Several chemical compounds consisting of two simple elements - Cr and O - belong to the class of inorganic compounds - oxides. Their common name is chromium oxide; then in parentheses it is customary to indicate the valence of the metal in Roman numerals. Their other names and chemical formulas:
- chromium (II) oxide - chromium oxide, CrO;
- chromium (III) oxide - chrome green, chromium sesquioxide, Cr2O3;
- chromium (IV) oxide - chromium oxide, CrO2;
- chromium (VI) oxide - chromium anhydride, chromium trioxide, CrO3.
The compound in which the metal is hexavalent is the highest chromium oxide. This is an odorless solid substance, which in appearance is (they dissolve in air due to strong hygroscopicity). Molar mass - 99.99 g/mol. Density at 20 °C is 2.70 g/cm³. Melting point - 197 °C, boiling point - 251 °C. At 0 °C, 61.7 g/100 dissolves in water, at 25 °C - 63 g/100 ml, at 100 °C - 67.45 g/100 ml. The oxide also dissolves in sulfuric acid (this is a chromic mixture that is used in laboratory practice for washing chemical glassware) and ethyl alcohol, ethyl ether, acetic acid, and acetone. At 450 °C it decomposes to Cr2O3.
Chromium (VI) oxide is used in the electrolysis process (to obtain pure chromium), for chromating galvanized products, in electrolytic chrome plating, as a strong oxidizing agent (for the production of indigo and isatin). chromium is used to detect alcohol in exhaled air. The interaction proceeds according to the following scheme: 4CrO3 + 6H2SO4 + 3C2H5OH → 2Cr2(SO4)3 + 3CH3COOH + 9H2O. The presence of alcohol is indicated by a change in the color of the solution (it turns green).
Chromium (VI) oxide, like all compounds of hexavalent Cr, is a strong poison (lethal dose - 0.1 g). Due to its high activity, CrO3 causes fire (with explosions) upon contact with them. Despite its low volatility, higher chromium oxide is dangerous if inhaled, as it causes lung cancer. Upon contact with skin (even if removed quickly), it causes irritation, dermatitis, eczema, and provokes the development of cancer.
Oxide with tetravalent chromium CrO2 in appearance is a solid in the form of black tetrahedral ferromagnetic crystals. Chromium oxide 4 has a molar mass of 83.9949 g/mol and a density of 4.89 g/cm³. The substance melts, simultaneously decomposing, at a temperature of 375 ° C. Does not dissolve in water. Used in magnetic recording media as a working substance. With the rise in popularity of CDs and DVDs, the use of chromium(IV) oxide has decreased. It was first synthesized in 1956 by EI DuPont chemist Norman L. Cox by decomposing chromium trioxide in the presence of water at a temperature of 640 °C and a pressure of 200 MPa. DuPont is produced under license by Sony in Japan and BASF in Germany.
Chromium oxide 3 Cr2O3 is a solid, finely crystalline substance of light to dark green color. The molar mass is 151.99 g/mol. Density - 5.22 g/cm³. Melting point - 2435 °C, boiling point - 4000 °C. The refractive index of the pure substance is 2.551. This oxide does not dissolve in water, alcohol, acetone, or acid. Since its density approaches the density of corundum, it is introduced into polishing compositions (for example, GOI paste). It is one of chromium which is used as a pigment. It was first obtained using secret technology in 1838 in the form of a transparent hydrated form. It occurs in nature in the form of chromium iron ore FeO.Cr2O3.
Divalent chromium oxide is a black or red solid with a melting point of 1550 °C. Melts with decomposition. Molar mass - 67.996 g/mol. Chromium (II) oxide, which is red in color, is not pyrophoric, but the same substance, which is black in color, is pyrophoric. The powder spontaneously ignites in air, so it can only be stored under a layer of water, since it does not interact with it. It is very difficult to obtain black chromium oxide in its pure form.
Chromium oxides with lower valency are characterized by basic properties, while oxides with higher valency are characterized by acidic properties.
Chromium forms three oxides: CrO, Cr 2 O 3, CrO 3.
Chromium (II) oxide CrO is a pyrophoric black powder. Has basic properties.
In redox reactions it behaves as a reducing agent:
CrO is obtained by decomposition of chromium carbonyl Cr(CO) 6 in vacuum at 300°C.
Chromium (III) oxide Cr 2 O 3 is a refractory green powder. It is close to corundum in hardness, which is why it is included in polishing agents. Formed by the interaction of Cr and O 2 at high temperatures. In the laboratory, chromium(III) oxide can be prepared by heating ammonium dichromate:
(N -3 H 4) 2 Cr +6 2 O 7 =Cr +3 2 O 3 +N 0 2 +4H 2 O
Chromium(III) oxide has amphoteric properties. When interacting with acids, chromium (III) salts are formed: Cr 2 O 3 +3H 2 SO 4 =Cr 2 (SO 4) 3 +3H 2 O
When interacting with alkalis in the melt, chromium (III) compounds are formed - chromites (in the absence of oxygen): Cr 2 O 3 + 2NaOH = 2NaCrO 2 + H 2 O
Chromium(III) oxide is insoluble in water.
In redox reactions, chromium(III) oxide behaves as a reducing agent:
Chromium (VI) oxide CrO 3 - chromic anhydride, is a dark red needle-shaped crystals. When heated to about 200°C, it decomposes:
4CrO 3 =2Cr 2 O 3 +3O 2
Easily dissolves in water, being acidic in nature, it forms chromic acids. With excess water, chromic acid H 2 CrO 4 is formed:
CrO 3 +H 2 O=H 2 CrO 4
At a high concentration of CrO 3, dichromic acid H 2 Cr 2 O 7 is formed:
2CrO 3 +H 2 O=H 2 Cr 2 O 7
which, when diluted, turns into chromic acid:
H 2 Cr 2 O 7 +H 2 O=2H 2 CrO 4
Chromic acids exist only in aqueous solution; none of these acids are isolated in a free state. However, their salts are very stable.
Chromium(VI) oxide is a strong oxidizing agent:
3S+4CrO 3 =3SO 2 +2Cr 2 O 3
Oxidizes iodine, sulfur, phosphorus, coal, turning into Cr 2 O 3. CrO 3 is obtained by the action of an excess of concentrated sulfuric acid on a saturated aqueous solution of sodium dichromate: Na 2 Cr 2 O 7 +2H 2 SO 4 =2CrO 3 +2NaHSO 4 +H 2 O It should be noted that chromium (VI) oxide is highly toxic.
] numerous R-shaded bands are attributed to the CrO molecule, observed in the range 4800 – 7100 Å in the emission spectrum of an electric arc in air when metallic chromium or Cr 2 Cl 6 salt is placed in it. Vibrational analysis showed that the bands belong to one system (electronic transition) with a 0-0 band around 6000 Å, and the vibrational constants of the upper and lower electronic states were determined. The “orange” system also includes bands in the range 7100–8400Å, measured in [32FER]. In [55NIN], a partial analysis of the rotational structure of the bands was carried out, on the basis of which the type of electronic transition 5 Π - 5 Π was established. In the reference book [84HUGH/GER], the lower state of the system is designated as the ground state of the X 5 Π molecule.
A complete rotational analysis of the five bands of the system (2-0, 1-0, 0-0, 0-1 and 0-2) was performed in [80HOC/MER]. The bands were recorded with high resolution in the discharge emission spectrum and in the laser excitation spectrum of CrO molecules in a flow of inert carrier gas. The lower state of the system is confirmed as the ground state of the molecule (the laser excitation spectrum was obtained at a carrier gas temperature just below room temperature).
Another weaker system of CrO bands was detected in the discharge emission spectrum in the near-infrared region [84CHE/ZYR]. The spectrum was obtained using a Fourier spectrometer. Rotational analysis of the 0-0 band located around 8000 cm -1 showed that the system belongs to the 5 Σ - X 5 Π transition.
The third system of CrO bands, centered at about 11800 cm -1, was detected in the chemiluminescence spectrum during the reaction of chromium atoms with ozone [89DEV/GOL]. The bands of this system are also marked in the atlas [57GAT/JUN]. In [93BAR/HAJ], the 0-0 and 1-1 bands are obtained with high resolution in the laser excitation spectrum. A rotational analysis was carried out, which showed that the system is formed by the 5 Δ - X 5 Π transition.
In the chemiluminescence spectrum [89DEV/GOL], a system of bands was detected in the region of 4510 Å (ν 00 = 22163 cm ‑1), vibrational analysis was carried out. The system probably belongs to an electronic transition with charge transfer, because the vibrational range in the upper state is much smaller than the vibrational ranges in other states of CrO. The preliminary electronic transition is designated as C 5 Π - X 5 Π.
Photoelectron spectra of the CrO anion were obtained in [96WEN/GUN] and [2001GUT/JEN]. The most complete and reliable interpretation of the spectra, based on MRCI calculations of the anion and molecule, is presented in [2002BAU/GUT]. According to the calculation, the anion has a ground state X 4 Π and a first excited state 6 Σ +. The spectra show one-electron transitions from these states to the ground and 5 excited states of the neutral molecule: X 5 Π ← 6 Σ + (1.12 eV), X 5 Π ← X 4 Π (1.22 eV), 3 Σ – ← X 4 Π (1.82 eV), 5 Σ + ← 6 Σ + (2.13 eV), 3 Π ← X 4 Π (2.28 eV), 5 Δ ← 6 Σ + (2.64 eV), 3 Φ ← X 4 Π (3.03 eV). The energies of the quintet states of CrO are consistent with the optical spectra data. The triplet states 3 Σ – (0.6 eV), 3 Π (1.06 eV) and 3 Φ (1.81 eV) were not observed in the optical spectra.
Quantum mechanical calculations of CrO were performed in [82GRO/WAH, 84HUZ/KLO, 85BAU/NEL, 85NEL/BAU, 87AND/GRI, 87DOL/WED, 88JAS/STE, 89STE/NAC, 95BAU/MAI, 96BAK/STI, 2000BRI /ROT, 2000GUT/RAO, 2001GUT/JEN, 2002BAU/GUT, 2003GUT/AND, 2003DAI/DEN, 2006FUR/PER, 2007JEN/ROO, 2007WAG/MIT ]. The [85BAU/NEL] calculation showed and confirmed in subsequent calculations that the ground state of the molecule is 5 Π. The energies of excited states are given directly or indirectly (in the form of dissociation energy or electron affinity) in [85BAU/NEL, 85NEL/BAU, 96BAK/STI, 2000BRI/ROT, 2001GUT/JEN, 2002BAU/GUT, 2003DAI/DEN].
The calculation of thermodynamic functions included: a) the lower component Ω = -1 of the X 5 Π state, as the ground state; b) the remaining Ω-components X 5 Π, as separate excited states; c) excited states, the energies of which are determined experimentally or calculated; d) synthetic states, which take into account all other states of the molecule with an estimated energy up to 40000 cm -1.
Equilibrium constants for the X 5 Π CrO state were obtained in [80HOC/MER]. They are given in table Cr.D1 as constants for the lower component X 5 Π –1, although they relate to the entire state as a whole. The differences in the values of ω e for the components of the X 5 Π state are insignificant and are taken into account in the error of ± 1 cm -1.
The energies of excited states are given according to spectroscopic data [84CHE/ZYR] (5 Π 0, 5 Π 1, 5 Π 2, 5 Π 3, A 5 Σ +), [93BAR/HAJ] ( A΄ 5 Δ), [ 80HOC/MER ] (B 5 Π), [ 89DEV/GOL ] (C 5 Π); interpretation of photoelectron spectra [2002BAU/GUT] (3 Σ - , 3 Π, 3 Φ); according to the calculations of [2002BAU/GUT] (5 Σ – , 3 Δ) and [2003DAI/DEN] (3 Σ).
The vibrational and rotational constants of the excited states of CrO were not used in the calculations of thermodynamic functions and are given in table Cr.D1 for reference. For states A 6 Σ + , A΄ 5Δ, B 5 Π, C(5 Π) spectroscopic constants are given according to the data of [84CHE/ZYR, 93BAR/HAJ, 80HOC/MER, 89DEV/GOL], respectively. For the 3 Σ -, 3 Π, 3 Φ states, the values of ω e obtained from the photoelectron spectrum of the anion in [96WEN/GUN] are given. Values of ω e for states 5 Σ - , 3 Δ and r e for 3 Σ - , 3 Π, 3 Φ, 5 Σ - , 3 Δ are given according to the results of the MRCI calculation [2002BAU/GUT].
The statistical weights of the synthetic states are estimated using the ionic model. The observed and calculated states of CrO are assigned to three ionic configurations: Cr 2+ (3d 4)O 2- , Cr 2+ (3d 3 4s)O 2- and Cr + (3d 5)O - . The energies of other states of these configurations were estimated using data [71MOO] on the position of the terms of singly and doubly charged chromium ions. Estimates from [2001GUT/JEN] for the energies of states 7 Π, 7 Σ + configuration Cr + (3d 5)O - were also used.
Thermodynamic functions CrO(g) were calculated using equations (1.3) - (1.6) , (1.9) , (1.10) , (1.93) - (1.95) . Values Q int and its derivatives were calculated using equations (1.90) - (1.92) taking into account nineteen excited states under the assumption that Q kol.vr ( i) = (p i /p X)Q kol.vr ( X) . The vibrational-rotational partition function of the state X 5 Π -1 and its derivatives were calculated using equations (1.70) - (1.75) by direct summation over vibrational levels and integration over rotational energy levels using an equation like (1.82). The calculations took into account all energy levels with values J< J max,v , where J max,v was found from conditions (1.81). The vibrational-rotational levels of the state X 5 Π -1 were calculated using equations (1.65), the values of the coefficients Y kl in these equations were calculated using relations (1.66) for the isotopic modification corresponding to the natural mixture of chromium and oxygen isotopes from the molecular constants 52 Cr 16 O given in table Cr.D1. Coefficient values Y kl , as well as the quantities v max and J lim are given in table Cr.D2.
At room temperature the following values were obtained:
C p o (298.15 K) = 32.645 ± 0.26 J × K ‑1 × mol ‑1
S o (298.15 K) = 238.481 ± 0.023 J× K‑1 × mol‑1
H o (298.15 K)- H o (0) = 9.850 ± 0.004 kJ× mol ‑1
The main contribution to the error of the calculated thermodynamic functions of CrO(g) at temperatures of 298.15 and 1000 K comes from the calculation method. At 3000 and 6000 K, the error is mainly due to the uncertainty in the energies of the excited electronic states. Errors in the values of Φº( T) at T= 298.15, 1000, 3000 and 6000 K are estimated to be 0.02, 0.04, 0.2 and 0.4 J× K‑1 × mol‑1, respectively.
Previously, thermodynamic functions of CrO(g) were calculated for tables by JANAF [85CHA/DAV], Schneider [74SCH] (T = 1000 – 9000 K), Brewer and Rosenblat [69BRE/ROS] (values Φº( T) for T ≤ 3000 K). Discrepancies between JANAF tables and table. CrO at low temperatures are due to the fact that the authors of [85CHA/DAV] could not take into account the multiplet splitting of the X 5 Π state; the discrepancy in the values of Φº(298.15) is 4.2 J × K ‑1 × mol ‑1. In the region of 1000 – 3000 K, discrepancies in the values of Φº( T) do not exceed 1.5 J × K ‑1 × mol ‑1, but by 6000 K they reach 3.1 J × K ‑1 × mol ‑1 due to the fact that in [
Among the variety of chemical elements and their compounds, it is difficult to single out the most useful substance for humanity. Each is unique in its properties and application possibilities. Technological progress greatly facilitates the research process, but also poses new challenges. Chemical elements, discovered several hundred years ago and studied in all their manifestations, are being used in more technologically advanced ways in the modern world. This trend extends to compounds that exist in nature and those created by humans.
Oxide
In the earth's crust and in the vastness of the Universe there are many chemical compounds that differ in classes, types, and characteristics. One of the most common types of compounds is oxide (oxide, oxide). It includes sand, water, carbon dioxide, i.e., fundamental substances for the existence of humanity and the entire biosphere of the Earth. Oxides are substances that contain oxygen atoms with an oxidation state of -2, and the bond between the elements is binary. Their formation occurs as a result of a chemical reaction, the conditions of which vary depending on the composition of the oxide.
The characteristic features of this substance are three positions: the substance is complex, consists of two atoms, one of them is oxygen. The large number of existing oxides is explained by the fact that many chemical elements form several substances. They are identical in composition, but the atom that reacts with oxygen exhibits several degrees of valence. For example, chromium oxide (2, 3, 4, 6), nitrogen (1, 2, 3, 4, 5), etc. Moreover, their properties depend on the degree of valence of the element entering the oxidative reaction.
According to the accepted classification, oxides are basic and acidic. An amphoteric species is also distinguished, which exhibits the properties of a basic oxide. Acidic oxides are compounds of non-metals or elements with high valency; their hydrates are acids. Basic oxides include all substances that have an oxygen + metal bond; their hydrates are bases.
Chromium
In the 18th century, the chemist I. G. Lehman discovered an unknown mineral, which was called red Siberian lead. Professor Vaukelin, a professor at the Paris Mineralogical School, carried out a series of chemical reactions with the resulting sample, as a result of which an unknown metal was isolated. The main properties identified by the scientist were its resistance to acidic environments and refractoriness (heat resistance). The name "chrome" (Chromium) arose due to the wide range of colors that are characterized by the compounds of the element. The metal is quite inert and is not found in its pure form in natural conditions.
The main minerals containing chromium are: chromite (FeCr 2 O 4), melanochroite, vokelenite, ditzeite, tarapacaite. The chemical element Cr is located in group 6 of the periodic system of D.I. Mendeleev, has atomic number 24. The electronic configuration of the chromium atom allows the element to have a valence of +2, +3, +6, with the most stable compounds being trivalent metals. Reactions are possible in which the oxidation state is +1, +5, +4. Chromium is not chemically active; the metal surface is covered with a film (passivation effect), which prevents reactions with oxygen and water under normal conditions. Chromium oxide formed on the surface protects the metal from interaction with acids and halogens in the absence of catalysts. Compounds with simple substances (not metals) are possible at temperatures from 300 o C (chlorine, bromine, sulfur).
When interacting with complex substances, additional conditions are required, for example, with an alkali solution the reaction does not occur, with its melts the process occurs very slowly. Chromium reacts with acids when high temperature is present as a catalyst. Chromium oxide can be obtained from various minerals by exposure to temperature. Depending on the future oxidation state of the element, concentrated acids are used. In this case, the valence of chromium in the compound varies from +2 to +6 (highest chromium oxide).
Application
Due to their unique anti-corrosion properties and heat resistance, chromium-based alloys are of great practical importance. At the same time, in percentage terms, its share should not exceed half of the total volume. The big disadvantage of chromium is its brittleness, which reduces the processing capabilities of the alloys. The most common way to use metal is in the production of coatings (chrome plating). The protective film may be a layer of 0.005 mm, but it will reliably protect the metal product from corrosion and external influences. Chromium compounds are used for the manufacture of heat-resistant structures in the metallurgical industry (smelting furnaces). Anti-corrosion decorative coatings (metal-ceramics), special alloy steel, electrodes for welding machines, alloys based on silicon and aluminum are in demand on world markets. Chromium oxide, due to its low oxidation potential and high heat resistance, serves as a catalyst for many chemical reactions occurring at high temperatures (1000 o C).
Divalent compounds
Chromium (2) oxide CrO (nitrous oxide) is a bright red or black powder. It is insoluble in water, does not oxidize under normal conditions, and exhibits pronounced basic properties. The substance is solid, refractory (1550 o C), non-toxic. During heating to 100 o C it is oxidized to Cr 2 O 3. It does not dissolve in weak solutions of nitric and sulfuric acids; the reaction occurs with hydrochloric acid.
Receipt, application
This substance is considered a lower oxide. It has a fairly narrow scope of application. In the chemical industry, chromium oxide 2 is used to purify hydrocarbons from oxygen, which it attracts during the oxidation process at temperatures above 100 o C. Chromium oxide can be obtained in three ways:
- Decomposition of carbonyl Cr(CO) 6 in the presence of high temperature as a catalyst.
- Reducing chromium oxide with phosphoric acid 3.
- Chromium amalgam is oxidized by oxygen or nitric acid.
Trivalent compounds
For chromium oxides, the +3 oxidation state is the most stable form of the substance. Cr 2 O 3 (chrome green, sesquioxide, escolaid) is chemically inert, insoluble in water, and has a high melting point (more than 2000 o C). Chromium oxide 3 is a green, refractory powder, very hard, and has amphoteric properties. The substance is soluble in concentrated acids, reaction with alkalis occurs as a result of fusion. Can be reduced to pure metal when reacted with a strong reducing agent.
Receipt and use
Due to its high hardness (comparable to corundum), the substance is most widely used in abrasive and polishing materials. Chromium oxide (formula Cr 2 O 3) has a green color, so it is used as a pigment in the manufacture of glasses, paints, and ceramics. For the chemical industry, this substance is used as a catalyst for reactions with organic compounds (ammonia synthesis). Trivalent chromium oxide is used to create artificial gemstones and spinels. To obtain it, several types of chemical reactions are used:
- Oxidation of chromium oxide.
- Heating (calcination) of ammonium dichromate or ammonium chromate.
- Decomposition of trivalent chromium hydroxide or hexavalent oxide.
- Calcination of mercury chromate or dichromate.
Hexavalent compounds
The formula of the highest chromium oxide is CrO 3. The substance is purple or dark red in color and can exist in the form of crystals, needles, plates. Chemically active, toxic, when interacting with organic compounds there is a danger of spontaneous combustion and explosion. Chromium oxide 6 - chromic anhydride, chromium trioxide - is highly soluble in water, under normal conditions it interacts with air (dissolves), melting point - 196 o C. The substance has pronounced acidic characteristics. During a chemical reaction with water, dichromic or chromic acid is formed; without additional catalysts it reacts with alkalis (yellow chromates). For halogens (iodine, sulfur, phosphorus) it is a strong oxidizing agent. As a result of heating above 250 o C, free oxygen and trivalent chromium oxide are formed.
How to get it and where to use it
Chromium oxide 6 is obtained by treating sodium or potassium chromates (dichromates) with concentrated sulfuric acid or by reacting silver chromate with hydrochloric acid. The high chemical activity of the substance determines the main directions of its use:
- Obtaining pure metal - chromium.
- In the process of chrome plating surfaces, including electrolytic methods.
- Oxidation of alcohols (organic compounds) in the chemical industry.
- In rocket technology it is used as a fuel igniter.
- In chemical laboratories, it cleans glassware from organic compounds.
- Used in the pyrotechnic industry.
Chromium is an element of the side 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 (lat. Chromium). The simple substance chromium is a hard metal of a bluish-white color.
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 are formed.
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 surface of the metal explains its passivity in relation to concentrated solutions of acids - oxidizers.
Chromium compounds
Chromium(II) oxide and chromium(II) hydroxide are basic in nature.
Cr(OH) 2 + 2HCl → CrCl 2 + 2H 2 O
Chromium (II) compounds are strong reducing agents; transform into chromium (III) compounds under the influence 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. Can be obtained by calcination of 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 (volcano reaction)
Amphoteric oxide. When Cr 2 O 3 is fused with alkalis, soda and acid salts, chromium compounds with an oxidation state of (+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 fused with a mixture of alkali and 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) hydroxide C r (OH) 3 . Amphoteric hydroxide. Gray-green, decomposes when heated, losing water and forming green metahydroxide CrO(OH). Does not dissolve in water. Precipitates from solution as a gray-blue 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 Cr(OH) 3 + ZN + = Cr 3+ + 3H 2 O
Cr(OH) 3 + KOH → K, Cr(OH) 3 + ZON - (conc.) = [Cr(OH) 6 ] 3-
Cr(OH) 3 + KOH → KCrO 2 + 2H 2 O Cr(OH) 3 + MOH = MSrO 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 C –H 2 O) Cr2O3
2Cr(OH) 3 + 4NaOH (conc.) + ZN 2 O 2 (conc.) = 2Na 2 CrO 4 + 8H 2 0
Receipt: 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 alkali - the precipitate dissolves)
Chromium (III) salts have a purple or dark green color. Their chemical properties 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 - bright red crystals, soluble in water.
Obtained 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 is an acidic oxide, with alkalis it 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:
K 2 Cr 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 + 6KI = 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 O 4 . Oxosol. Yellow, non-hygroscopic. Melts without decomposition, thermally stable. Very soluble in water ( yellow the color of the solution corresponds to the CrO 4 2- ion), slightly hydrolyzes the anion. In an acidic environment it turns into K 2 Cr 2 O 7 . Oxidizing agent (weaker than K 2 Cr 2 O 7). Enters into ion exchange reactions.
Qualitative reaction on the CrO 4 2- ion - the precipitation of a yellow precipitate of barium chromate, which decomposes in a strongly acidic environment. It is used as a mordant for dyeing fabrics, a leather tanning agent, a 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 (concentration, horizon) = 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 reaction:
K 2 CrO 4 + BaCl 2 = 2KCl + BaCrO 4 ↓
2BaCrO 4 (t) + 2HCl (dil.) = BaCr 2 O 7 (p) + BaC1 2 + H 2 O
Receipt: sintering of chromite with potash in air:
4(Cr 2 Fe ‖‖)O 4 + 8K 2 CO 3 + 7O 2 = 8K 2 CrO 4 + 2Fe 2 O 3 + 8СO 2 (1000 °C)
Potassium dichromate K 2 Cr 2 O 7 . Oxosol. Technical name chrome peak. Orange-red, non-hygroscopic. Melts without decomposition, and decomposes upon further heating. Very soluble in water ( orange The color of the solution corresponds to the Cr 2 O 7 2- ion. In an alkaline environment it forms K 2 CrO 4 . A typical oxidizing agent in solution and during fusion. Enters into ion exchange reactions.
Qualitative reactions- blue color of an ethereal solution in the presence of H 2 O 2, blue color of an aqueous solution under the action of atomic hydrogen.
It is used as a leather tanning agent, a mordant for dyeing fabrics, a component of pyrotechnic compositions, a reagent in analytical chemistry, a metal corrosion inhibitor, in a mixture 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 (conc) = 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 (red) ↓
Cr 2 O 7 2- (dil.) +H 2 O +Pb 2+ =2H + + 2PbCrO 4 (red) ↓
K 2 Cr 2 O 7(t) +6HCl+8H 0 (Zn)=2CrCl 2(syn) +7H 2 O+2KCl
Receipt: treatment of K 2 CrO 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
