Chromium (III) Sulfate Composition and Molar Mass. Mobilni pregled Chromium - general characteristics of the element, chemical properties of chromium and its compounds Chromium brief description

DEFINITION

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

Chromium is contained in earth crust in the 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 chemical interaction chromium donates 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

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Absorbed Dose Rate Converter ionizing radiation Radioactivity. Radioactive decay Radiation converter. Exposure Dose Converter Radiation. Absorbed Dose Converter Decimal Prefix Converter Data Transfer Typography and Image Processing Unit Converter Timber Volume Unit Converter Molar Mass Calculation Periodic Table chemical elements D. I. Mendeleeva Chemical formula Molar mass Cr 2 (SO 4) 3, chromium (III) sulfate 392.18 g / mol 51.9961 2 + (32.065 + 15.9994 4) 3 Mass fraction of elements in the compound Using the molar mass calculator Chemical formulas must be entered case sensitive Indices are entered as regular numbers The point on the midline (multiplication sign), used, for example, in the formulas of crystalline hydrates, is replaced by an ordinary point. Example: instead of CuSO₄ · 5H₂O, the converter uses the spelling CuSO4.5H2O for ease of input. Microphones and their specifications Molar mass calculator Moth All substances are made up of atoms and molecules. In chemistry, it is important to accurately measure the mass of substances that react and result from it. By definition, a mole is the SI unit of the amount of a substance. One mole contains exactly 6.02214076 × 10²³ elementary particles... This value is numerically equal to the Avogadro constant N A, if expressed in units of mol and is called the Avogadro number. The amount of substance (symbol n) of the system is a measure of the number of structural elements. A building block can be an atom, molecule, ion, electron, or any particle or group of particles. Avogadro's constant N A = 6.02214076 × 10²³ mol⁻¹. Avogadro's number is 6.02214076 × 10²³. In other words, a mole is an amount of a substance equal in mass to the sum of the atomic masses of atoms and molecules of a substance, multiplied by Avogadro's number. The unit of amount of a substance, mol, is one of the seven basic units of the SI system and is denoted by mol. Since the name of the unit and its symbol are the same, it should be noted that the symbol is not declined, unlike the name of the unit, which can be declined according to the usual rules of the Russian language. One mole of pure carbon-12 is exactly 12 g. Molar mass Molar mass is a physical property of a substance, defined as the ratio of the mass of this substance to the amount of substance in moles. In other words, it is the mass of one mole of a substance. In SI, the unit of molar mass is kilogram / mol (kg / mol). However, chemists are accustomed to using a more convenient unit of g / mol. molar mass= g / mol Molar mass of elements and compounds Compounds are substances made up of different atoms that are chemically bonded to each other. For example, the following substances, which can be found in the kitchen of any housewife, are chemical compounds: salt (sodium chloride) NaCl sugar (sucrose) C₁₂H₂₂O₁₁ vinegar (acetic acid solution) CH₃COOH The molar mass of chemical elements in grams per mole numerically coincides with the mass of the element's atoms, expressed in atomic mass units (or daltons). The molar mass of compounds is equal to the sum of the molar masses of the elements that make up the compound, taking into account the number of atoms in the compound. For example, the molar mass of water (H₂O) is approximately 1 × 2 + 16 = 18 g / mol. Molecular mass Molecular weight (formerly called molecular weight) is the mass of a molecule, calculated as the sum of the masses of each atom in a molecule multiplied by the number of atoms in that molecule. Molecular weight is dimensionless physical quantity, numerically equal to the molar mass. That is, the molecular weight differs from the molar weight in dimension. Despite the fact that molecular weight is a dimensionless quantity, it still has a quantity called atomic mass unit (amu) or dalton (Da), and approximately equal to the mass of one proton or neutron. The atomic mass unit is also numerically equal to 1 g / mol. Calculating molar mass The molar mass is calculated as follows: determine the atomic masses of elements according to the periodic table; determine the number of atoms of each element in the compound formula; determine the molar mass by adding the atomic masses of the elements included in the compound, multiplied by their number. For example, let's calculate the molar mass of acetic acid It consists of: two carbon atoms four hydrogen atoms two oxygen atoms carbon C = 2 × 12.0107 g / mol = 24.0214 g / mol hydrogen H = 4 × 1.00794 g / mol = 4.03176 g / mol oxygen O = 2 × 15.9994 g / mol = 31.9988 g / mol molar mass = 24.0214 + 4.03176 + 31.9988 = 60.05196 g / mol Our calculator does just that. You can enter the acetic acid formula into it and check what happens. Do you find it difficult to translate a unit of measurement from one language to another? Colleagues are ready to help you. Post a question to TCTerms and you will receive an answer within a few minutes.

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 (Cr), chemical element of group VI periodic system Mendeleev. Refers to a transition metal with atomic number 24 and atomic mass 51.996. Translated from Greek, the name of the metal means "color". The metal owes this name to a variety of colors, which is inherent in its various compounds.

Physical characteristics of chromium

The metal has sufficient hardness and brittleness at the same time. On the Mohs scale, the hardness of chromium is 5.5. This figure means that chromium has the highest hardness of all metals known today, after uranium, iridium, tungsten and beryllium. For simple substance chrome is characterized by a bluish-white color.

Metal is not a rare element. Its concentration in the earth's crust reaches 0.02% of the mass. shares. Chromium is never found in its pure form. It is found in minerals and ores, which are the main source of metal mining. Chromite (chromium iron ore, FeO * Cr 2 O 3) is considered the main compound of chromium. Another fairly common, but less important mineral is crocoite PbCrO 4.

The metal is easy to melt at a temperature of 1907 0 C (2180 0 K or 3465 0 F). At a temperature of 2672 0 С - it boils. The atomic mass of the metal is 51.996 g / mol.

Chromium is a unique metal due to its magnetic properties. Antiferromagnetic ordering is inherent in it at room temperature, while other metals exhibit it at extremely low temperatures. However, if chromium is heated above 37 0 С, physical properties chromium changes. So, the electrical resistance and the coefficient of linear expansion change significantly, the modulus of elasticity reaches a minimum value, and the internal friction increases significantly. This phenomenon is associated with the passage of the Néel point, at which the antiferromagnetic properties of the material can change to paramagnetic. This means that the first level has been passed, and the substance has sharply increased in volume.

The structure of chromium is a body-centered lattice, due to which the metal is characterized by the temperature of the brittle-ductile period. However, in the case of this metal, the degree of purity is of great importance, therefore, the value is in the range -50 0 С - +350 0 С. As practice shows, the crystallized metal does not have any ductility, but soft annealing and molding make it malleable.

Chemical properties of chromium

The atom has the following external configuration: 3d 5 4s 1. As a rule, in compounds, chromium has the following oxidation states: +2, +3, +6, among which Cr 3+ exhibits the greatest stability. In addition, there are other compounds in which chromium exhibits a completely different oxidation state, namely: +1 , +4, +5.

The metal is not particularly reactive. While chromium is under normal conditions, the metal exhibits resistance to moisture and oxygen. However, this characteristic does not apply to the compound of chromium and fluorine - CrF 3, which, when exposed to temperatures exceeding 600 0 C, interacts with water vapor, forming as a result of the reaction Cr 2 O 3, as well as nitrogen, carbon and sulfur.

When metallic chromium is heated, it interacts with halogens, sulfur, silicon, boron, carbon, and some other elements, resulting in the following chemical reactions chrome:

Cr + 2F 2 = CrF 4 (doped with CrF 5)

2Cr + 3Cl 2 = 2CrCl 3

2Cr + 3S = Cr 2 S 3

Chromates can be obtained by heating chromium with molten soda in air, nitrates or chlorates of alkali metals:

2Cr + 2Na 2 CO 3 + 3O 2 = 2Na 2 CrO 4 + 2CO 2.

Chromium has no toxicity, which cannot be said about some of its compounds. As you know, the dust of this metal, when it enters the body, can irritate the lungs, it is not absorbed through the skin. But, since it does not occur in its pure form, its entry into the human body is impossible.

Trivalent chromium enters environment during the mining and processing of chrome ore. The ingestion of chromium in the human body is likely in the form of a dietary supplement used in weight loss programs. Chromium with a valency of +3 is an active participant in glucose synthesis. Scientists have found that excessive use of chromium does not cause much harm to the human body, since it is not absorbed, however, it is able to accumulate in the body.

Compounds in which a hexavalent metal is involved are extremely toxic. The likelihood of their entering the human body appears during the production of chromates, chrome plating of objects, during some welding work. The ingestion of such chromium into the body is fraught with serious consequences, since the compounds in which the hexavalent element is present are strong oxidants. Therefore, it can cause bleeding in the stomach and intestines, sometimes with intestinal perforation. When such compounds get on the skin, strong chemical reactions occur in the form of burns, inflammation, and ulcers.

Depending on the quality of chromium, which must be obtained at the outlet, there are several methods of metal production: electrolysis of concentrated aqueous solutions of chromium oxide, electrolysis of sulfates, and reduction with silicon oxide. However, the last method is not very popular, since it produces chrome with huge amount impurities. Moreover, it is also economically disadvantageous.

Typical oxidation states of chromium

Oxidation state	Oxide	Hydroxide	Character	Prevailing forms in solutions	Notes (edit)
+2	CrO (black)	Cr (OH) 2 (yellow)	Basic	Cr2 + (blue salts)	Very strong reducing agent
	Cr2O3 (green)	Cr (OH) 3 (gray-green)	Amphoteric	Cr3 + (green or purple salts) - (green)
+4	CrO2	does not exist	Non-salt-forming	-	Rare, uncommon
+6	CrO3 (red)	H2CrO4 H2Cr2O7	Acid	CrO42- (chromates, yellow) Cr2O72- (dichromates, orange)	The transition depends on the pH of the medium. The strongest oxidizing agent, hygroscopic, very toxic.

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 of hydrochloric acid potassium chromate was converted to 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 the works of Vauquelin 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.) So this name was affirmed 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 coming for the most part for smelting 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.

Royal 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 on South 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 is an element of a secondary subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 24. It is designated by the symbol Cr (Latin Chromium). A simple substance chromium is a bluish-white hard metal.

Chemical properties of chromium

Under normal conditions, chromium reacts only with fluorine. At high temperatures (above 600 ° C), it interacts with oxygen, halogens, nitrogen, silicon, boron, sulfur, phosphorus.

4Cr + 3O 2 - t ° → 2Cr 2 O 3

2Cr + 3Cl 2 - t ° → 2CrCl 3

2Cr + N 2 - t ° → 2CrN

2Cr + 3S - t ° → Cr 2 S 3

When heated, it reacts with water vapor:

2Cr + 3H 2 O → Cr 2 O 3 + 3H 2

Chromium dissolves in dilute strong acids (HCl, H 2 SO 4)

In the absence of air, Cr 2+ salts are formed, and in air, Cr 3+ salts.

Cr + 2HCl → CrCl 2 + H 2

2Cr + 6HCl + O 2 → 2CrCl 3 + 2H 2 O + H 2

The presence of a protective oxide film on the metal surface explains its passivity in relation to concentrated solutions of acids - oxidizing agents.

Chromium compounds

Chromium (II) oxide and chromium (II) hydroxide are basic.

Cr (OH) 2 + 2HCl → CrCl 2 + 2H 2 O

Chromium (II) compounds are strong reducing agents; are converted into chromium (III) compounds under the action of atmospheric oxygen.

2CrCl 2 + 2HCl → 2CrCl 3 + H 2

4Cr (OH) 2 + O 2 + 2H 2 O → 4Cr (OH) 3

Chromium oxide (III) Cr 2 O 3 is a green, water-insoluble powder. It can be obtained by calcining chromium (III) hydroxide or potassium and ammonium dichromates:

2Cr (OH) 3 - t ° → Cr 2 O 3 + 3H 2 O

4K 2 Cr 2 O 7 - t ° → 2Cr 2 O 3 + 4K 2 CrO 4 + 3O 2

(NH 4) 2 Cr 2 O 7 - t ° → Cr 2 O 3 + N 2 + 4H 2 O (reaction "volcano")

Amphoteric oxide. When Cr 2 O 3 is fused with alkalis, soda and acidic salts, chromium compounds with the oxidation state (+3) are obtained:

Cr 2 O 3 + 2NaOH → 2NaCrO 2 + H 2 O

Cr 2 O 3 + Na 2 CO 3 → 2NaCrO 2 + CO 2

When fusion with a mixture of alkali and an oxidizing agent, chromium compounds are obtained in the oxidation state (+6):

Cr 2 O 3 + 4KOH + KClO 3 → 2K 2 CrO 4 + KCl + 2H 2 O

Chromium (III) C hydroxide r (OH) 3. Amphoteric hydroxide. Gray-green, decomposes when heated, losing water and forming green metahydroxide CrO (OH). Does not dissolve in water. It precipitates from the solution in the form of a blue-gray and bluish-green hydrate. Reacts with acids and alkalis, does not interact with ammonia hydrate.

It has amphoteric properties - it dissolves in both acids and alkalis:

2Cr (OH) 3 + 3H 2 SO 4 → Cr 2 (SO 4) 3 + 6H 2 O Сr (ОН) 3 + ЗН + = Сr 3+ + 3H 2 O

Cr (OH) 3 + KOH → K, Cr (OH) 3 + ZOH - (conc.) = [Cr (OH) 6] 3-

Cr (OH) 3 + KOH → KCrO 2 + 2H 2 O Cr (OH) 3 + MOH = MCrO 2 (green) + 2H 2 O (300-400 ° C, M = Li, Na)

Cr (OH) 3 →(120 o C – H 2 O) CrO (OH) → (430-1000 0 С -H 2 O) Cr 2 O 3

2Сr (ОН) 3 + 4NаОН (conc.) + ЗН 2 O 2 (conc.) = 2Na 2 СrO 4 + 8Н 2 0

Receiving: precipitation with ammonia hydrate from a solution of chromium (III) salts:

Cr 3+ + 3 (NH 3 H 2 O) = WITHr(OH) 3 ↓+ ЗNН 4+

Cr 2 (SO 4) 3 + 6NaOH → 2Cr (OH) 3 ↓ + 3Na 2 SO 4 (in excess of alkali - the precipitate dissolves)

Chromium (III) salts are purple or dark green in color. In terms of chemical properties, they resemble colorless aluminum salts.

Cr (III) compounds can exhibit both oxidizing and reducing properties:

Zn + 2Cr +3 Cl 3 → 2Cr +2 Cl 2 + ZnCl 2

2Cr +3 Cl 3 + 16NaOH + 3Br 2 → 6NaBr + 6NaCl + 8H 2 O + 2Na 2 Cr +6 O 4

Hexavalent chromium compounds

Chromium (VI) oxide CrO 3 are bright red crystals, soluble in water.

Prepared from potassium chromate (or dichromate) and H 2 SO 4 (conc.).

K 2 CrO 4 + H 2 SO 4 → CrO 3 + K 2 SO 4 + H 2 O

K 2 Cr 2 O 7 + H 2 SO 4 → 2CrO 3 + K 2 SO 4 + H 2 O

CrO 3 - acid oxide, with alkalis forms yellow chromates CrO 4 2-:

CrO 3 + 2KOH → K 2 CrO 4 + H 2 O

In an acidic environment, chromates turn into orange dichromates Cr 2 O 7 2-:

2K 2 CrO 4 + H 2 SO 4 → K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

In an alkaline environment, this reaction proceeds in the opposite direction:

K 2 Cr 2 O 7 + 2KOH → 2K 2 CrO 4 + H 2 O

Potassium dichromate is an oxidizing agent in an acidic environment:

К 2 Сr 2 O 7 + 4H 2 SO 4 + 3Na 2 SO 3 = Cr 2 (SO 4) 3 + 3Na 2 SO 4 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 4H 2 SO 4 + 3NaNO 2 = Cr 2 (SO 4) 3 + 3NaNO 3 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6 KI = Cr 2 (SO 4) 3 + 3I 2 + 4K 2 SO 4 + 7H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6FeSO 4 = Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O

Potassium chromate K 2 Cr About 4 . Oxosol. Yellow, non-absorbent. It melts without decomposition, thermally stable. Let's well dissolve in water ( yellow the color of the solution corresponds to the CrO 4 2- ion), slightly hydrolyzed by the anion. In an acidic environment, it transforms into K 2 Cr 2 O 7. Oxidizing agent (weaker than K 2 Cr 2 O 7). It enters into ion exchange reactions.

Qualitative reaction on the CrO 4 2- ion - precipitation of a yellow precipitate of barium chromate, which decomposes in a strongly acidic medium. It is used as a mordant for dyeing fabrics, leather tanning agent, selective oxidizing agent, and a reagent in analytical chemistry.

Equations of the most important reactions:

2K 2 CrO 4 + H 2 SO 4 (30%) = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

2K 2 CrO 4 (t) + 16HCl (end, hot) = 2CrCl 3 + 3Cl 2 + 8H 2 O + 4KCl

2K 2 CrO 4 + 2H 2 O + 3H 2 S = 2Cr (OH) 3 ↓ + 3S ↓ + 4KOH

2K 2 CrO 4 + 8H 2 O + 3K 2 S = 2K [Cr (OH) 6] + 3S ↓ + 4KOH

2K 2 CrO 4 + 2AgNO 3 = KNO 3 + Ag 2 CrO 4 (red) ↓

Qualitative response:

К 2 СгO 4 + ВаСl 2 = 2КСl + ВаCrO 4 ↓

2ВаСrO 4 (t) + 2HCl (dil.) = ВаСr 2 O 7 (p) + ВаС1 2 + Н 2 O

Receiving: sintering of chromite with potash in air:

4 (Сr 2 Fe ‖‖) O 4 + 8К 2 CO 3 + 7O 2 = 8К 2 СrO 4 + 2Fе 2 O 3 + 8СO 2 (1000 ° С)

Potassium dichromate K 2 Cr 2 O 7 ... Oxosol. Technical name chrompeak... Orange red, non-absorbent. It melts without decomposition, decomposes upon further heating. Let's well dissolve in water ( orange the color of the solution corresponds to the ion Cr 2 O 7 2-). In an alkaline environment forms K 2 CrO 4. Typical oxidizing agent in solution and fusion. It enters into ion exchange reactions.

Qualitative reactions- blue coloration of the ethereal solution in the presence of H 2 O 2, blue coloration aqueous solution under the action of atomic hydrogen.

It is used as a tanning agent for leather, a mordant for dyeing fabrics, a component of pyrotechnic compositions, a reagent in analytical chemistry, an inhibitor of metal corrosion, mixed with H 2 SO 4 (conc.) - for washing chemical dishes.

Equations of the most important reactions:

4K 2 Cr 2 O 7 = 4K 2 CrO 4 + 2Cr 2 O 3 + 3O 2 (500-600 o C)

K 2 Cr 2 O 7 (t) + 14HCl (end) = 2CrCl 3 + 3Cl 2 + 7H 2 O + 2KCl (boiling)

K 2 Cr 2 O 7 (t) + 2H 2 SO 4 (96%) ⇌2KHSO 4 + 2CrO 3 + H 2 O (“chromium mixture”)

K 2 Cr 2 O 7 + KOH (conc) = H 2 O + 2K 2 CrO 4

Cr 2 O 7 2- + 14H + + 6I - = 2Cr 3+ + 3I 2 ↓ + 7H 2 O

Cr 2 O 7 2- + 2H + + 3SO 2 (g) = 2Cr 3+ + 3SO 4 2- + H 2 O

Cr 2 O 7 2- + H 2 O + 3H 2 S (g) = 3S ↓ + 2OH - + 2Cr 2 (OH) 3 ↓

Cr 2 O 7 2- (conc.) + 2Ag + (dil.) = Ag 2 Cr 2 O 7 (t. Red) ↓

Cr 2 O 7 2- (dil.) + H 2 O + Pb 2+ = 2H + + 2PbCrO 4 (red) ↓

K 2 Cr 2 O 7 (s) + 6HCl + 8H 0 (Zn) = 2CrCl 2 (syn) + 7H 2 O + 2KCl

Receiving: treatment of К 2 СrO 4 with sulfuric acid:

2K 2 CrO 4 + H 2 SO 4 (30%) = K 2Cr 2 O 7 + K 2 SO 4 + H 2 O

 

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