Chromium is a chemical element with atomic number 24. It is a hard, shiny, steel-gray metal that polishes well and does not tarnish. Used in alloys such as stainless steel and as a coating. The human body requires small amounts of trivalent chromium to metabolize sugar, but Cr(VI) is highly toxic.
Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and used in paints and pigments. The red color of ruby is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (together with sulfuric acid) for cleaning laboratory glassware. In addition, chromium (VI) oxide is used in the production of magnetic tape.
The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it “Siberian red lead.” Although it was erroneously identified as a compound of lead with selenium and iron, the material was actually lead chromate with chemical formula PbCrO4. Today it is known as the mineral croconte.
In 1770, Peter Simon Pallas visited the site where Lehmann found a red lead mineral that had a very beneficial features pigment in paints. The use of Siberian red lead as paint developed rapidly. In addition, the bright yellow color of crocont has become fashionable.
In 1797, Nicolas-Louis Vauquelin obtained samples of red. By mixing croconte with hydrochloric acid, he obtained CrO 3 oxide. Chromium was isolated as a chemical element in 1798. Vauquelin obtained it by heating the oxide with charcoal. He was also able to detect traces of chromium in precious stones, such as ruby and emerald.
In the 1800s, Cr was primarily used in dyes and tanning salts. Today, 85% of the metal is used in alloys. The rest is applied in chemical industry, production of refractory materials and foundry industry.
The pronunciation of the chemical element chromium corresponds to the Greek χρῶμα, meaning "color", due to the variety of colored compounds that can be obtained from it.
The element is produced from chromite (FeCr 2 O 4). Approximately half of the world's ore is mined in South Africa. In addition, Kazakhstan, India and Türkiye are its major producers. There are enough explored deposits of chromite, but geographically they are concentrated in Kazakhstan and southern Africa.
Deposits of native chromium metal are rare, but they do exist. For example, it is mined at the Udachnaya mine in Russia. It is rich in diamonds, and the reducing environment helped produce pure chromium and diamonds.
For industrial production Metal chromite ores are treated with molten alkali (caustic soda, NaOH). In this case, sodium chromate (Na 2 CrO 4) is formed, which is reduced by carbon to the oxide Cr 2 O 3. The metal is produced by heating the oxide in the presence of aluminum or silicon.
In 2000, approximately 15 million tons of chromite ore were mined and processed into 4 million tons of ferrochrome, a 70% chromium-iron alloy, with an approximate market value of US$2.5 billion.
The characteristics of the chemical element chromium are due to the fact that it is a transition metal of the fourth period of the periodic table and is located between vanadium and manganese. Included in group VI. Melts at a temperature of 1907 °C. In the presence of oxygen, chromium quickly forms a thin layer of oxide, which protects the metal from further interaction with oxygen.
As a transition element, it reacts with substances in different proportions. Thus, it forms compounds in which it has different oxidation states. Chromium is a chemical element with the basic states +2, +3 and +6, of which +3 is the most stable. In addition, in rare cases conditions +1, +4 and +5 are observed. Chromium compounds in the +6 oxidation state are strong oxidizing agents.
What color is chrome? The chemical element gives the ruby hue. The Cr 2 O 3 used for is also used as a pigment called chrome green. Its salts color glass emerald green. Chromium is the chemical element whose presence makes rubies red. Therefore, it is used in the production of synthetic rubies.
Isotopes of chromium have atomic weights from 43 to 67. Typically, this chemical element consists of three stable forms: 52 Cr, 53 Cr and 54 Cr. Of these, 52 Cr is the most common (83.8% of all natural chromium). In addition, 19 radioisotopes have been described, of which the most stable is 50 Cr with a half-life exceeding 1.8x10 17 years. 51 Cr has a half-life of 27.7 days, while all others radioactive isotopes it does not exceed 24 hours, and for most of them it lasts less than one minute. The element also has two meta states.
Isotopes of chromium in earth's crust, as a rule, accompany manganese isotopes, which is used in geology. 53 Cr is formed during the radioactive decay of 53 Mn. The Mn/Cr isotope ratio supports other information about early history solar system. Changes in the 53 Cr/ 52 Cr and Mn/Cr ratios from different meteorites prove that new atomic nuclei were created just before the formation of the Solar System.
Chromium(III) oxide Cr 2 O 3, also known as sesquioxide, is one of the four oxides of this chemical element. It is obtained from chromite. The green color compound is commonly called "chrome green" when used as a pigment for enamel and glass painting. The oxide can dissolve in acids, forming salts, and in molten alkali - chromites.
K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as a means for cleaning laboratory glassware from organic matter. For this purpose, its saturated solution is used. Sometimes, however, it is replaced with sodium bichromate, based on the higher solubility of the latter. In addition, it can regulate the oxidation process of organic compounds, converting primary alcohol into aldehyde and then into carbon dioxide.
Potassium dichromate can cause chrome dermatitis. Chromium is likely to cause sensitization leading to the development of dermatitis, especially of the hands and forearms, which is chronic and difficult to cure. Like other Cr(VI) compounds, potassium bichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.
The compound has the hypothetical structure H 2 CrO 4 . Neither chromic nor dichromic acids occur in nature, but their anions are found in various substances. The “chromic acid” that can be found on sale is actually its acid anhydride - CrO 3 trioxide.
PbCrO 4 has a bright yellow color and is practically insoluble in water. For this reason, it has found application as coloring pigment called "yellow crown".
Chromium is distinguished by its ability to form pentavalent bonds. The compound is created by Cr(I) and a hydrocarbon radical. A pentavalent bond is formed between two chromium atoms. Its formula can be written as Ar-Cr-Cr-Ar, where Ar represents a specific aromatic group.
Chromium is a chemical element whose properties have given it many various options applications, some of which are listed below.
It gives metals resistance to corrosion and glossy surface. Therefore, chromium is included in alloys such as stainless steel, used, for example, in cutlery. It is also used for chrome plating.
Chromium is a catalyst for various reactions. It is used to make molds for firing bricks. Its salts are used to tan leather. Potassium bichromate is used for the oxidation of organic compounds such as alcohols and aldehydes, as well as for cleaning laboratory glassware. It serves as a fixing agent for fabric dyeing and is also used in photography and photo printing.
CrO 3 is used for the manufacture of magnetic tapes (for example, for audio recordings), which have best characteristics than films with iron oxide.
Trivalent chromium is a chemical element necessary for the metabolism of sugar in the human body. In contrast, hexavalent Cr is highly toxic.
Chromium metal and Cr(III) compounds are generally not considered a health hazard, but substances containing Cr(VI) can be toxic if ingested or inhaled. Most of these substances are irritating to the eyes, skin and mucous membranes. With chronic exposure, chromium(VI) compounds can cause eye damage if not treated properly. In addition, it is a recognized carcinogen. The lethal dose of this chemical element is about half a teaspoon. According to the recommendations of the World Health Organization, the maximum permissible concentration of Cr (VI) in drinking water is 0.05 mg per liter.
Because chromium compounds are used in dyes and leather tanning, they are often found in soil and groundwater abandoned industrial facilities, requiring environmental cleanup and restoration. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.
The main physical properties of chromium are as follows:
Chromium
CHROMIUM-A; m.[from Greek chrōma - color, paint]
1. Chemical element (Cr), hard metal gray-steel color (used in the manufacture hard alloys and for covering metal products).
2. Soft thin leather tanned with salts of this metal. Boots made of chrome.
3. A type of yellow paint obtained from chromates.
◁ Chrome (see).
chromium(lat. Chromium), chemical element of group VI periodic table. Named from the Greek. chrōma - color, paint (due to the bright color of the compounds). Bluish-silver metal; density 7.19 g/cm 3, t pl 1890°C. Does not oxidize in air. The main minerals are chrome spinels. Chromium is an essential component of stainless, acid-resistant, heat-resistant steels and a large number of other alloys (nichrome, chrome, stellite). Used for chrome plating. Chromium compounds are oxidizing agents, inorganic pigments, tanning agents.
CHROMIUMCHROME (lat. chromium, from the Greek chromium - color, color, chromium compounds are characterized by a wide color palette), Cr (pronounced "chrome"), a chemical element with atomic number 24, atomic mass 51.9961. Located in group VIB in the 4th period of the periodic table of elements.
Natural chromium consists of a mixture of four stable nuclides: 50 Cr (mixture content 4.35%), 52 Cr (83.79%), 53 Cr (9.50%) and 54 Cr (2.36%). Configuration of two outer electronic layers 3s 2
R 6
d 5
4s 1
. Oxidation states range from 0 to +6, the most typical are +3 (the most stable) and +6 (valences III and VI).
Neutral atom radius 0.127 nm, ion radius (coordination number 6): Cr 2+ 0.073 nm, Cr 3+ 0.0615 nm, Cr 4+ 0.055 nm, Cr 5+ 0.049 nm and Cr 6+ 0.044 nm. The sequential ionization energies are 6.766, 16.49, 30.96, 49.1, 69.3 and 90.6 eV. Electron affinity 1.6 eV. Electronegativity according to Pauling (cm. PAULING Linus) 1,66.
History of discovery
In 1766, a mineral was discovered in the vicinity of Yekaterinburg, which was called “Siberian red lead,” PbCrO 4. The modern name is crocoite. In 1797, the French chemist L. N. Vauquelin (cm. VAUCLIN Louis Nicolas) isolated a new refractory metal from it (most likely Vauquelin obtained chromium carbide).
Being in nature
Content in the earth's crust is 0.035% by weight. IN sea water chromium content 2·10 -5 mg/l. Chromium is practically never found in free form. It is part of more than 40 different minerals (chromite FeCr 2 O 4, volkonskoite, uvarovite, vokelenite, etc.). Some meteorites contain chromium sulfide compounds.
Receipt
The industrial raw material for the production of chromium and chromium-based alloys is chromite. By reducing chromite melting with coke (reducing agent), iron ore and other components, ferrochrome with a chromium content of up to 80% (by weight) is obtained.
To obtain pure metal chromium, chromite is fired with soda and limestone in furnaces:
2Cr 2 O 3 + 2Na 2 CO 3 + 3O 2 = 4Na 2 CrO 4 + 4CO 2
The resulting sodium chromate Na 2 CrO 4 is leached with water, the solution is filtered, evaporated and treated with acid. In this case, Na 2 CrO 4 chromate transforms into Na 2 Cr 2 O 7 dichromate:
2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O
The resulting dichromate is reduced with sulfur:
Na 2 Cr 2 O 7 + 3S = Na 2 S + Cr 2 O 3 + 2SO 2,
The resulting pure chromium(III) oxide Cr 2 O 3 is subjected to aluminothermy:
Cr 2 O 3 + 2Al = Al 2 O 3 + 2Cr.
Silicon is also used:
2Cr 2 O 3 + 3Si = 3SiO 2 + 4Cr
To obtain high-purity chromium, technical chromium is electrochemically purified from impurities.
Physical and chemical properties
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 state (Néel point). Melting point 1890°C, boiling point 2680°C. Density 7.19 kg/dm3.
Stable in air. At 300°C it burns to form green chromium (III) oxide Cr 2 O 3, which has amphoteric properties. By fusing Cr 2 O 3 with alkalis, chromites are obtained:
Cr 2 O 3 + 2NaOH = 2NaCrO 2 + H 2 O
Uncalcined chromium(III) oxide easily dissolves in alkaline solutions and acids:
Cr 2 O 3 + 6HCl = 2CrCl 3 + 3H 2 O
The thermal decomposition of chromium carbonyl Cr(OH) 6 produces red basic chromium(II) oxide CrO. Brown or yellow hydroxide Cr(OH) 2 with weakly basic properties is precipitated when alkalis are added to solutions of chromium(II) salts.
Careful decomposition of chromium(VI) oxide CrO 3 under hydrothermal conditions produces chromium(IV) dioxide CrO 2, which is ferromagnetic and has metallic conductivity.
When concentrated sulfuric acid reacts with solutions of dichromates, red or violet-red crystals of chromium(VI) oxide CrO 3 are formed. A typically acidic oxide, when interacting with water it forms strong unstable chromic acids: chromic H 2 CrO 4 , dichromic H 2 Cr 2 O 7 and others.
Halides are known corresponding to to different degrees chromium oxidation. Chromium dihalides CrF 2, CrCl 2, CrBr 2 and CrI 2 and trihalides CrF 3, CrCl 3, CrBr 3 and CrI 3 were synthesized. However, unlike similar compounds of aluminum and iron, CrCl 3 trichloride and chromium tribromide CrBr 3 are non-volatile.
Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapors. Chromium hexafluoride CrF 6 is known.
Chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 were obtained and characterized.
Compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), were synthesized. with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).
Chromium(III) compounds are the most stable in solutions. In this oxidation state, chromium corresponds to both the cationic form and the anionic forms, for example, the 3- anion existing in an alkaline environment.
When chromium(III) compounds are oxidized in an alkaline medium, chromium(VI) compounds are formed:
2Na 3 + 3H 2 O 2 = 2Na 2 CrO 4 + 2NaOH + 8H 2 O
Cr (VI) corresponds to a number of acids that exist only in aqueous solutions: chromic H 2 CrO 4 , dichromic H 2 Cr 2 O 7 , trichromic H 3 Cr 3 O 10 and others that form salts - chromates, dichromates, trichromates, etc. .
Depending on the acidity of the environment, the anions of these acids easily convert into each other. For example, when a yellow solution of potassium chromate K 2 CrO 4 is acidified, orange potassium dichromate K 2 Cr 2 O 7 is formed:
2K 2 CrO 4 + 2HCl = K 2 Cr 2 O 7 + 2KCl + H 2 O
But if an alkali solution is added to the orange solution of K 2 Cr 2 O 7, the color turns yellow again because potassium chromate K 2 CrO 4 is formed again:
K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O
When a barium salt solution is added to a yellow solution containing chromate ions, a yellow precipitate of barium chromate BaCrO 4 precipitates:
Ba 2+ + CrO 4 2- = BaCrO 4
Chromium(III) compounds are strong oxidizing agents, for example:
K 2 Cr 2 O 7 + 14 HCl = 2CrCl 3 + 2KCl + 3Cl 2 + 7H 2 O
Application
The use of chromium is based on its heat resistance, hardness and corrosion resistance. Used to produce alloys: of stainless steel, nichrome, etc. A large number of chromium is used for decorative corrosion-resistant coatings. Chromium compounds are fire-resistant materials. Chromium (III) oxide is a green paint pigment, also included in abrasive materials (GOI pastes). The color change upon reduction of chromium(VI) compounds is used to conduct a rapid analysis of the alcohol content in exhaled air.
The Cr 3+ cation is part of potassium chromium KCr(SO 4) 2 ·12H 2 O alum used in leather tanning.
Physiological action
Chromium is one of the biogenic elements and is constantly included in the tissues of plants and animals. In animals, chromium is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), and carbohydrates. A decrease in chromium content in food and blood leads to a decrease in growth rate and an increase in cholesterol in the blood.
Chromium metal is practically non-toxic, but metal dust chromium irritates lung tissue. Chromium(III) compounds cause dermatitis. Chromium(VI) compounds lead to various human diseases, including cancer. The maximum permissible concentration of chromium(VI) in atmospheric air is 0.0015 mg/m3.
encyclopedic Dictionary. 2009 .
Synonyms:chromium- chrome, and... Russian spelling dictionary
chromium- chrome/… Morphemic-spelling dictionary
- (from the Greek chroma color, paint). A grayish metal mined from chrome ore. Dictionary foreign words, included in the Russian language. Chudinov A.N., 1910. CHROME metal of grayish color; V pure form X. not used; connections with... Dictionary of foreign words of the Russian language
CHROMIUM- see CHROME (Cr). Chromium compounds are found in wastewater many industrial enterprises producing chrome salts, acetylene, tannins, aniline, linoleum, paper, paints, pesticides, plastics, etc. Trivalent compounds are found in water... ... Fish Diseases: A Guide
CHROME, ah, husband. 1. Chemical element, hard light gray shiny metal. 2. Kind of yellow paint (special). | adj. chromium, aya, oh (to 1 value) and chrome, aya, oh. Chromium steel. Chrome ore. II. CHROME, ah, husband. A type of soft, thin leather. | adj... Dictionary Ozhegova
chromium- a, m. chrome m. novolat. chromium lat. chroma gr. dye. 1. Chemical element is a hard silvery metal used in the manufacture of hard alloys and for coating metal products. BAS 1. Metal discovered by Vauquelin... ... Historical Dictionary of Gallicisms of the Russian Language
CHROMIUM- CHROME, Chromium (from the Greek chroma paint), I symbol. SG, chem. element with at. weighing 52.01 (isotopes 50, 52, 53, 54); serial number 24, for! occupies a place in the even subgroup VI of group j of the periodic table. Compounds X. are often found in nature... Great Medical Encyclopedia
- (lat. Chromium) Cr, chemical element of group VI of Mendeleev’s periodic table, atomic number 24, atomic mass 51.9961. Name from Greek. chroma color, paint (due to the bright color of the Compound). Bluish silver metal; density 7.19… … Big Encyclopedic Dictionary
CHROME 1, a, m. Ozhegov’s Explanatory Dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary
CHROME 2, a, m. A type of soft, thin leather. Ozhegov's explanatory dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary
Cr2+. The charge concentration of the divalent chromium cation corresponds to the charge concentration of the magnesium cation and the divalent iron cation, therefore a number of properties, especially the acid-base behavior of these cations, are close. Moreover, as already mentioned, Cr 2+ is a strong reducing agent, so the solution contains following reactions: 2CrCl 2 + 2HCl = 2CrCl 3 + H 2 4CrCl 2 + 4HCl + O 2 = 4CrCl 3 + 2H 2 O. Quite slowly, but even oxidation with water occurs: 2CrSO 4 + 2H 2 O = 2Cr(OH)SO 4 + H 2. The oxidation of divalent chromium occurs even more easily than the oxidation of divalent iron; salts also undergo cation hydrolysis to a moderate extent (i.e., the first step is dominant).
CrO is a basic oxide, black in color, pyrophoric. At 700 o C it disproportions: 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 – insoluble base yellow color. 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 produced 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 chemical properties similar to compounds of aluminum and iron (III). 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, so chromium (III) sulfide exists and can be obtained under anhydrous conditions, although not from simple substances, since it decomposes when heated, but according to 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 with zinc such a reaction takes place: 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 amorphous state. Reacts mainly when alloyed with acidic 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 = 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) – amphoteric hydroxide of gray-blue color. Dissolves in both 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.
Hydroxo complexes are easily decomposed by acids, while the processes with strong and weak acids are different:
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 in relation to the conversion to Cr(VI) compounds. The reaction occurs especially easily 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 that of sulfur compounds in the same oxidation state. Such similarity in the properties of compounds of elements of the main and secondary subgroups in the maximum positive oxidation state is characteristic of most groups of the periodic system.
CrO3 - a dark red compound, a typical acid oxide. At the melting point it decomposes: 4CrO 3 = 2Cr 2 O 3 + 3O 2.
Example of oxidizing action: CrO 3 + NH 3 = Cr 2 O 3 + N 2 + H 2 O (When heated).
Chromium(VI) oxide easily dissolves in water, adding it and turning into hydroxide:
H2CrO4 - chromic acid is a strong dibasic acid. It is not allocated in free form, because at a concentration above 75%, a condensation reaction occurs 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, 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) orange color. Most dichromates are soluble, and the solubility of chromates closely corresponds to the solubility of sulfates of the corresponding metals. Interconversion of the corresponding salts is possible in solutions:
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, insoluble in it:
K 2 Cr 2 O 7 (crystalline) + + 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 (“birthing salts”)
Halogens are the elements of the main subgroup of group VII of the periodic table. 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 at the outer electronic level, and they lack only one electron to reach the stable shell of the 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 high electronegativity and high ionization potential. Halogens are strong oxidizing agents; they are capable of accepting an electron, becoming an anion with a "1-" charge, or exhibiting a "-1" oxidation state when covalently bonding with less electronegative elements. At the same time, when moving through the group from top to bottom, the atomic radius 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 restorative 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 period II, it has only 4 atomic orbitals at 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-electron sublevel, where an excited electron can go. Each lone pair gives two electrons when paired, so the main oxidation states of chlorine, bromine and iodine, in addition to “-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 single bonds between the atoms. The dissociation energies of bonds 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 bond energy when going from chlorine to iodine is easily explained by an increase in 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 on 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 orbitals do not completely overlap, and the bond order in the fluorine molecule is slightly less than one. According to the second explanation, in the molecules of the remaining halogens there is an additional donor-acceptor overlap between 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 the molecules of chlorine, bromine and iodine is defined as almost triple in terms of the presence of interactions. But donor-acceptor overlap occurs only partially, and the bond has an order (for a chlorine molecule) of 1.12.
Physical properties: Under normal conditions, fluorine is a difficult to liquefy gas (boiling point of which is -187 0 C) of light yellow color, chlorine is an easily liquefied gas (boiling point is -34.2 0 C) yellow-green gas, bromine is a brown, easily evaporating liquid. , iodine – solid gray with a metallic sheen. In the solid state, all halogens form a molecular crystal lattice characterized by weak intermolecular interactions. In connection with this, iodine has a tendency to sublimate - when heated at atmospheric pressure, it goes 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 the molecular weight of the substances and due to the strengthening of the van der Waals forces acting between the molecules. The magnitude of these forces is greater, the greater the polarizability of the molecule, which, in turn, increases with increasing radius of the atom.
All halogens are poorly soluble in water, but well soluble in non-polar organic solvents, for example, carbon tetrachloride. Poor solubility in water is due to the fact that when a cavity is formed for the dissolution of a halogen molecule, water loses fairly strong hydrogen bonds, in return for which there are no bonds between its polar molecule and the non-polar halogen molecule strong interactions does not arise. 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.
Historical reference
Metallic chromium is obtained by reducing it from its oxide using aluminum (aluminothermy):
For this purpose, chromium iron ore is used. First, it is fused with soda in the presence of oxygen, and then the resulting sodium chromate is reduced with carbon to chromium oxide:
Properties of chromium and its compounds. Chrome is a white, shiny metal with a grayish tint, having great hardness and elasticity. At room temperature resistant to water and air.
Chemically, chromium as a metal is a reducing agent. Depending on the reaction conditions, it can exhibit variable oxidation states; states +2, +3, +6 are stable.
Under normal conditions, chromium is resistant to oxygen, interaction with which occurs only when heated. Under the same conditions, chromium also reacts with chlorine, sulfur, nitrogen, and silicon. For example:
Typically, the surface of chromium contains a dense layer of Cr 20 3 oxide, which protects the metal from further oxidation. This passivated surface is the reason that at ordinary temperatures there is no interaction of chromium with nitric acid and aqua regia.
Chromium reacts with dilute hydrochloric and sulfuric acids to release hydrogen and form Cr(II) salts, which, quickly oxidizing, turn into Cr(III) salts: 
Chromium compounds most often have the following spatial structure:
With oxygen, chromium forms a series of oxides, which, depending on the degree of oxidation of the metal, exhibit basic, amphoteric or acidic properties.
Chromium(II) oxide CrO has basic properties. When interacting with HC1, it forms CrCl 2.
Under the influence of hydrogen, CrO is reduced to metallic chromium; when heated under the influence of atmospheric oxygen, it transforms into Cr 203.
The oxide CrO corresponds to the hydroxide Cr(OH), formed from CrCl 2:
Cr(OH) 2 is a yellow substance. It is basic in nature and in reactions with acids forms the corresponding Cr(P) salts.
The Cr 2+ ion is such a strong reducing agent that it is capable of displacing hydrogen from water:
Cr(P) is easily oxidized by atmospheric oxygen, so a solution of CrCl:! , for example, can be used to absorb oxygen:
Aqueous solutions of Cr(P) compounds are blue.
Chromium(III) oxide Cr 2 0 3 belongs to amphoteric oxides.
It is obtained by calcination of chromium oxide (U1), or decomposition of ammonium dichromate, or thermal decomposition of chromium (III) hydroxide:
Chromium(III) hydroxide Cr(OH) ;j is obtained by the action of alkalis on chromium salts; in this case, Cr(OH) 3 is released in the form of a bluish-gray precipitate:
Cr(OH) 3 has amphoteric properties. Like aluminum hydroxide, Cr(OH) 3 reacts with acids to form Cr(III) salts, and with alkalis to form chromites:
Meta- or orthochromites, which are salts of the corresponding acids - HCl0 2 (metachromic) and H 3 Cr0 3 (orthochromic), are formed by fusing chromium oxide (III) with alkalis or soda:
Therefore, Cr(OH) 3 should be considered an amphoteric hydroxide:
Under the influence of strong oxidizing agents in an alkaline environment, chromium(III) compounds transform into chromium(U1) compounds - chromates:
The Cr 3 * ion is characterized by numerous complex compounds in which, with rare exceptions, a coordination number of 6 appears. The main feature of these complex compounds is their kinetic stability in aqueous solutions.
The blue-violet hexaaqua ion [Cr(H 2 0) 6 ] 3+ is part of many crystalline hydrates: CrCl 3 -6H 2 0, KCr(S0 4) 2 -12H 2 0, etc. The preparation of this cationic complex can be expressed by the following equation:
The composition of cationic Cr(III) complexes can vary depending on pH, temperature and concentration, and therefore their color changes from violet to green. As H 2 0 molecules in a complex cation are replaced, for example, with chlorine, various isomeric forms of CrCl 3 6H 2 0 can form:
The most numerous are complexes with amines as ligands. Among them, compounds with all possible types of isomerism were found. In addition to mononuclear complexes, for example 2+, there can also be polynuclear complexes, in which two or more metal atoms are connected via hydroxyl bridges.
Anionic complexes - chromates - are varied in composition and can be obtained using the following reactions:
The color of anionic complexes depends on the nature of the ligand: 3_ - emerald green, [CrCl 6 ] 3_ - pink-red, and 3_ - yellow.
The anionic complex [Cr(OH) 6 ]:1 “ forms numerous salts - hydroxochromates, stable in the solid state, and in solutions - only in a strongly alkaline environment.
Anhydrous Cr(III) compounds differ in structure and properties from crystalline hydrates. Thus, the anhydrous salt CrCl 3 has a polymer layer structure, while CrCl 3 -6H 2 0 has an island structure. CrCl 3, unlike CrC1 3 -6H 2 0, dissolves very slowly in water. Cr(PT) compounds in aqueous solutions are usually hydrolyzed, and at the first stage of this process the complex ion [Cr(H 2 0)0H| 3+:
Subsequently, polymerization of these complexes may occur. Cr 2 S 3 sulfide and carbonate Cr 2 (C0 3) 3 are characterized by even greater instability. Thus, Cr 2 S 3 and Cr 2 (C0 3) 3 cannot be obtained from an aqueous solution by exchange reactions, because these compounds, due to their greater solubility compared to Cr (OH) 3, are completely hydrolyzed:
Chromium oxide (U1) Cr0 3 is crystalline substance dark red color. It is obtained by the action of concentrated H 2 S0 4 on dichromates:
Cr0 3 has a chain structure formed by Cr0 4 tetrahedra.
Cr0 3 is a typical acidic oxide. It easily dissolves in water to form a solution of chromic acid H 2 Cr0 4 and dichromic acid 11 2 Cr 20 7, between which an equilibrium is established:
With increasing dilution, the equilibrium shifts towards the formation of HCr0 4
In alkaline solutions at pH > 7, Cr0 3 forms the tetrahedral chromate ion Cr() 4 of yellow color. In the pH range from 2 to 6, the HCl0 4 ion and the orange-red dichromate ion Cr 2 0| .
The following processes occur in an alkaline environment: 
The equilibrium position depends not only on pH, but also on the nature of the cations that can form insoluble chromates (Ba 2+, Pb 2+ and Ag* cations form chromates).
Thus, the addition of acids shifts the equilibrium to the left, and the addition of alkalis shifts the equilibrium to the right:
This is the basis for the production of chromates from dichromates, and vice versa:
Cr(VI) compounds are oxidizing agents. In an acidic environment, the dichromate ion Cr 2 0 2 exhibits strong oxidizing properties, restoring to Cr(W):
The high oxidative activity of Cr(VI) is manifested in the reaction between K 2 Cr 2 0 7 and concentrated HC1 when heated:
This reaction is convenient for producing chlorine in small quantities. When heating stops, the release of chlorine also stops. By the action of very strong reducing agents, Cr(VI) derivatives can be reduced in neutral and slightly alkaline media. For example, interaction with (NH^S occurs upon heating:
It should be noted that the oxidizing properties of Cr(VI) in an alkaline environment are much less pronounced than in an acidic environment. Thus, in acidic and alkaline solutions, the compounds Cr(III) and Cr(VI) exist in different forms: in an acidic environment, Cr 3+ or Cr 7 0 2- ions predominate, and in an alkaline environment, |Cr(OH) (.| 3 or Cl 2 ions predominate), and therefore the interconversion of Cr(III) compounds into Cr(VI) and on the contrary, it occurs depending on the reaction of the environment:
in an acidic environment
in an alkaline environment
It follows from this that in an acidic environment the oxidizing properties of Cr(VI) are expressed, and in an alkaline environment the reducing properties of Cr(III) are expressed:
Chromic acid H 2 Cr0 4 is much weaker than dichromic acid. So, for H 2 CrO, TO,= 3 10 7, and for H 2 Cr 2 0 7 TO, = 2 10" 2 .
H 2 Cr 2 0 7 - the simplest representative of chromium isopolyacids, corresponding general formulaеЭ0 3 *тН 2 0 (where p > t) and known as salts of iolichromates. So, except for orange-red dichromates (T = 1, P= 2) dark red trichromates were obtained (t = 1, n = 3) and brown-red ts-trachromats (w = 1, P = 4).
Polychromates are formed by the action of acids on chromates:
When alkalis act on solutions of iolichromates, the reverse process occurs with the eventual formation of chromates.
Cr(VI) does not form large series of polyacids and polyanions, which is explained by the size of the ion and its tendency to form multiple Cr=0 bonds.
Chromium is characterized by the formation of non-oxide compounds when interacting with H 2 0 2:
In addition to the blue oxide-diperoxide chromium (U1), CrO-chromium forms salts of peroxoacids H 2 Cr 2 0 12,11 2 Cr 2 0 8 and H 2 Cr 0 6 of the following structure (Fig. 6.1).
Rice. 6.1. Structure of pentaieroxodichromic acid H,Cr 2 O l2
Acid H 2 Cr 2 0 |2 forms salts colored in Blue colour, and P,Cr,0 8 - in red.
Chromium peroxide compounds are stable in ethereal solution; in aqueous solutions they are unstable and easily decompose with the release of oxygen and the formation of CrO2 ions (in an alkaline environment) or Cr(111) compounds (in an acidic environment). It is assumed that the stability of chromium(U1) oxide-dineroxide Cr0 5 in ether is due to the formation of a complex in the shape of a psn-tagonal pyramid with an oxygen atom at the apex (Fig. 6.2).
Rice. 6.2. Structure of chromium(U1) oxide-diperoxide Cr0 3 in ether, where L is an ether or water molecule
This complex can be obtained by treating a dichromate solution with hydrogen peroxide in an acidic medium:
By coloring the ether layer blue, one can judge the formation of a peroxo complex. This reaction is very sensitive and specific and is therefore widely used in analytical chemistry for detection of dichromate ion.
Qualitative reactions to chromate ion (Cr0 4 ~)
The technical use of chromium is well known: as an alloying additive, chromium is widely used to produce high-strength steels, nickel and copper alloys. Chromates and dichromates are widely used in the leather, textile, paint and varnish industries. pharmaceutical industry. Lead chromate PBCrO 4, called yellow crown, is used to make paints. Dichromates K 2 Cr 2 0 7 and Na 2 Cr 2 0 7 -2H 2 0, known as chromium peaks, are used in analytical chemistry.
A mixture of equal volumes of a solution of K 2 Cr 2 0 7 saturated in the cold and concentrated H 2 S0 1 is called a chromium mixture and is used for vigorous oxidation.
All chromium compounds are very poisonous!
DEFINITION
Chromium- twenty-fourth element periodic table. Designation - Cr from the Latin "chromium". Located in the fourth period, VIB group. Refers to metals. The nuclear charge is 24.
Chromium is contained in the earth's crust in an amount of 0.02% (mass.). In nature, it is found 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, chromium is resistant to both water and air. Diluted sulfuric and hydrochloric acid chromium is dissolved to release hydrogen. In cold concentrated nitric acid chromium is insoluble and becomes passive after treatment.
Rice. 1. Chrome. Appearance.
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 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 greater 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.
It is known that in nature chromium can be found 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 an 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.
On the outside energy level The chromium atom has six electrons, which are valence:
1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 .
As a result 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+ .
In the free state, chromium exists in the form of monoatomic Cr molecules. Here are some properties characterizing the chromium atom and molecule:
Chromium metal is used for chrome plating and as one of the most important components of alloy steels. The introduction of chromium into steel increases its resistance to corrosion as in aquatic environments at normal temperatures and in gases at elevated temperatures. In addition, chromium steels have increased hardness. Chromium is part of stainless acid-resistant and heat-resistant steels.
EXAMPLE 1
EXAMPLE 2
| Exercise | Chromium (VI) oxide 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 the reaction equation for the production of chromic acid from chromium (VI) oxide: CrO 3 + H 2 O = H 2 CrO 4. Let's 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 to ( 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%. |
