translated as "a painted body" or "a body that is painted." The structural element of the chromosome, which forms in the interphase of the cell nucleus as a result of the duplication of chromosomes, is called "chromatid". “Chromatin” is a chromasome substance found in the nuclei of plant and animal cells, which is intensely stained with nuclear dyes. "Chromatophores" are pigment cells in animals and humans. In music, the concept of "chromatic scale" is used. "Khromka" is one of the types of Russian accordion. In optics, there are the concepts of "chromatic aberration" and "chromatic polarization". "Chromatography" is a physicochemical method for the separation and analysis of mixtures. "Chromoscope" - a device for obtaining a color image by optical alignment of two or three color-separated photographic images, illuminated through specially selected differently colored light filters.
The most poisonous is chromium oxide (VI) CrO3, it belongs to the I hazard class. A lethal dose for humans (oral) 0.6 g. Ethyl alcohol ignites in contact with freshly prepared CrO3!
The most common grade of stainless steel contains 18% Cr, 8% Ni, about 0.1% C. It resists corrosion and oxidation, and retains its strength at high temperatures. It was from this steel that the sheets were made that were used in the construction of the sculptural group of V.I. Mukhina "Worker and Collective Farm Woman".
Ferrochrome, used in the metallurgical industry for the production of chromium steels, was of very poor quality at the end of the 9th century. This is due to the low chromium content in it - only 7-8%. Then it was called "Tasmanian cast iron" in view of the fact that the original iron-chrome ore was imported from Tasmania.
It was previously mentioned that chrome alum is used in leather tanning. Thanks to this, the concept of "chrome" boots appeared. Leather tanned with chromium compounds gains shine, gloss and durability.
Many laboratories use a "chromium mixture" - a mixture of a saturated solution of potassium dichromate with concentrated sulfuric acid. It is used in degreasing glass and steel laboratory glassware. It oxidizes fat and removes residues. It is only necessary to handle this mixture with care, because it is a mixture of a strong acid and a strong oxidizing agent!
Nowadays, wood is still used as a building material, because it is inexpensive and easy to process. But it also has many negative properties - susceptibility to fires, fungal diseases that destroy it. To avoid all these troubles, the tree is impregnated with special compounds containing chromates and dichromates plus zinc chloride, copper sulfate, sodium arsenate and some other substances. Thanks to such compositions, wood increases its resistance to fungi and bacteria, as well as to open fire.
Chrome has taken a special niche in the printing industry. In 1839, it was found that paper impregnated with sodium dichromate suddenly turns brown after being illuminated with bright light. Then it turned out that bichromate coatings on paper after curing do not dissolve in water, but, when moistened, acquire a bluish tint. This property was used by printers. The desired pattern was photographed on a plate with a colloidal coating containing dichromate. The illuminated places did not dissolve during washing, and the non-illuminated ones dissolved, and a drawing remained on the plate from which it was possible to print.
Story
The history of the discovery of element No. 24 began in 1761, when an unusual red mineral was found in the Berezovsky mine (the eastern foot of the Ural Mountains) near Yekaterinburg, which, when ground into dust, gave a yellow color. The find belonged to the professor of St. Petersburg University Johann Gottlob Lehmann. Five years later, the scientist delivered the samples to the city of St. Petersburg, where he conducted a number of experiments on them. In particular, he treated the unusual crystals with hydrochloric acid, producing a white precipitate in which lead was found. Based on the results obtained, Lehman called the mineral Siberian red lead. This is the story of the discovery of crocoite (from the Greek "krokos" - saffron) - a natural lead chromate PbCrO4.
Interested in this find, Peter Simon Pallas, a German naturalist and traveler, organized and led the expedition of the St. Petersburg Academy of Sciences in the heart of Russia. In 1770, the expedition reached the Urals and visited the Berezovsky mine, where samples of the studied mineral were taken. Here is how the traveler himself describes it: “This amazing red lead mineral is not found in any other deposit. When ground into powder it turns yellow and can be used in artistic miniatures. " German entrepreneurship overcame all the difficulties of harvesting and delivering crocoite to Europe. Despite the fact that these operations took at least two years, soon the carriages of the noble gentlemen of Paris and London rode painted with finely crushed crocoite. Collections of mineralogical museums of many universities of the old world have been enriched with the best samples of this mineral from the Russian interior. However, European scientists could not figure out the composition of the mysterious mineral.
This lasted for thirty years, until a sample of Siberian red lead fell into the hands of the professor of chemistry at the Paris Mineralogical School Nicolas Louis Vauquelin in 1796. After analyzing the crocoite, the scientist did not find anything in it except for the oxides of iron, lead and aluminum. Subsequently, Vauquelin treated the crocoite with a solution of potash (K2CO3) and, following the precipitation of a white precipitate of lead carbonate, he isolated a yellow solution of an unknown salt. After conducting a series of experiments on processing the mineral with salts of various metals, the professor, using hydrochloric acid, isolated a solution of "red lead acid" - chromium oxide and water (chromic acid exists only in dilute solutions). By evaporating this solution, he obtained ruby-red crystals (chromic anhydride). Further heating of the crystals in a graphite crucible in the presence of coal yielded a multitude of intergrown gray needle-like crystals - a new, hitherto unknown metal. The next series of experiments showed the high refractoriness of the obtained element and its resistance to acids. The Paris Academy of Sciences immediately witnessed the discovery, the scientist, at the insistence of his friends, gave a name to the new element - chrome (from the Greek "color", "color") due to the variety of shades of compounds formed by it. In his further works, Vauquelin confidently stated that the emerald color of some gemstones, as well as natural silicates of beryllium and aluminum, is explained by the admixture of chromium compounds in them. An example is emerald, which is a green-colored beryl in which aluminum is partially replaced by chromium.
It is clear that Vauquelin did not receive a pure metal, most likely its carbides, which is confirmed by the acicular shape of light gray crystals. Pure metallic chromium was later obtained by F. Tassert, presumably in 1800.
Also, independently of Vauquelin, chromium was discovered by Klaproth and Lovitz in 1798.
Being in nature
In the bowels of the earth, chromium is a fairly common element, despite the fact that it is not found in free form. Its clarke (average content in the earth's crust) is 8.3.10-3% or 83 ppm. However, its distribution across breeds is uneven. This element is mainly characteristic of the Earth's mantle, the fact is that ultrabasic rocks (peridotites), which are supposedly close in composition to the mantle of our planet, are richest in chromium: 2 10-1% or 2 kg / t. In such rocks, Cr forms massive and disseminated ores; the formation of the largest deposits of this element is associated with them. The chromium content is also high in basic rocks (basalts, etc.) 2 10-2% or 200 g / t. Much less Cr in acidic rocks: 2.5 10-3%, sedimentary (sandstones) - 3.5 10-3%, shales also contain chromium - 9 10-3%.
It can be concluded that chromium is a typical lithophilic element and almost all of it is contained in deeply buried minerals in the Earth's interior.
There are three main chromium minerals: magnochromite (Mn, Fe) Cr2O4, chromopicotite (Mg, Fe) (Cr, Al) 2O4, and alumochromite (Fe, Mg) (Cr, Al) 2O4. These minerals have a single name - chromium spinel and the general formula (Mg, Fe) O (Cr, Al, Fe) 2O3. They are indistinguishable in appearance and are inaccurately referred to as "chromites". Their composition is changeable. The content of the most important components varies (wt%): Cr2O3 from 10.5 to 62.0; Al2O3 4 to 34.0; Fe2O3 1.0 to 18.0; FeO 7.0 to 24.0; MgO 10.5 to 33.0; SiO2 from 0.4 to 27.0; TiO2 impurities up to 2; V2O5 up to 0.2; ZnO up to 5; MnO up to 1. Some chromium ores contain 0.1-0.2 g / t of elements of the platinum group and up to 0.2 g / t of gold.
In addition to various chromites, chromium is a part of a number of other minerals - chromvesuvian, chromium chlorite, chromium tourmaline, chromium mica (fuchsite), chrome garnet (uvarovite), etc., which often accompany ores, but are not of industrial importance themselves. Chromium is a relatively weak water migrant. Under exogenous conditions, chromium, like iron, migrates in the form of suspensions and can be deposited in clays. Chromates are the most mobile form.
Of practical importance is, perhaps, only chromite FeCr2O4, which belongs to spinels - isomorphic minerals of the cubic system with the general formula MO Me2O3, where M is a divalent metal ion, and Me is a trivalent metal ion. In addition to spinels, chromium is found in many much less common minerals, for example, melanochroite 3PbO 2Cr2O3, vokelenite 2 (Pb, Cu) CrO4 (Pb, Cu) 3 (PO4) 2, tarapakaite K2CrO4, ditzeite CaIO3 CaCrO4, and others.
Chromites are usually found in the form of black granular masses, less often in the form of octahedral crystals, have a metallic luster, and lie in the form of continuous massifs.
At the end of the 20th century, the reserves of chromium (identified) in almost fifty countries of the world with deposits of this metal amounted to 1,674 million tons. The leading position is occupied by the Republic of South Africa - 1,050 million tons, where the main contribution is made by the Bushveld complex (about 1000 million tons ). The second place in terms of chromium resources belongs to Kazakhstan, where very high quality ore is mined in the Aktobe region (Kempirsay massif). Other countries also have stocks of this element. Turkey (in Guleman), the Philippines on the island of Luzon, Finland (Kemi), India (Sukinda), etc.
Our country has its own developed chromium deposits - in the Urals (Donskoye, Saranovskoye, Khalilovskoye, Alapaevskoye and many others). Moreover, at the beginning of the 19th century, it was the Ural deposits that were the main sources of chrome ores. Only in 1827 the American Isaac Tison discovered a large deposit of chrome ore on the border of Maryland and Pennsylvania, seizing the mining monopoly for many years. In 1848, high-quality chromite deposits were found in Turkey, near Bursa, and soon (after the depletion of the Pennsylvania deposit), it was this country that took over the role of a monopolist. This continued until 1906, when rich deposits of chromite were discovered in South Africa and India.
Application
The total consumption of pure chromium metal today is approximately 15 million tonnes. The production of electrolytic chromium - the purest - accounts for 5 million tons, which is one third of the total consumption.
Chromium is widely used for alloying steels and alloys, giving them corrosion and heat resistance. More than 40% of the resulting pure metal is consumed for the manufacture of such "superalloys". The most famous resistance alloys are nichrome with 15-20% Cr, heat-resistant alloys - 13-60% Cr, stainless - 18% Cr and ball bearing steels 1% Cr. The addition of chromium to common steels improves their physical properties and makes the metal more susceptible to heat treatment.
Metallic chromium is used for chromium plating - applying a thin layer of chromium to the surface of steel alloys in order to increase the corrosion resistance of these alloys. The chrome-plated coating perfectly resists the effects of humid atmospheric air, salty sea air, water, nitric and most organic acids. Such coatings can be used for two purposes: protective and decorative. The thickness of the protective coatings is about 0.1 mm, they are applied directly to the product and give it increased wear resistance. Decorative coatings have aesthetic value, they are applied to a layer of another metal (copper or nickel), which actually performs a protective function. The thickness of such a coating is only 0.0002–0.0005 mm.
Chromium compounds are also actively used in various fields.
The main chromium ore, chromite FeCr2O4, is used in the production of refractories. Magnesite-chromite bricks are chemically passive and heat-resistant, they withstand sudden multiple changes in temperature, therefore they are used in the structures of the roofs of open-hearth furnaces and the working space of other metallurgical devices and structures.
The hardness of crystals of chromium (III) oxide - Cr2O3 is comparable to the hardness of corundum, which ensured its use in the compositions of grinding and lapping pastes used in mechanical engineering, jewelry, optical and watch industries. It is also used as a catalyst for the hydrogenation and dehydrogenation of certain organic compounds. Cr2O3 is used in painting as a green pigment and for coloring glass.
Potassium chromate - K2CrO4 is used in leather tanning, as a mordant in the textile industry, in the production of dyes, and in wax bleaching.
Potassium dichromate (chromopik) - K2Cr2O7 is also used for tanning leather, staining for fabric dyeing, and is a corrosion inhibitor for metals and alloys. It is used in the manufacture of matches and for laboratory purposes.
Chromium (II) chloride CrCl2 is a very strong reducing agent, easily oxidized even by atmospheric oxygen, which is used in gas analysis for the quantitative absorption of O2. In addition, it is limitedly used in the production of chromium by electrolysis of molten salts and chromatometry.
Potassium chromium alum K2SO4.Cr2 (SO4) 3 24H2O is used mainly in the textile industry - for tanning leather.
Anhydrous chromium chloride CrCl3 is used for the deposition of chromium coatings on the surface of steels by chemical vapor deposition, and is an integral part of some catalysts. Hydrates CrCl3 - mordant for dyeing fabrics.
Various dyes are made from lead chromate PbCrO4.
The surface of the steel wire is cleaned and etched with a solution of sodium dichromate before galvanizing, and the brass is also clarified. Chromic acid is obtained from sodium bichromate, which is used as an electrolyte in the chromium plating of metal parts.
Production
In nature, chromium occurs mainly in the form of chromium iron ore FeO ∙ Cr2O3, when it is reduced with coal, an alloy of chromium with iron is obtained - ferrochrome, which is directly used in the metallurgical industry in the production of chromium steels. The chromium content in this composition reaches 80% (by weight).
Reduction of chromium (III) oxide with coal is intended to produce high-carbon chromium, which is necessary for the production of special alloys. The process is carried out in an electric arc furnace.
To obtain pure chromium, chromium (III) oxide is preliminarily obtained, and then it is reduced by the aluminothermal method. In this case, a preliminary mixture of powder or in the form of aluminum (Al) chips and a charge of chromium oxide (Cr2O3) is heated to a temperature of 500-600 ° C. ... In this process, it is important that the generated heat energy is sufficient to melt the chromium and separate it from the slag.
Cr2O3 + 2Al = 2Cr + 2Al2O3
The chromium obtained in this way contains a certain amount of impurities: iron 0.25-0.40%, sulfur 0.02%, carbon 0.015-0.02%. The content of the pure substance is 99.1–99.4%. Such chromium is brittle and easily ground into powder.
The reality of this method was proven and demonstrated back in 1859 by Friedrich Wöhler. On an industrial scale, alumothermal reduction of chromium became possible only after a method for producing cheap aluminum became available. Goldschmidt was the first to develop a safe way to control the highly exothermic (hence explosive) reduction process.
If it is necessary to obtain high-purity chromium in industry, electrolytic methods are used. A mixture of chromic anhydride, chromium ammonium alum or chromium sulfate with dilute sulfuric acid is subjected to electrolysis. Chromium deposited during electrolysis on aluminum or stainless cathodes contains dissolved gases as impurities. The purity of 99.90–99.995% can be achieved with the help of high-temperature (1500-1700 ° C) purification in a stream of hydrogen and vacuum degassing. Advanced electrolytic chromium refining techniques remove sulfur, nitrogen, oxygen and hydrogen from the "crude" product.
In addition, it is possible to obtain metallic Cr by electrolysis of CrCl3 or CrF3 melts in a mixture with potassium, calcium, sodium fluorides at a temperature of 900 ° C in an argon atmosphere.
The possibility of an electrolytic method for producing pure chromium was proved by Bunsen in 1854 by electrolysis of an aqueous solution of chromium chloride.
The industry also uses a silicothermal method for producing pure chromium. In this case, chromium is reduced from oxide by silicon:
2Cr2O3 + 3Si + 3CaO = 4Cr + 3CaSiO3
Silicothermally, chromium is smelted in arc furnaces. The addition of quicklime makes it possible to convert refractory silicon dioxide into low-melting calcium silicate slag. The purity of silicothermal chromium is approximately the same as that of aluminothermic, however, naturally, the content of silicon in it is slightly higher, and aluminum is somewhat lower.
Cr can also be obtained by reduction of Cr2O3 with hydrogen at 1500 ° C, reduction of anhydrous CrCl3 with hydrogen, alkali or alkaline earth metals, magnesium and zinc.
To obtain chromium, they tried to use other reducing agents - carbon, hydrogen, magnesium. However, these methods are not widely used.
In the Van Arkel - Kuchman - De Boer process, the decomposition of chromium (III) iodide is used on a wire heated to 1100 ° C with the deposition of pure metal on it.
Physical properties
Chromium is a hard, very heavy, refractory, malleable metal of a steel-gray color. Pure chromium is quite plastic, crystallizes in a body-centered lattice, a = 2.885 Å (at a temperature of 20 ° C). At a temperature of about 1830 ° C, the probability of transformation into a modification with a face-centered lattice is high, a = 3.69 Å. Atomic radius 1.27 Å; ionic radii Cr2 + 0.83 Å, Cr3 + 0.64 Å, Cr6 + 0.52 Å.
The melting point of chromium directly depends on its purity. Therefore, the determination of this indicator for pure chromium is a very difficult task - after all, even a small content of nitrogen or oxygen impurities can significantly change the value of the melting point. Many researchers for more than one decade have been dealing with this issue and obtained results that are far from each other: from 1513 to 1920 ° C. It was previously believed that this metal melts at a temperature of 1890 ° C, but modern research indicates a temperature of 1907 ° C. chromium boils at temperatures above 2500 ° C - the data also vary: from 2199 ° C to 2671 ° C. The density of chromium is less than that of iron; it is 7.19 g / cm3 (at a temperature of 200 ° C).
Chromium has all the basic characteristics of metals - it conducts heat well, its resistance to electric current is very small, like most metals, chromium has a characteristic luster. In addition, this element has one very interesting feature: the fact is that at a temperature of 37 ° C its behavior defies explanation - there is a sharp change in many physical properties, this change is of an abrupt nature. Chromium, like a sick person at a temperature of 37 ° C, begins to be capricious: the internal friction of chromium reaches a maximum, the modulus of elasticity drops to minimum values. The value of electrical conductivity jumps, the thermoelectromotive force, the coefficient of linear expansion are constantly changing. Scientists cannot yet explain this phenomenon.
The specific heat capacity of chromium is 0.461 kJ / (kg.K) or 0.11 cal / (g ° C) (at a temperature of 25 ° C); thermal conductivity coefficient 67 W / (m K) or 0.16 cal / (cm sec ° С) (at a temperature of 20 ° С). Thermal coefficient of linear expansion 8.24 10-6 (at 20 ° C). Chromium at a temperature of 20 ° C has a specific electrical resistance of 0.414 mOhm m, and its thermal coefficient of electrical resistance in the range of 20-600 ° C is 3.01 10-3.
It is known that chromium is very sensitive to impurities - the smallest fractions of other elements (oxygen, nitrogen, carbon) can make chromium very brittle. It is extremely difficult to obtain chromium without these impurities. For this reason, this metal is not used for structural purposes. But in metallurgy, it is actively used as an alloying material, since its addition to the alloy makes steel hard and wear-resistant, because chromium is the hardest of all metals - it cuts glass like diamond! Brinell hardness of high-purity chromium is 7-9 Mn / m2 (70-90 kgf / cm2). Spring, spring, tool, die and ball bearing steels are alloyed with chromium. In them (except for ball bearing steels) chromium is present together with manganese, molybdenum, nickel, vanadium. The addition of chromium to common steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment.
Chromium is antiferromagnetic, specific magnetic susceptibility 3.6 10-6. Specific electrical resistance 12.710-8 Ohm. The temperature coefficient of linear expansion of chromium is 6,210-6. The heat of vaporization of this metal is 344.4 kJ / mol.
Chromium is resistant to corrosion in air and water.
Chemical properties
Chemically, chromium is quite inert; this is due to the presence of a strong thin oxide film on its surface. Cr does not oxidize in air, even in the presence of moisture. When heated, oxidation occurs exclusively on the metal surface. At 1200 ° C, the film breaks down and oxidation proceeds much faster. At 2000 ° C, chromium burns to form the green chromium (III) oxide Cr2O3, which has amphoteric properties. By fusing Cr2O3 with alkalis, chromites are obtained:
Cr2O3 + 2NaOH = 2NaCrO2 + H2O
Uncalcined chromium (III) oxide dissolves easily in alkaline solutions and acids:
Cr2O3 + 6HCl = 2CrCl3 + 3H2O
In compounds, chromium mainly exhibits the oxidation states Cr + 2, Cr + 3, Cr + 6. The most stable are Cr + 3 and Cr + 6. There are also some compounds where chromium has oxidation states Cr + 1, Cr + 4, Cr + 5. Chromium compounds are very diverse in color: white, blue, green, red, purple, black and many others.
Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chromium chloride and sulfate and release hydrogen:
Cr + 2HCl = CrCl2 + H2
Tsarskaya vodka and nitric acid passivate chromium. Moreover, chromium passivated with nitric acid does not dissolve in dilute sulfuric and hydrochloric acids even with prolonged boiling in their solutions, but at some point dissolution still occurs, accompanied by violent foaming from the released hydrogen. This process is explained by the fact that chromium passes from a passive state to an active one, in which the metal is not protected by a protective film. Moreover, if nitric acid is added again during the dissolution process, the reaction will stop, since chromium is again passivated.
Under normal conditions, chromium reacts with fluorine to form CrF3. At temperatures above 600 ° C, interaction with water vapor occurs, the result of this interaction is chromium (III) oxide Сr2О3:
4Cr + 3O2 = 2Cr2O3
Cr2O3, is a green microcrystal with a density of 5220 kg / m3 and a high melting point (2437 ° C). Chromium (III) oxide exhibits amphoteric properties, but is very inert, it is difficult to dissolve in aqueous acids and alkalis. Chromium (III) oxide is quite toxic. When it gets on the skin, it can cause eczema and other skin conditions. Therefore, when working with chromium (III) oxide, it is imperative to use personal protective equipment.
In addition to oxide, other compounds with oxygen are known: CrO, CrO3, obtained indirectly. The greatest hazard is inhaled oxide aerosol, which causes severe disease of the upper respiratory tract and lungs.
Chromium forms a large number of salts with oxygen-containing components.
And fat.
Scientists claim that cholesterol levels are affected by chromium. Element it is considered biogenic, that is, it is necessary for the body, not only for humans, but also for all mammals.
With a lack of chromium, their growth slows down and cholesterol "jumps". The norm is 6 milligrams of chromium from the total mass of a person.
Ions of matter are present in all tissues of the body. 9 micrograms should be received per day.
You can take them from seafood, pearl barley, beets, liver and duck meat. While you are purchasing products, we will tell you about other purposes and properties of chromium.
Chromium properties
Chromium is a chemical element related to metals. The color of the substance is silvery-blue.
The element is under the 24th ordinal, or, as they say, atomic number.
The number indicates the number of protons in the nucleus. As for the electrons orbiting around it, they have a special property - to fall through.
This means that one or two particles can move from one sublevel to another.
As a result, the 24th element is able to fill half of the 3rd sublevel. The result is a stable electronic configuration.
Electron sinking is rare. In addition to chromium, perhaps only,, and are remembered.
Like the 24th substance, they are chemically inactive. Then the atom does not come to a stable state in order to react with everyone in a row.
Under normal conditions chrome - an element of the periodic table, Which can only be "stirred up".
The latter, is the antipode of the 24th substance, is maximally active. During the reaction, fluoride is formed chrome.
Element, properties which are discussed, does not oxidize, is not afraid of moisture and is refractory.
The latter characteristic "delays" the reactions possible during heating. So, interaction with water vapor starts only at 600 degrees Celsius.
It turns out chromium oxide. Reaction c is also triggered, giving nitride of the 24th element.
At 600 degrees, several compounds with and the formation of sulfide are also possible.
If the temperature is brought up to 2000, chromium will ignite on contact with oxygen. Combustion will result in a dark green oxide.
This precipitate reacts readily with solutions and acids. The result of the interaction is chromium chloride and sulfide. All compounds of the 24th substance are usually brightly colored.
In its pure form, the main chromium element characteristic- toxicity. Metal dust irritates lung tissue.
Dermatitis, that is, allergic diseases, may appear. Accordingly, it is better not to exceed the norm of chromium for the body.
There is also a norm for the content of the 24th element in the air. There should be 0.0015 milligrams per cubic meter of atmosphere. Exceeding the standard is considered contamination.
Chromium metal has a high density - over 7 grams per cubic centimeter. This means the substance is quite heavy.
The metal is pretty high too. It depends on the temperature of the electrolyte and the current density. For fungi and mold, this seems to command respect.
If wood is impregnated with a chromium composition, microorganisms will not undertake to destroy it. This is used by builders.
They are also satisfied with the fact that the treated wood burns worse, because chromium is a refractory metal. We will tell you how and where else it can be applied.
Application of chromium
Chromium - alloying element when smelting. Remember that under normal conditions the 24th metal does not oxidize, does not rust?
The base of steels is. It cannot boast of such properties. Therefore, chromium is added, which increases the corrosion resistance.
In addition, the addition of the 24th substance lowers the critical cooling rate point.
Silicothermal chromium is used for smelting. This is the duet of the 24th element with nickel.
As additives are silicon,. Nickel is responsible for ductility, while chromium is responsible for its oxidation resistance and hardness.
Connect chrome and c. It turns out superhard stellite. Additives to it are molybdenum and.
The composition is expensive, but necessary for surfacing machine parts in order to increase their wear resistance. Stellite is sprayed on working machines,.
In decorative corrosion-resistant coatings, as a rule, chromium compounds.
A bright range of colors comes in handy. In cermets, color is not needed, therefore, powder chrome is used. It is added, for example, for strength to the bottom layer of crowns for.
Chromium formula- component . This is a mineral from the group, but it does not have the usual color.
Uvarovite is a stone, and it is chrome that makes it so. It is no secret that they are used.
The green variety of the stone is no exception, and it is valued higher than the red one, since it is rare. Also, uvarovit a little standard.
This is also a plus, because mineral inserts are harder to scratch. The stone is cut faceted, that is, forming corners, which increases the play of light.
Chromium mining
It is not profitable to extract chromium from minerals. Most with the 24th element are used in their entirety.
In addition, the chromium content in, as a rule, is low. The substance is extracted, in the base, from ores.
One of them is associated opening of chrome. They found him in Siberia. Crocoite was found there in the 18th century. It is a red lead ore.
Its basis is, the second element is chrome. It was discovered by a German chemist named Lehmann.
At the time of the discovery of the crocoite, he was visiting St. Petersburg, where he conducted experiments. Now, the 24th element is obtained by electrolysis of concentrated aqueous solutions of chromium oxide.
The electrolysis of sulfate is also possible. These are 2 ways to get the cleanest chrome. Molecule oxide or sulfate is destroyed in the crucible, where the starting compounds are ignited.
The 24th element is separated, the rest goes into the slag. It remains to smelt chromium in the arc. This is how the purest metal is recovered.
There are other ways to get chromium element, for example, the reduction of its oxide with silicon.
But, this method gives a metal with a large amount of impurities and, moreover, is more expensive than electrolysis.
Chromium price
In 2016, the cost of chrome is still going down. January started at $ 7,450 per ton.
By the middle of summer, they ask for only 7100 conventional units for 1000 kilograms of metal. Data provided by Infogeo.ru.
That is, the Russian prices have been considered. Globally, the cost of chromium reached almost $ 9,000 per ton.
The smallest summer mark differs from the Russian one by only $ 25 upward.
If it is not an industrial sphere, for example, metallurgy, is considered, but benefits of chromium for the body, you can study the offers of pharmacies.
So, "Picolinat" of the 24th substance costs about 200 rubles. They ask for 320 rubles for Kartnitin Chrome Forte. That's the price tag for a 30-pill pack.
Turamine Chromium can also fill the deficit of the 24th element. Its cost is 136 rubles.
Chromium, by the way, is included in the tests to identify drugs, in particular, marijuana. One test costs 40-45 rubles.
Chemical properties of chromium compounds.
Cr 2+. The concentration of the charge of the divalent chromium cation corresponds to the concentration of the charge of the magnesium cation and the divalent iron cation; therefore, a number of properties, especially the acid-base behavior of these cations are similar. At the same time, as already mentioned, Cr 2+ is a strong reducing agent, therefore, the following reactions take place in the solution: 2CrCl 2 + 2HCl = 2CrCl 3 + H 2 4CrCl 2 + 4HCl + O 2 = 4CrCl 3 + 2H 2 O. Slow enough, but even oxidation with water occurs: 2CrSO 4 + 2H 2 O = 2Cr (OH) SO 4 + H 2. The oxidation of bivalent chromium is even easier than the oxidation of bivalent iron; salts are also moderately hydrolyzed by the cation (i.e., the first stage is dominant).
CrO - basic oxide, black, pyrophoric. At 700 ° C it disproportionates: 3CrO = 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. Reacts with acids, while oxidizing acids simultaneously with acid-base interaction oxidize bivalent chromium, under certain conditions this also happens with non-oxidizing acids (oxidizing agent - H +). When received by the exchange reaction, chromium (II) hydroxide quickly turns green due to oxidation:
4Cr (OH) 2 + O 2 = 4CrO (OH) + 2H 2 O.
Oxidation is accompanied by the decomposition of chromium (II) hydroxide in the presence of oxygen: 4Cr (OH) 2 = 2Cr 2 O 3 + 4H 2 O.
Cr 3+. Chromium (III) compounds are similar in chemical properties 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 = 2Cr (OH) 3 ↓ + 3H 2 S + 6KCl; Cr 2 S 3 + 6H 2 O = 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 upon heating, but by the reaction: 2CrCl 3 (cr) + 2H 2 S (gas) = 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.
Cr 2 O 3 - amphoteric oxide of green color, has a very strong crystal lattice, therefore, it exhibits chemical activity only in the amorphous state. Reacts mainly when fusion with acidic and basic oxides, with acids and alkalis, as well as with compounds that have acidic or basic functions:
Cr 2 O 3 + 3K 2 S 2 O 7 = Cr 2 (SO 4) 3 + 3K 2 SO 4; Cr 2 O 3 + K 2 CO 3 = 2KCrO 2 + CO 2.
Cr (OH) 3 (CrO (OH), Cr 2 O 3 * nH 2 O) - gray-blue amphoteric hydroxide. It dissolves in acids and alkalis. When dissolved in alkalis, hydroxo complexes are formed, in which the chromium cation has a coordination number of 4 or 6:
Cr (OH) 3 + NaOH = Na; Cr (OH) 3 + 3NaOH = Na 3.
Hydroxocomplexes are readily decomposed by acids, while the processes are different with strong and weak acids:
Na + 4HCl = NaCl + CrCl 3 + 4H 2 O; Na + CO 2 = Cr (OH) 3 ↓ + NaHCO 3.
Cr (III) compounds are not only oxidizing agents, but also reducing agents with respect to the transformation into Cr (VI) compounds. The reaction is especially easy in an alkaline environment:
2Na 3 + 3Cl 2 + 4NaOH = 2Na 2 CrO 4 + 6NaCl + 8H 2 O E 0 = - 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 oxidizing agents. The acid-base behavior of these compounds is similar to the behavior of sulfur compounds in the same oxidation state. Such a similarity in the properties of compounds of elements of the main and secondary subgroups in the maximum positive oxidation state is typical for most groups of the periodic system.
CrO 3 - a compound of dark red color, a typical acidic oxide. Decomposes at melting point: 4CrO 3 = 2Cr 2 O 3 + 3O 2.
An example of oxidative action: CrO 3 + NH 3 = Cr 2 O 3 + N 2 + H 2 O (When heated).
Chromium (VI) oxide dissolves easily in water, attaching it and turning into hydroxide:
H 2 CrO 4 - chromic acid is a strong diacid. It does not stand out in free form, because at a concentration above 75%, a condensation reaction takes place with the formation of dichromic acid: 2H 2 CrO 4 (yellow) = 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, chromic acid salts 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 = 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 dichromate undergoes an intramolecular redox reaction: (NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O.
HALOGENS ("Giving birth salts")
Halogens are the elements of the main subgroup of the VII group of the periodic system. These are fluorine, chlorine, bromine, iodine, astatine. The structure of the outer electron layer of their atoms: ns 2 np 5. Thus, there are 7 electrons on the external electronic level, and they lack only one electron to reach a stable shell of a noble gas. 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 capable of accepting an electron, transforming into an anion with a "1-" charge, or exhibiting an oxidation state "-1" when covalently bonded to less electronegative elements. At the same time, when moving along 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. Firstly, it exhibits only a negative oxidation state, since 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 lone electron pairs, on the fourth there is an unpaired electron, which in most cases is the only valence electron. In the atoms of other elements, at the outer level, there is an unfilled d-electronic sublevel, where an excited electron can transfer. Each lone pair gives two electrons when steamed, therefore 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".
As simple substances, all halogens are diatomic molecules with a single bond between 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 on going from chlorine to iodine is easily explained by an increase in the bond length due to an increase in the atomic radius. The abnormally 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, electron-electron repulsion occurs, as a result of which the overlapping of the orbitals does not occur completely, and the bond order in the fluorine molecule is somewhat less than one. 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 another atom, two such opposite interactions per molecule. Thus, the bond in chlorine, bromine and iodine molecules is defined as almost triple in terms of the presence of interactions. But donor-acceptor overlaps occur only partially, and the bond has the order (for a chlorine molecule) 1.12.
Physical properties: Under normal conditions, fluorine is a light yellow gas that is difficult to liquefy (boiling point -187 0 С), chlorine is easily liquefied (boiling point –34.2 0 С), yellow-green gas, bromine is a brown, easily evaporating liquid , iodine is a gray solid with a metallic luster. In the solid state, all halogens form a molecular crystal lattice characterized by weak intermolecular interactions. In this connection, iodine has a tendency to sublimation - when heated at atmospheric pressure, it passes into a gaseous state (forms purple vapors), bypassing the liquid one. When moving through the group from top to bottom, the melting and boiling points increase both due to an increase in the molecular weight of substances and due to an increase in the van der Waals forces acting between molecules. The magnitude of these forces is the greater, the greater the polarizability of the molecule, which, in turn, increases with an increase in the radius of the atom.
All halogens are poorly soluble in water, but well-soluble in non-polar organic solvents, such as carbon tetrachloride. Poor solubility in water is due to the fact that when a cavity is formed for dissolving a halogen molecule, water loses sufficiently strong hydrogen bonds, instead of which there are no strong interactions between its polar molecule and a non-polar halogen molecule. The dissolution of halogens in non-polar solvents corresponds to the situation: “like dissolves in like”, when the nature of breaking and forming bonds is the same.