Chromium is a transition metal widely used in industry for its strength and resistance to heat and corrosion. This article will give you an understanding of some of the important properties and uses of this transition metal.
Chromium belongs to the category of transition metals. It is a hard but brittle steel-gray metal with atomic number 24. This shiny metal is placed in group 6 of the periodic table, and is designated by the symbol "Cr".
The name chromium is derived from the Greek word chroma, which means color.
True to its name, chromium forms several intensely colored compounds. Today, virtually all commercially used chromium is extracted from iron chromite ore or chromium oxide (FeCr2O4).
Chromium Properties
- Chromium is the most abundant element on the earth's crust, but it never occurs in its purest form. Mainly mined from mines such as chromite mines.
- Chromium is melted at 2180 K or 3465°F and the boiling point is 2944 K or 4840°F. its atomic weight is 51.996 g/mol, and is 5.5 on the Mohs scale.
- Chromium occurs in many oxidation states such as +1, +2, +3, +4, +5, and +6, of which +2, +3, and +6 are the most common, and +1, +4, A +5 is a rare oxidation. The +3 oxidation state is the most stable state of chromium. Chromium(III) can be obtained by dissolving elemental chromium in hydrochloric or sulfuric acid.
- This metallic element is known for its unique magnetic properties. At room temperature, it exhibits antiferromagnetic ordering, which is shown in other metals at relatively low temperatures.
- Antiferromagnetism is where nearby ions that behave like magnets attach to opposite or anti-parallel arrangements through the material. As a result, the magnetic field generated by the magnetic atoms or ions orient in one direction canceling out the magnetic atoms or ions aligned in the opposite direction, so that the material does not exhibit any harsh external magnetic fields.
- At temperatures above 38°C, chromium becomes paramagnetic, i.e. it is attracted to an externally applied magnetic field. In other words, chromium attracts an external magnetic field at temperatures above 38°C.
- Chromium does not undergo hydrogen embrittlement, i.e., does not become brittle when exposed to atomic hydrogen. But when exposed to nitrogen, it loses its plasticity and becomes brittle.
- Chromium is highly resistant to corrosion. A thin protective oxide film forms on the surface of a metal when it comes into contact with oxygen in the air. This layer prevents oxygen from diffusing into the base material and thus protects it from further corrosion. This process is called passivation, chromium passivation gives resistance to acids.
- There are three main isotopes of chromium, called 52Cr, 53Cr, and 54Cr, of which 52CR is the most common isotope. Chromium reacts with most acids but does not react with water. At room temperature, it reacts with oxygen to form chromium oxide.
Application
Stainless steel production
Chromium has found a wide range of applications due to its hardness and resistance to corrosion. It is mainly used in three industries - metallurgical, chemical and refractory. It is widely used for stainless steel production as it prevents corrosion. Today it is a very important alloying material for steels. It is also used to make nichrome, which is used in resistance heating elements due to its ability to withstand high temperatures.
Surface coating
Acid chromate or dichromate is also used to coat surfaces. This is usually done using the electroplating method, in which a thin layer of chromium is deposited on a metal surface. Another method is parts chromium plating, through which chromates are used to apply a protective layer to certain metals such as aluminum (Al), cadmium (CD), zinc (Zn), silver as well as magnesium (MG).
Preservation of wood and tanning of leather
Chromium(VI) salts are toxic, so they are used to keep wood from being damaged and destroyed by fungus, insects, and termites. Chromium(III), especially chromic alum or potassium sulfate is used in the leather industry as it helps to stabilize the skin.
Dyes and pigments
Chromium is also used to make pigments or dyes. Chrome yellow and lead chromate have been widely used as pigments in the past. Due to environmental concerns, its use dropped substantially, and then it was finally replaced by lead and chromium pigments. Other pigments based on chromium, red chromium, green chromium oxide, which is a mixture of yellow and Prussian blue. Chromium oxide is used to impart a greenish color to glass.
Synthesis of artificial rubies
Emeralds owe their green hue to chromium. Chromium oxide is also used for the production of synthetic rubies. Natural corundum rubies or aluminum oxide crystals that turn red due to the presence of chromium. Synthetic or artificial rubies are made by doping chromium(III) on synthetic corundum crystals.
biological functions
Chromium(III) or trivalent chromium is essential in the human body, but in very small amounts. It is believed to play an important role in lipid and sugar metabolism. It is currently used in many dietary supplements that are claimed to have several health benefits, however, this is a controversial issue. The biological role of chromium has not been adequately tested, and many experts believe that it is not important for mammals, while others consider it an essential trace element for humans.
Other uses
The high melting point and heat resistance make chromium an ideal refractory material. It has found its way into blast furnaces, cement kilns, and metal kilns. Many chromium compounds are used as catalysts for hydrocarbon processing. Chromium(IV) is used to manufacture magnetic tapes used in audio and video cassettes.
Hexavalent chromium or chromium(VI) is said to be toxic and mutagenic, and chromium(IV) is known to be carcinogenic. Salt chromate also causes allergic reactions in some people. Due to public health and environmental concerns, some restrictions have been placed on the use of chromium compounds in various parts of the world.
Cr2+. The charge concentration of the divalent chromium cation corresponds to the charge concentration of the magnesium cation and the divalent iron cation, so a number of properties, especially the acid-base behavior of these cations, are close. At the same time, as already mentioned, Cr 2+ is a strong reducing agent, therefore the following reactions take place in the solution: but even water oxidation occurs: 2CrSO 4 + 2H 2 O \u003d 2Cr (OH) SO 4 + H 2. The oxidation of divalent chromium occurs even more easily than the oxidation of ferrous iron, salts are also hydrolyzed by the cation to a moderate degree (i.e., the first step is dominant).
CrO - basic oxide, black, pyrophoric. At 700 ° C, it disproportionates: 3CrO \u003d Cr 2 O 3 + Cr. It can be obtained by thermal decomposition of the corresponding hydroxide in the absence of oxygen.
Cr(OH) 2 is an insoluble yellow base. It reacts with acids, while oxidizing acids simultaneously with acid-base interaction oxidize divalent chromium, under certain conditions this also happens with non-oxidizing acids (oxidizing agent - H +). When obtained by an exchange reaction, chromium (II) hydroxide quickly turns green due to oxidation:
4Cr(OH) 2 + O 2 = 4CrO(OH) + 2H 2 O.
Oxidation is also accompanied by the decomposition of chromium (II) hydroxide in the presence of oxygen: 4Cr(OH) 2 = 2Cr 2 O 3 + 4H 2 O.
Cr3+. Chromium(III) compounds are chemically similar to aluminum and iron(III) compounds. Oxide and hydroxide are amphoteric. Salts of weak unstable and insoluble acids (H 2 CO 3, H 2 SO 3, H 2 S, H 2 SiO 3) undergo irreversible hydrolysis:
2CrCl 3 + 3K 2 S + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3H 2 S + 6KCl; Cr 2 S 3 + 6H 2 O \u003d 2Cr (OH) 3 ↓ + 3H 2 S.
But the chromium (III) cation is not a very strong oxidizing agent, therefore chromium (III) sulfide exists and can be obtained under anhydrous conditions, however, not from simple substances, since it decomposes when heated, but by the reaction: 2CrCl 3 (cr) + 2H 2 S (gas) \u003d Cr 2 S 3 (cr) + 6HCl. The oxidizing properties of trivalent chromium are not enough for solutions of its salts to interact with copper, but such a reaction takes place with zinc: 2CrCl 3 + Zn = 2CrCl 2 + ZnCl 2.
Cr2O3 - amphoteric oxide of green color, has a very strong crystal lattice, therefore it exhibits chemical activity only in the amorphous state. Reacts mainly when fused with 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 \u003d Cr 2 (SO 4) 3 + 3K 2 SO 4; Cr 2 O 3 + K 2 CO 3 \u003d 2KCrO 2 + CO 2.
Cr(OH) 3 (CrO(OH), Cr 2 O 3 *nH 2 O) - amphoteric hydroxide of gray-blue color. It dissolves in both acids and alkalis. When dissolved in alkalis, hydroxocomplexes are formed, in which the chromium cation has a coordination number of 4 or 6:
Cr(OH) 3 + NaOH = Na; Cr(OH) 3 + 3NaOH \u003d Na 3.
Hydroxo complexes are easily decomposed by acids, while the processes are different with strong and weak acids:
Na + 4HCl \u003d NaCl + CrCl 3 + 4H 2 O; Na + CO 2 \u003d Cr (OH) 3 ↓ + NaHCO 3.
Cr(III) compounds are not only oxidizing agents, but also reducing agents with respect to transformation into Cr(VI) compounds. The reaction proceeds especially easily in an alkaline medium:
2Na 3 + 3Cl 2 + 4NaOH \u003d 2Na 2 CrO 4 + 6NaCl + 8H 2 O E 0 \u003d - 0.72 V.
In an acidic environment: 2Cr 3+ → Cr 2 O 7 2- E 0 = +1.38 V.
cr +6 . All Cr(VI) compounds are strong oxidizers. The acid-base behavior of these compounds is similar to that of sulfur compounds in the same oxidation state. Such 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.
CrO3 - a dark red compound, a typical acidic oxide. At the melting point, it decomposes: 4CrO 3 \u003d 2Cr 2 O 3 + 3O 2.
An example of an oxidizing action: CrO 3 + NH 3 = Cr 2 O 3 + N 2 + H 2 O (When heated).
Chromium(VI) oxide easily dissolves in water, attaching it and turning into hydroxide:
H2CrO4 - chromic acid, is a strong dibasic acid. It does not stand out in a free form, because. at a concentration above 75%, a condensation reaction occurs with the formation of dichromic acid: 2H 2 CrO 4 (yellow) \u003d H 2 Cr 2 O 7 (orange) + H 2 O.
Further concentration leads to the formation of trichromic (H 2 Cr 3 O 10) and even tetrachromic (H 2 Cr 4 O 13) acids.
Dimerization of the chromate anion also occurs upon acidification. As a result, salts of chromic acid at pH > 6 exist as yellow chromates (K 2 CrO 4), and at pH< 6 как бихроматы(K 2 Cr 2 O 7) оранжевого цвета. Большинство бихроматов растворимы, а растворимость хроматов чётко соответствует растворимости сульфатов соответствующих металлов. В растворах возможно взаимопревращения соответствующих солей:
2K 2 CrO 4 + H 2 SO 4 = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O; K 2 Cr 2 O 7 + 2KOH \u003d 2K 2 CrO 4 + H 2 O.
The interaction of potassium dichromate with concentrated sulfuric acid leads to the formation of chromic anhydride, which is insoluble in it:
K 2 Cr 2 O 7 (crystal) + + H 2 SO 4 (conc.) = 2CrO 3 ↓ + K 2 SO 4 + H 2 O;
When heated, ammonium bichromate undergoes an intramolecular redox reaction: (NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2 O.
HALOGENS ("giving birth to salts")
Halogens are called elements of the main subgroup of group VII of the periodic system. These are fluorine, chlorine, bromine, iodine, astatine. The structure of the outer electronic layer of their atoms: ns 2 np 5. Thus, there are 7 electrons in the outer electronic level, and only one electron is missing from them to the stable noble gas shell. Being the penultimate elements in the period, halogens have the smallest radius in the period. All this leads to the fact that halogens exhibit the properties of non-metals, have a high electronegativity and a high ionization potential. Halogens are strong oxidizing agents, they are able to accept an electron to become an anion with a charge of "1-" or exhibit an oxidation state of "-1" when covalently bonded to less electronegative elements. At the same time, when moving down the group from top to bottom, the radius of the atom increases and the oxidizing ability of halogens decreases. If fluorine is the strongest oxidizing agent, then iodine, when interacting with some complex substances, as well as with oxygen and other halogens, exhibits reducing properties.
The fluorine atom is different from the other members of the group. 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 on the outer electronic level, three of which are occupied by unshared 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, there is an unfilled d-electron sublevel on the outer level, where an excited electron can go. Each lone pair gives two electrons when steamed, so the main oxidation states of chlorine, bromine and iodine, except for "-1", are "+1", "+3", "+5", "+7". Less stable, but fundamentally achievable are the oxidation states "+2", "+4" and "+6".
As simple substances, all halogens are diatomic molecules with a single bond between the atoms. The bond dissociation energies in the series of molecules F 2 , Cl 2 , Br 2 , J 2 are as follows: 151 kJ/mol, 239 kJ/mol, 192 kJ/mol, 149 kJ/mol. The monotonic decrease in the binding energy upon passing from chlorine to iodine is easily explained by the increase in the bond length due to the increase in the atomic radius. The anomalously low binding energy in the fluorine molecule has two explanations. The first concerns the fluorine molecule itself. As already mentioned, fluorine has a very small atomic radius and as many as seven electrons at the outer level, therefore, when atoms approach each other during the formation of a molecule, interelectronic repulsion occurs, as a result of which the orbitals overlap incompletely, and the bond order in the fluorine molecule is slightly less than unity. According to the second explanation, in the molecules of the remaining halogens there is an additional donor-acceptor overlap of the lone electron pair of one atom and the free d-orbital of the other atom, two such opposite interactions per molecule. Thus, the bond in the molecules of chlorine, bromine and iodine is defined as almost triple in terms of the presence of interactions. But donor-acceptor overlaps occur only partially, and the bond has an order (for a chlorine molecule) of 1.12.
Physical properties: Under normal conditions, fluorine is a gas that is difficult to liquefy (boiling point of which is -187 0 C) of a light yellow color, chlorine is an easily liquefied gas of yellow-green color (boiling point is -34.2 0 C), 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 sublimate - when heated at atmospheric pressure, it passes into a gaseous state (forms purple vapors), bypassing the liquid state. When moving down the group, 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 increasing atomic radius.
All halogens are poorly soluble in water, but well - 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 the dissolution of the halogen molecule, water loses sufficiently strong hydrogen bonds, instead of which no strong interactions occur between its polar molecule and the nonpolar halogen molecule. The dissolution of halogens in non-polar solvents corresponds to the situation: “like dissolves in like”, when the nature of the breaking and forming bonds is the same.
Chromium is 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 are used in paints and pigments. The red color of a ruby is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (along with sulfuric acid) to clean laboratory glassware. In addition, chromium oxide (VI) is used in the production of magnetic tape.
Discovery and etymology
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 the chemical formula PbCrO 4 . Today it is known as the croconte mineral.
In 1770, Peter Simon Pallas visited the place where Leman found a red lead mineral that had very useful pigment properties in paints. The use of Siberian red lead as a paint developed rapidly. In addition, bright yellow from croconte has become fashionable.
In 1797, Nicolas-Louis Vauquelin obtained samples of red By mixing croconte with hydrochloric acid, he obtained the oxide CrO 3 . Chromium as a chemical element was isolated in 1798. Vauquelin obtained it by heating oxide with charcoal. He was also able to detect traces of chromium in gemstones such as ruby and emerald.
In the 1800s, Cr was mainly used in paints and leather salts. Today, 85% of the metal is used in alloys. The rest is used in the chemical industry, the production of refractory materials and the foundry industry.
The pronunciation of the chemical element chromium corresponds to the Greek χρῶμα, which means "color", because of the many colored compounds that can be obtained from it.
Mining and production
The element is made from chromite (FeCr 2 O 4). Approximately half of this ore in the world 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 form pure chromium and diamonds.
For the industrial production of 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 Cr 2 O 3 oxide. The metal is obtained by heating the oxide in the presence of aluminum or silicon.
In 2000, approximately 15 Mt of chromite ore was mined and processed into 4 Mt of ferrochromium, 70% chromium-iron, with an estimated market value of US$2.5 billion.
Main characteristics
The characteristic of the chemical element chromium is 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 the VI group. It 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 various proportions. Thus, it forms compounds in which it has various oxidation states. Chromium is a chemical element with ground states +2, +3 and +6, of which +3 is the most stable. In addition, states +1, +4 and +5 are observed in rare cases. Chromium compounds in the +6 oxidation state are strong oxidizing agents.
What color is chrome? The chemical element imparts a ruby hue. The Cr 2 O 3 used for is also used as a pigment called "chrome green". Its salts color glass in an emerald green color. Chromium is a chemical element whose presence makes a ruby red. Therefore, it is used in the production of synthetic rubies.
isotopes
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 50 Cr is the most stable, with a half-life exceeding 1.8 x 10 17 years. 51 Cr has a half-life of 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, and for most of them it lasts less than one minute. The element also has two metastates.
Chromium isotopes in the earth's crust, as a rule, accompany manganese isotopes, which finds application in geology. 53 Cr is formed during the radioactive decay of 53 Mn. The Mn/Cr isotope ratio reinforces other information about the early history of the solar system. Changes in the ratios of 53 Cr/ 52 Cr and Mn/Cr from different meteorites prove that new atomic nuclei were created just before the formation of the solar system.
Chemical element chromium: properties, formula of compounds
Chromium oxide (III) 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 compound is commonly referred to as "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.
Potassium bichromate
K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as a cleaning agent for laboratory glassware from organics. For this, its saturated solution is used. Sometimes, however, it is replaced with sodium dichromate, based on the higher solubility of the latter. In addition, it can regulate the process of oxidation of organic compounds, converting primary alcohol into aldehyde, and then into carbon dioxide.
Potassium dichromate can cause chromium dermatitis. Chromium is probably the cause of the sensitization leading to the development of dermatitis, especially of the hands and forearms, which is chronic and difficult to treat. Like other Cr(VI) compounds, potassium bichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.
Chromic acid
The compound has the hypothetical structure H 2 CrO 4 . Neither chromic nor dichromic acids are found in nature, but their anions are found in various substances. "Chromic acid", which can be found on sale, is actually its acid anhydride - CrO 3 trioxide.
Lead(II) chromate
PbCrO 4 has a bright yellow color and is practically insoluble in water. For this reason, it has found application as a coloring pigment under the name "yellow crown".
Cr and pentavalent bond
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 is a specific aromatic group.
Application
Chromium is a chemical element whose properties have provided it with many different uses, some of which are listed below.
It gives metals resistance to corrosion and a glossy surface. Therefore, chromium is included in alloys such as stainless steel, used in cutlery, for example. 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 tan the skin. Potassium bichromate is used to oxidize organic compounds such as alcohols and aldehydes, as well as to clean laboratory glassware. It serves as a fixing agent for dyeing fabric and is also used in photography and photo printing.
CrO 3 is used to make magnetic tapes (for example, for audio recording), which have better characteristics than iron oxide films.
Role in biology
Trivalent chromium is a chemical element essential for the metabolism of sugar in the human body. In contrast, hexavalent Cr is highly toxic.
Precautionary measures
Chromium metal and Cr(III) compounds are not generally considered hazardous to health, 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 properly treated. 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 allowable 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 the soil and groundwater of abandoned industrial sites that require environmental cleanup and remediation. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.
Element properties
The main physical properties of chromium are as follows:
- Atomic number: 24.
- Atomic weight: 51.996.
- Melting point: 1890 °C.
- Boiling point: 2482 °C.
- Oxidation state: +2, +3, +6.
- Electron configuration: 3d 5 4s 1 .
The discovery of chromium belongs to the period of rapid development of chemical-analytical studies of salts and minerals. In Russia, chemists took a special interest in the analysis of minerals found in Siberia and almost unknown in Western Europe. One of these minerals was the Siberian red lead ore (crocoite), described by Lomonosov. The mineral was investigated, but nothing but oxides of lead, iron and aluminum was found in it. However, in 1797, Vauquelin, by boiling a finely ground sample of the mineral with potash and precipitating lead carbonate, obtained an orange-red solution. From this solution, he crystallized a ruby-red salt, from which an oxide and a free metal, different from all known metals, were isolated. Vauquelin called him Chromium ( Chrome ) from the Greek word- coloring, color; True, here it was not the property of the metal that was meant, but its brightly colored salts.
Finding in nature.
The most important chromium ore of practical importance is chromite, the approximate composition of which corresponds to the formula FeCrO 4.
It is found in Asia Minor, in the Urals, in North America, in southern Africa. The above-mentioned mineral crocoite - PbCrO 4 - is also of technical importance. Chromium oxide (3) and some of its other compounds are also found in nature. In the earth's crust, the chromium content in terms of metal is 0.03%. Chromium is found on the Sun, stars, meteorites.
Physical Properties.
Chromium is a white, hard and brittle metal, exceptionally chemically resistant to acids and alkalis. It oxidizes in air and has a thin transparent oxide film on the surface. Chromium has a density of 7.1 g / cm 3, its melting point is +1875 0 C.
Receipt.
With strong heating of chromium iron ore with coal, chromium and iron are reduced:
FeO * Cr 2 O 3 + 4C = 2Cr + Fe + 4CO
As a result of this reaction, an alloy of chromium with iron is formed, which is characterized by high strength. To obtain pure chromium, it is reduced from chromium(3) oxide with aluminum:
Cr 2 O 3 + 2Al \u003d Al 2 O 3 + 2Cr
Two oxides are usually used in this process - Cr 2 O 3 and CrO 3
Chemical properties.
Thanks to a thin protective oxide film covering the surface of chromium, it is highly resistant to aggressive acids and alkalis. Chromium does not react with concentrated nitric and sulfuric acids, as well as with phosphoric acid. Chromium interacts with alkalis at t = 600-700 o C. However, chromium interacts with dilute sulfuric and hydrochloric acids, displacing hydrogen:
2Cr + 3H 2 SO 4 \u003d Cr 2 (SO 4) 3 + 3H 2
2Cr + 6HCl = 2CrCl 3 + 3H 2
At high temperatures, chromium burns in oxygen to form oxide(III).
Hot chromium reacts with water vapor:
2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2
Chromium also reacts with halogens at high temperatures, halogens with hydrogens, sulfur, nitrogen, phosphorus, coal, silicon, boron, for example:
Cr + 2HF = CrF 2 + H 2
2Cr + N2 = 2CrN
2Cr + 3S = Cr2S3
Cr + Si = CrSi
The above physical and chemical properties of chromium have found their application in various fields of science and technology. For example, chromium and its alloys are used to obtain high-strength, corrosion-resistant coatings in mechanical engineering. Alloys in the form of ferrochrome are used as metal cutting tools. Chrome-plated alloys have found application in medical technology, in the manufacture of chemical process equipment.
The position of chromium in the periodic table of chemical elements:
Chromium heads the side subgroup of group VI of the periodic system of elements. Its electronic formula is as follows:
24 Cr IS 2 2S 2 2P 6 3S 2 3P 6 3d 5 4S 1
In filling the orbitals with electrons at the chromium atom, the regularity is violated, according to which the 4S orbital should have been filled first to the state 4S 2 . However, due to the fact that the 3d orbital occupies a more favorable energy position in the chromium atom, it is filled up to the value 4d 5 . Such a phenomenon is observed in the atoms of some other elements of the secondary subgroups. Chromium can exhibit oxidation states from +1 to +6. The most stable are chromium compounds with oxidation states +2, +3, +6.
Divalent chromium compounds.
Chromium oxide (II) CrO - pyrophoric black powder (pyrophoric - the ability to ignite in air in a finely divided state). CrO dissolves in dilute hydrochloric acid:
CrO + 2HCl = CrCl 2 + H 2 O
In air, when heated above 100 0 C, CrO turns into Cr 2 O 3.
Divalent chromium salts are formed by dissolving chromium metal in acids. These reactions take place in an atmosphere of an inactive gas (for example, H 2), because in the presence of air, Cr(II) is easily oxidized to Cr(III).
Chromium hydroxide is obtained in the form of a yellow precipitate by the action of an alkali solution on chromium (II) chloride:
CrCl 2 + 2NaOH = Cr(OH) 2 + 2NaCl
Cr(OH) 2 has basic properties, is a reducing agent. The hydrated Cr2+ ion is colored pale blue. An aqueous solution of CrCl 2 has a blue color. In air in aqueous solutions, Cr(II) compounds transform into Cr(III) compounds. This is especially pronounced for Cr(II) hydroxide:
4Cr(OH) 2 + 2H 2 O + O 2 = 4Cr(OH) 3
Trivalent chromium compounds.
Chromium oxide (III) Cr 2 O 3 is a refractory green powder. It is close to corundum in hardness. In the laboratory, it can be obtained by heating ammonium dichromate:
(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2
Cr 2 O 3 - amphoteric oxide, when fused with alkalis, forms chromites: Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O
Chromium hydroxide is also an amphoteric compound:
Cr(OH) 3 + HCl = CrCl 3 + 3H 2 O
Cr(OH) 3 + NaOH = NaCrO 2 + 2H 2 O
Anhydrous CrCl 3 has the appearance of dark purple leaves, is completely insoluble in cold water, and dissolves very slowly when boiled. Anhydrous chromium sulfate (III) Cr 2 (SO 4) 3 pink, also poorly soluble in water. In the presence of reducing agents, it forms purple chromium sulfate Cr 2 (SO 4) 3 *18H 2 O. Green chromium sulfate hydrates are also known, containing a smaller amount of water. Chrome alum KCr(SO 4) 2 *12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. A solution of chromic alum turns green when heated due to the formation of sulfates.
Reactions with chromium and its compounds
Almost all chromium compounds and their solutions are intensely colored. Having a colorless solution or a white precipitate, we can conclude with a high degree of probability that chromium is absent.
- We strongly heat in the flame of a burner on a porcelain cup such an amount of potassium dichromate that will fit on the tip of a knife. Salt will not release water of crystallization, but will melt at a temperature of about 400 0 C with the formation of a dark liquid. Let's heat it for a few more minutes on a strong flame. After cooling, a green precipitate forms on the shard. Part of it is soluble in water (it turns yellow), and the other part is left on the shard. The salt decomposed when heated, resulting in the formation of soluble yellow potassium chromate K 2 CrO 4 and green Cr 2 O 3 .
- Dissolve 3g of powdered potassium dichromate in 50ml of water. To one part add some potassium carbonate. It will dissolve with the release of CO 2 , and the color of the solution will become light yellow. Chromate is formed from potassium dichromate. If we now add a 50% solution of sulfuric acid in portions, then the red-yellow color of the bichromate will appear again.
- Pour into a test tube 5 ml. potassium dichromate solution, boil with 3 ml of concentrated hydrochloric acid under draft. Yellow-green poisonous gaseous chlorine is released from the solution, because chromate will oxidize HCl to Cl 2 and H 2 O. The chromate itself will turn into green trivalent chromium chloride. It can be isolated by evaporating the solution, and then, fusing with soda and nitrate, converted to chromate.
- When a solution of lead nitrate is added, yellow lead chromate precipitates; when interacting with a solution of silver nitrate, a red-brown precipitate of silver chromate is formed.
- Add hydrogen peroxide to a solution of potassium bichromate and acidify the solution with sulfuric acid. The solution acquires a deep blue color due to the formation of chromium peroxide. Peroxide, when shaken with some ether, will turn into an organic solvent and turn it blue. This reaction is specific for chromium and is very sensitive. It can be used to detect chromium in metals and alloys. First of all, it is necessary to dissolve the metal. With prolonged boiling with 30% sulfuric acid (hydrochloric acid can also be added), chromium and many steels partially dissolve. The resulting solution contains chromium (III) sulfate. To be able to conduct a detection reaction, we first neutralize it with caustic soda. Gray-green chromium (III) hydroxide precipitates, which dissolves in excess NaOH and forms green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow, as chromite is oxidized to chromate. Acidification will result in a blue color of the solution. The colored compound can be extracted by shaking with ether.
Analytical reactions for chromium ions.
- To 3-4 drops of a solution of chromium chloride CrCl 3 add a 2M solution of NaOH until the initial precipitate dissolves. Note the color of the sodium chromite formed. Heat the resulting solution in a water bath. What is happening?
- To 2-3 drops of CrCl 3 solution add an equal volume of 8M NaOH solution and 3-4 drops of 3% H 2 O 2 solution. Heat the reaction mixture in a water bath. What is happening? What precipitate is formed if the resulting colored solution is neutralized, CH 3 COOH is added to it, and then Pb (NO 3) 2 ?
- Pour 4-5 drops of solutions of chromium sulfate Cr 2 (SO 4) 3, IMH 2 SO 4 and KMnO 4 into a test tube. Heat the reaction site for several minutes on a water bath. Note the change in color of the solution. What caused it?
- To 3-4 drops of K 2 Cr 2 O 7 solution acidified with nitric acid, add 2-3 drops of H 2 O 2 solution and mix. The blue color of the solution that appears is due to the appearance of perchromic acid H 2 CrO 6:
Cr 2 O 7 2- + 4H 2 O 2 + 2H + = 2H 2 CrO 6 + 3H 2 O
Pay attention to the rapid decomposition of H 2 CrO 6:
2H 2 CrO 6 + 8H+ = 2Cr 3+ + 3O 2 + 6H 2 O
blue color green color
Perchromic acid is much more stable in organic solvents.
- To 3-4 drops of K 2 Cr 2 O 7 solution acidified with nitric acid, add 5 drops of isoamyl alcohol, 2-3 drops of H 2 O 2 solution and shake the reaction mixture. The layer of organic solvent that floats to the top is colored bright blue. The color fades very slowly. Compare the stability of H 2 CrO 6 in organic and aqueous phases.
- When CrO 4 2- and Ba 2+ ions interact, a yellow precipitate of barium chromate BaCrO 4 precipitates.
- Silver nitrate forms brick red precipitate of silver chromate with CrO 4 2 ions.
- Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution in one of them, the same volume of K 2 CrO 4 solution in the second, and three drops of both solutions in the third. Then add three drops of potassium iodide solution to each tube. Explain the result. Acidify the solution in the second tube. What is happening? Why?
Entertaining experiments with chromium compounds
- A mixture of CuSO 4 and K 2 Cr 2 O 7 turns green when alkali is added, and turns yellow in the presence of acid. By heating 2 mg of glycerol with a small amount of (NH 4) 2 Cr 2 O 7 and then adding alcohol, a bright green solution is obtained after filtration, which turns yellow when acid is added, and turns green in a neutral or alkaline medium.
- Place in the center of the can with thermite "ruby mixture" - thoroughly ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). So that the jar does not cool down longer, it is necessary to bury it under the upper edge in the sand, and after the thermite is ignited and the reaction begins, cover it with an iron sheet and cover it with sand. Bank to dig out in a day. The result is a red-ruby powder.
- 10 g of potassium bichromate is triturated with 5 g of sodium or potassium nitrate and 10 g of sugar. The mixture is moistened and mixed with collodion. If the powder is pressed in a glass tube, and then the stick is pushed out and set on fire from the end, then a “snake” will begin to crawl out, first black, and after cooling - green. A stick with a diameter of 4 mm burns at a speed of about 2 mm per second and lengthens 10 times.
- If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, then an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will fall out, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If further alcohol is added to this solution, a green precipitate will form, which, after filtration, becomes blue, and after drying, blue-violet with red sparkles, clearly visible in strong light.
- The chromium oxide left after the “volcano” or “pharaoh snake” experiments can be regenerated. To do this, it is necessary to fuse 8 g of Cr 2 O 3 and 2 g of Na 2 CO 3 and 2.5 g of KNO 3 and treat the cooled alloy with boiling water. Soluble chromate is obtained, which can also be converted into other Cr(II) and Cr(VI) compounds, including the original ammonium dichromate.
Examples of redox transitions involving chromium and its compounds
1. Cr 2 O 7 2- -- Cr 2 O 3 -- CrO 2 - -- CrO 4 2- -- Cr 2 O 7 2-
a) (NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O 
b) Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O
c) 2NaCrO 2 + 3Br 2 + 8NaOH = 6NaBr + 2Na 2 CrO 4 + 4H 2 O
d) 2Na 2 CrO 4 + 2HCl = Na 2 Cr 2 O 7 + 2NaCl + H 2 O
2. Cr(OH) 2 -- Cr(OH) 3 -- CrCl 3 -- Cr 2 O 7 2- -- CrO 4 2-
a) 2Cr(OH) 2 + 1/2O 2 + H 2 O = 2Cr(OH) 3
b) Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O
c) 2CrCl 3 + 2KMnO 4 + 3H 2 O = K 2 Cr 2 O 7 + 2Mn(OH) 2 + 6HCl
d) K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O
3. CrO - Cr (OH) 2 - Cr (OH) 3 - Cr (NO 3) 3 - Cr 2 O 3 - CrO - 2
Cr2+
a) CrO + 2HCl = CrCl 2 + H 2 O
b) CrO + H 2 O \u003d Cr (OH) 2
c) Cr(OH) 2 + 1/2O 2 + H 2 O = 2Cr(OH) 3
d) Cr(OH) 3 + 3HNO 3 = Cr(NO 3) 3 + 3H 2 O
e) 4Cr (NO 3) 3 \u003d 2Cr 2 O 3 + 12NO 2 + O 2
f) Cr 2 O 3 + 2 NaOH = 2NaCrO 2 + H 2 O
Chrome element as an artist
Chemists quite often turned to the problem of creating artificial pigments for painting. In the 18th-19th centuries, the technology for obtaining many pictorial materials was developed. Louis Nicolas Vauquelin in 1797, who discovered the previously unknown element chromium in Siberian red ore, prepared a new, remarkably stable paint - chrome green. Its chromophore is aqueous chromium (III) oxide. Under the name "emerald green" it began to be produced in 1837. Later, L. Vauquelen proposed several new paints: barite, zinc and chrome yellow. Over time, they were replaced by more persistent yellow, orange pigments based on cadmium.
Chrome green is the most durable and lightfast paint that is not affected by atmospheric gases. Rubbed in oil, chrome green has great hiding power and is capable of drying quickly, therefore, since the 19th century. it is widely used in painting. It is of great importance in porcelain painting. The fact is that porcelain products can be decorated with both underglaze and overglaze painting. In the first case, paints are applied to the surface of only a slightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: for sintering the porcelain mass and melting the glaze, the products are heated to 1350 - 1450 0 C. Very few paints can withstand such a high temperature without chemical changes, and in the old days there were only two of them - cobalt and chromium. Black oxide of cobalt, applied to the surface of a porcelain item, fuses with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. This cobalt blue chinaware is well known to everyone. Chromium oxide (III) does not interact chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze with a "deaf" layer.
In addition to chrome green, artists use paints derived from Volkonskoite. This mineral from the group of montmorillonites (a clay mineral of the subclass of complex silicates Na (Mo, Al), Si 4 O 10 (OH) 2) was discovered in 1830 by the Russian mineralogist Kemmerer and named after M.N. Volkonskaya, the daughter of the hero of the Battle of Borodino, General N N. Raevsky, wife of the Decembrist S. G. Volkonsky Volkonskoite is a clay containing up to 24% chromium oxide, as well as oxides of aluminum and iron (III). determines its diverse coloration - from the color of a darkened winter fir to the bright green color of a swamp frog.
Pablo Picasso turned to the geologists of our country with a request to study the reserves of Volkonskoite, which gives the paint a uniquely fresh tone. At present, a method has been developed for obtaining artificial wolkonskoite. It is interesting to note that, according to modern research, Russian icon painters used paints from this material as early as the Middle Ages, long before its “official” discovery. Guinier's green (created in 1837), whose chromoform is a hydrate of chromium oxide Cr 2 O 3 * (2-3) H 2 O, where part of the water is chemically bound and part adsorbed, was also popular with artists. This pigment gives the paint an emerald hue.
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