Acidic chromium oxide formula. Chromium and its compounds. Chemical properties of chromium

"National Research Tomsk Polytechnic University"

Institute of Natural Resources Geoecology and Geochemistry

Chromium

By discipline:

Chemistry

Completed:

student of group 2G41 Tkacheva Anastasia Vladimirovna 10/29/2014

Checked:

teacher Stas Nikolay Fedorovich

Position in the periodic system

Chromium - an element of a 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). Simple substance chromium - solid metal, bluish-white. Chromium is sometimes referred to as ferrous metals.

Atom structure

17 Cl) 2) 8) 7 - atomic structure diagram

1s2s2p3s3p- electronic formula

The atom is located in the III period, and has three energy levels

The atom is located in VII in the group, in the main subgroup - at the external energy level of 7 electrons

Element properties

Physical properties

Chromium is a white, shiny metal with a cubic body-centered lattice, a \u003d 0.28845 nm, characterized by hardness and fragility, with a density of 7.2 g / cm 3, one of the hardest pure metals (second only to beryllium, tungsten and uranium), with a melting point of 1903 degrees. And with a boiling point of about 2570 degrees. C. In air, the surface of chromium is covered with an oxide film, which protects it from further oxidation. The addition of carbon to chromium further increases its hardness.

Chemical properties

Chromium under normal conditions is an inert metal, when heated it becomes quite active.

Interaction with non-metals

When heated above 600 ° C, chromium burns out in oxygen:

4Cr + 3O 2 \u003d 2Cr 2 O 3.

It reacts with fluorine at 350 ° С, with chlorine - at 300 ° С, with bromine - at the temperature of red heat, forming chromium (III) halides:

2Cr + 3Cl 2 \u003d 2CrCl 3.

Reacts with nitrogen at temperatures above 1000 ° C with the formation of nitrides:

2Cr + N 2 \u003d 2CrN

or 4Cr + N 2 \u003d 2Cr 2 N.

2Cr + 3S \u003d Cr 2 S 3.

Reacts with boron, carbon and silicon to form borides, carbides and silicides:

Cr + 2B \u003d CrB 2 (formation of Cr 2 B, CrB, Cr 3 B 4, CrB 4 is possible),

2Cr + 3C \u003d Cr 2 C 3 (formation of Cr 23 C 6, Cr 7 B 3 is possible),

Cr + 2Si \u003d CrSi 2 (formation of Cr 3 Si, Cr 5 Si 3, CrSi is possible).

Does not directly interact with hydrogen.

Interaction with water

In a finely divided incandescent state, chromium reacts with water, forming chromium (III) oxide and hydrogen:

2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2

5interactions with acids

In the electrochemical series of voltages of metals, chromium is up to hydrogen, it displaces hydrogen from solutions of non-oxidizing acids:

Cr + 2HCl \u003d CrCl 2 + H 2;

Cr + H 2 SO 4 \u003d CrSO 4 + H 2.

In the presence of atmospheric oxygen, chromium (III) salts are formed:

4Cr + 12HCl + 3O 2 \u003d 4CrCl 3 + 6H 2 O.

Concentrated nitric and sulfuric acids passivate chromium. Chromium can dissolve in them only with strong heating, chromium (III) salts and acid reduction products are formed:

2Cr + 6H 2 SO 4 \u003d Cr 2 (SO 4) 3 + 3SO 2 + 6H 2 O;

Cr + 6HNO 3 \u003d Cr (NO 3) 3 + 3NO 2 + 3H 2 O.

Interaction with alkaline reagents

In aqueous solutions of alkalis, chromium does not dissolve, slowly reacts with alkali melts with the formation of chromites and the release of hydrogen:

2Cr + 6KOH \u003d 2KCrO 2 + 2K 2 O + 3H 2.

Reacts with alkaline melts of oxidizing agents, for example potassium chlorate, while chromium goes into potassium chromate:

Cr + KClO 3 + 2KOH \u003d K 2 CrO 4 + KCl + H 2 O.

Recovery of metals from oxides and salts

Chromium is an active metal capable of displacing metals from solutions of their salts: 2Cr + 3CuCl 2 \u003d 2CrCl 3 + 3Cu.

Properties of a simple substance

Stable in air due to passivation. For the same reason, it does not react with sulfuric and nitric acids. At 2000 ° C, it burns to form green chromium (III) oxide Cr 2 O 3, which has amphoteric properties.

Compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).

Cr compounds (+2)

The oxidation state +2 corresponds to the basic oxide CrO (black). Cr 2+ salts (blue solutions) are obtained by reducing Cr 3+ salts or dichromates with zinc in an acidic medium ("with hydrogen at the time of separation"):

All these Cr 2+ salts are strong reducing agents to the extent that they displace hydrogen from water upon standing. Oxygen in the air, especially in an acidic environment, oxidizes Cr 2+, as a result of which the blue solution quickly turns green.

Brown or yellow hydroxide Cr (OH) 2 precipitates when alkalis are added to solutions of chromium (II) salts.

Chromium dihalides CrF 2, CrCl 2, CrBr 2 and CrI 2 were synthesized

Cr (+3) compounds

The oxidation state +3 corresponds to amphoteric oxide Cr 2 O 3 and hydroxide Cr (OH) 3 (both are green). This is the most stable oxidation state of chromium. Chromium compounds in this oxidation state have a color from dirty lilac (ion 3+) to green (anions are present in the coordination sphere).

Cr 3+ tends to form double sulfates of the type M I Cr (SO 4) 2 12H 2 O (alum)

Chromium (III) hydroxide is obtained by acting with ammonia on solutions of chromium (III) salts:

Cr + 3NH + 3H2O → Cr (OH) ↓ + 3NH

You can use solutions of alkalis, but in their excess, a soluble hydroxo complex is formed:

Cr + 3OH → Cr (OH) ↓

Cr (OH) + 3OH →

By fusing Cr 2 O 3 with alkalis, chromites are obtained:

Cr2O3 + 2NaOH → 2NaCrO2 + H2O

Uncalcined chromium (III) oxide dissolves in alkaline solutions and acids:

Cr2O3 + 6HCl → 2CrCl3 + 3H2O

When chromium (III) compounds are oxidized in an alkaline medium, chromium (VI) compounds are formed:

2Na + 3HO → 2NaCrO + 2NaOH + 8HO

The same happens when chromium (III) oxide is fused with alkali and oxidizing agents, or with alkali in air (the melt becomes yellow in this case):

2Cr2O3 + 8NaOH + 3O2 → 4Na2CrO4 + 4H2O

Chromium compounds (+4)[

With the careful decomposition of chromium (VI) oxide CrO 3 under hydrothermal conditions, chromium (IV) oxide CrO 2 is obtained, which is ferromagnet and has metallic conductivity.

Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapors.

Chromium compounds (+6)

The oxidation state +6 corresponds to the acidic chromium (VI) oxide CrO 3 and a number of acids between which there is an equilibrium. The simplest of them are chromic H 2 CrO 4 and two-chromic H 2 Cr 2 O 7. They form two series of salts: yellow chromates and orange dichromates, respectively.

Chromium oxide (VI) CrO 3 is formed by the interaction of concentrated sulfuric acid with dichromate solutions. A typical acidic oxide, when interacting with water, it forms strong unstable chromic acids: chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and other isopolyacids with the general formula H 2 Cr n O 3n + 1. An increase in the degree of polymerization occurs with a decrease in pH, that is, an increase in acidity:

2CrO + 2H → Cr2O + H2O

But if an alkali solution is added to the orange solution of K 2 Cr 2 O 7, the color again turns yellow, since the chromate K 2 CrO 4 is again formed:

Cr2O + 2OH → 2CrO + HO

It does not reach a high degree of polymerization, as it happens in tungsten and molybdenum, since polychromic acid decomposes into chromium (VI) oxide and water:

H2CrnO3n + 1 → H2O + nCrO3

The solubility of chromates roughly corresponds to the solubility of sulfates. In particular, the yellow chromate of barium BaCrO 4 precipitates when barium salts are added both to chromate solutions and to dichromate solutions:

Ba + CrO → BaCrO ↓

2Ba + CrO + H2O → 2BaCrO ↓ + 2H

The formation of blood-red, poorly soluble silver chromate is used to detect silver in alloys using assay acid.

Known chromium pentafluoride CrF 5 and unstable chromium hexafluoride CrF 6. Volatile chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 (chromyl chloride) were also obtained.

Chromium (VI) compounds are strong oxidizing agents, for example:

K2Cr2O7 + 14HCl → 2CrCl3 + 2KCl + 3Cl2 + 7H2O

The addition of hydrogen peroxide, sulfuric acid and an organic solvent (ether) to the dichromates leads to the formation of blue chromium peroxide CrO 5 L (L is a solvent molecule), which is extracted into the organic layer; this reaction is used as an analytical one.

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 brackets it is customary to indicate the valency of the metal in Roman numerals. Their other names and chemical formulas:

chromium (II) oxide - chromium oxide, CrO;
chromium (III) oxide - chromium green, chromium sesquioxide, Cr2O3;
chromium (IV) oxide - chromium oxide, CrO2;
chromium (VI) oxide - chromic anhydride, chromium trioxide, CrO3.

The compound in which the metal is hexavalent is the higher chromium oxide. This is a solid, odorless substance, which in appearance is (in air, they spread out due to strong hygroscopicity). The molar mass is 99.99 g / mol. The density at 20 ° C is 2.70 g / cm³. Melting point - 197 ° С, boiling point - 251 ° С. 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 chromium mixture that is used in laboratory practice for washing chemical dishes) and ethyl alcohol, ethyl ether, acetic acid, acetone. Decomposes to Cr2O3 at 450 ° C.

Chromium (VI) oxide is used in the electrolysis process (to obtain pure chromium), for chromating galvanized products, in electrolytic chromium plating, as a strong oxidant (for the production of indigo and isatin). chromium is used to detect alcohol in breath. The interaction proceeds according to the scheme: 4CrO3 + 6H2SO4 + 3C2H5OH → 2Cr2 (SO4) 3 + 3CH3COOH + 9H2O. The presence of alcohol is indicated by a change in the color of the solution (becomes 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 combustion (with explosions) when in contact with them. Despite its low volatility, higher chromium oxide is hazardous by inhalation as it causes lung cancer. Upon contact with the skin (even if it is removed soon), 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 while decomposing at a temperature of 375 ° C. It 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 declined. It was first synthesized in 1956 by Norman L. Cox, chemist from EI DuPont, by decomposing chromium trioxide in the presence of water at 640 ° C and a pressure of 200 MPa. Manufactured under license from Sony in Japan and BASF in Germany, DuPont.

Chromium oxide 3 Cr2O3 is a solid fine-crystalline substance from light to dark green color. The molar mass is 151.99 g / mol. Density - 5.22 g / cm³. Melting point - 2435 ° С, boiling point - 4000 ° С. The refractive index of a pure substance is 2.551. This oxide does not dissolve in water, alcohol, acetone, acid. Since its density approaches the density of corundum, it is introduced into the compositions of polishing agents (for example, GOI pastes). It is one of the chromium that is used as a pigment. For the first time using a secret technology, it was obtained in 1838 in the form of a transparent hydrated form. It occurs naturally 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. The molar mass is 67.996 g / mol. Red chromium (II) oxide is not pyrophoric, while the same black substance is pyrophoric. The powder ignites spontaneously in air, so it can only be stored under a layer in water, since it does not interact with it. It is very difficult to obtain black chromium oxide in its pure form.

For chromium oxides with a lower valence, basic properties are characteristic, and for an oxide with a higher valency, acidic ones.

] the CrO molecule is attributed to numerous R-shaded bands observed in the range 4800 - 7100Å in the emission spectrum of an electric arc in air when metallic chromium or a Cr 2 Cl 6 salt is placed in it. Vibrational analysis showed that the bands belong to the same system (electronic transition) with a 0-0 band at about 6000 Å, and the vibrational constants of the upper and lower electronic states were determined. The "orange" system also includes the bands in the range 7100 - 8400 Å, 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 was established. In the reference book [84XYU / 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) is performed in [80HOC / MER]. The bands were recorded with high resolution in the emission spectrum of the discharge and in the spectrum of laser excitation of CrO molecules in the flow of an inert carrier gas. The lower state of the system was confirmed as the ground state of the molecule (the laser excitation spectrum was obtained at a carrier gas temperature slightly below room temperature).

Another weaker system of CrO bands was found in the emission spectrum of the near infrared discharge [84CHE / ZYR]. The spectrum was obtained using a Fourier spectrometer. Rotational analysis of the 0-0 band located at about 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 found in the chemiluminescence spectrum of the reaction of chromium atoms with ozone [89DEV / GOL]. The bands of this system are also noted in the atlas [57GAT / JUN]. In [93BAR / HAJ], bands 0-0 and 1-1 were obtained with high resolution in the laser excitation spectrum. Rotational analysis was carried out, which showed that the system is formed by the transition 5 Δ - X 5 Π.

In the chemiluminescence spectrum [89DEV / GOL], a system of bands in the region of 4510 Å (ν 00 \u003d 22163 cm ‑1) was found; vibrational analysis was performed. The system probably belongs to an electronic charge transfer transition, since 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 anion and molecule is presented in [2002BAU / GUT]. According to the calculation, the anion has the ground state X 4 Π and the first excited state 6 Σ +. The spectra exhibit one-electron transitions from these states to the ground state and 5 excited states of a 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 CrO quintet states agree with the data of the optical spectra. 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 calculation [85BAU / NEL] showed and confirmed in subsequent calculations that the ground state of the molecule is 5 Å. 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 \u003d -1 of the state X 5 Π, as the ground state; b) other 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 of up to 40,000 cm -1.

Equilibrium constants for the X 5 Π CrO state were obtained in [80HOC / MER]. They are given in the table Cr.D1 as constants for the lower component X 5 Π –1, although they refer to the entire state as a whole. Differences in the values \u200b\u200bof ω e for the components of the state X 5 Π 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 2.5 3, A 5 Σ +), [93BAR / HAJ] ( A΄ 5 Δ), [80HOC / MER] (B 5), [89DEV / GOL] (C 5); interpretation of photoelectron spectra [2002BAU / GUT] (3 Σ -, 3 Π, 3 Φ); according to calculations [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 the thermodynamic functions and are given in the table Cr.D1 for reference. For states A 6 Σ +, A΄ 5 Δ, B 5 Π, C(5 Π) shows the spectroscopic constants according to the data of [84CHE / ZYR, 93BAR / HAJ, 80HOC / MER, 89DEV / GOL], respectively. For the states 3 Σ -, 3 Π, 3 Φ, the values \u200b\u200bof ω e obtained from the photoelectron spectrum of the anion in [96WEN / GUN] are given. The values \u200b\u200bof ω e for the 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 MRCI calculation [2002BAU / GUT].

The statistical weights of the synthetic states were 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 the data from [71MOO] on the positions of the terms of singly and doubly charged chromium ions. We also used the [2001GUT / JEN] estimates for the energies of the 7 Π, 7 Σ + states of the Cr + (3d 5) O - configuration.

The thermodynamic functions CrO (r) were calculated using equations (1.3) - (1.6), (1.9), (1.10), (1.93) - (1.95). The values Q ext and its derivatives were calculated using equations (1.90) - (1.92) taking into account nineteen excited states under the assumption that Q count.vr ( i) = (p i / p X) Q count.vr ( X). The vibrational-rotational partition function of the state X 5 Π -1 and its derivatives were calculated by equations (1.70) - (1.75) by direct summation over vibrational levels and integration over rotational energy levels using an equation of the type (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 \u200b\u200bof 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 the table Cr.D1. Coefficient values Y kl, as well as the quantities v max and J lim are given in Table Cr D2.

The following values \u200b\u200bare obtained at room temperature:

C p o (298.15 K) \u003d 32.645 ± 0.26 J × K ‑1 × mol ‑1

S o (298.15 K) \u003d 238.481 ± 0.023 J × K ‑1 × mol ‑1

H o (298.15 K) - H o (0) \u003d 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 excited electronic states. Errors in the values \u200b\u200bof Φº ( T) at T \u003d298.15, 1000, 3000, and 6000 K are estimated at 0.02, 0.04, 0.2, and 0.4 J × K ‑1 × mol ‑1, respectively.

Previously, the thermodynamic functions CrO (r) were calculated for the tables JANAF [85CHA / DAV], Schneider [74SCH] (T \u003d 1000 - 9000 K), Brewer and Rosenblatt [69BRE / ROS] (Φº ( T) for T ≤ 3000 K). The discrepancies between the JANAF tables and tab. CrO at low temperatures 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 \u200b\u200bof Φº (298.15) is 4.2 J × K ‑1 × mol ‑1. In the region of 1000 - 3000 K, the discrepancies in the values \u200b\u200bof 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 [

The discovery of chromium refers to the period of rapid development of chemical and analytical research of salts and minerals. In Russia, chemists have shown particular 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, Vauckelin, having boiled a finely ground sample of the mineral with potash and precipitated lead carbonate, received 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. Vaukelen named him Chromium (Chrome ) from the Greek word- coloring, color; the truth here was not the property of the metal, but of its brightly colored salts.

Being in nature.

The most important chromium ore of practical importance is chromite, the approximate composition of which corresponds to the formula FeCrO \u200b\u200b4.

It is found in Asia Minor, in the Urals, in North America, in southern Africa. The aforementioned crocoite mineral, PbCrO 4, is also of technical importance. Chromium (3) oxide 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 in the Sun, stars, meteorites.

Physical properties.

Chromium is a white, hard and brittle metal, extremely chemically resistant to acids and alkalis. It oxidizes in air and has a thin transparent oxide film on its surface. Chromium has a density of 7.1 g / cm 3, its melting point is +1875 0 С.

Receiving.

With strong heating of chromium iron ore with coal, chromium and iron are reduced:

FeO * Cr 2 O 3 + 4C \u003d 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

This process usually uses two oxides - Cr 2 O 3 and CrO 3

Chemical properties.

Thanks to the thin protective oxide film covering the chromium surface, 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 reacts with alkalis at t \u003d 600-700 ° 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 \u003d 2CrCl 3 + 3H 2

At high temperatures, chromium burns in oxygen, forming oxide (III).

Hot chrome reacts with water vapor:

2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2

Chromium at high temperatures also reacts with halogens, halogen - with hydrogen, sulfur, nitrogen, phosphorus, coal, silicon, boron, for example:

Cr + 2HF \u003d CrF 2 + H 2
2Cr + N2 \u003d 2CrN
2Cr + 3S \u003d Cr 2 S 3
Cr + Si \u003d CrSi

The above physical and chemical properties of chromium have found their application in various fields of science and technology. So, for example, chromium and its alloys are used to obtain high-strength, corrosion-resistant coatings in mechanical engineering. Ferrochrome alloys are used as metal cutting tools. Chromium-plated alloys have found application in medical technology, in the manufacture of chemical processing equipment.

The position of chromium in the periodic table of chemical elements:

Chromium heads the subgroup VI of group of the periodic table 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 first be filled up to the 4S 2 state. However, due to the fact that the 3d - orbital occupies a more favorable energy position in the chromium atom, it is filled up to a value of 4d 5. This phenomenon is observed in the atoms of some other elements of 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 is a pyrophoric black powder (pyrophoricity is the ability to ignite in air in a finely divided state). CrO dissolves in dilute hydrochloric acid:

CrO + 2HCl \u003d CrCl 2 + H 2 O

In air, when heated above 100 0 С, CrO turns into Cr 2 O 3.

Divalent chromium salts are formed by dissolving metallic chromium in acids. These reactions take place in an atmosphere of low-activity 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 \u003d Cr (OH) 2 + 2NaCl

Cr (OH) 2 has basic properties and is a reducing agent. The hydrated Cr2 + ion is pale blue. An aqueous solution of CrCl 2 is blue in color. In air, in aqueous solutions, Cr (II) compounds are converted to Cr (III) compounds. This is especially pronounced for Cr (II) hydroxide:

4Cr (OH) 2 + 2H 2 O + O 2 \u003d 4Cr (OH) 3

Trivalent chromium compounds.

Chromium (III) oxide Cr 2 O 3 is a refractory green powder. Hardness is close to corundum. 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 fusion 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 \u003d CrCl 3 + 3H 2 O
Cr (OH) 3 + NaOH \u003d 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 (III) sulfate Cr 2 (SO 4) 3 pink, also poorly soluble in water. In the presence of reducing agents, it forms violet chromium sulfate Cr 2 (SO 4) 3 * 18H 2 O. Green chromium sulfate hydrates are also known, containing less water. Chromium alum KCr (SO 4) 2 * 12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. Chromium alum solution 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 most likely conclude that there is no chromium.

Strongly heat in a burner flame on a porcelain cup such an amount of potassium dichromate that will fit on the tip of a knife. The salt will not release crystallization water, but will melt at a temperature of about 400 0 С with the formation of a dark liquid. We will heat it for a few more minutes on a strong flame. After cooling, a green precipitate forms on the shard. We will dissolve part of it in water (it becomes yellow), and leave the other part on the shard. The salt decomposed on heating, resulting in the formation of a soluble yellow potassium chromate K 2 CrO 4 and green Cr 2 O 3.
Dissolve 3g of powdered potassium dichromate in 50ml of water. Add some potassium carbonate to one part. It will dissolve with the evolution of CO 2, and the color of the solution will turn light yellow. Chromate is formed from potassium dichromate. If you now add a 50% solution of sulfuric acid in portions, then the red-yellow color of the dichromate will again appear.
Pour 5 ml into a test tube. potassium dichromate solution, boil with 3 ml of concentrated hydrochloric acid under draft. Yellow-green toxic 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 chloride of trivalent chromium. It can be isolated by evaporation of the solution, and then, melted with soda and nitrate, converted into 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 the potassium dichromate solution and acidify the solution with sulfuric acid. The solution takes on a deep blue color due to the formation of chromium peroxide. The peroxide, when shaken with a certain amount of ether, will go into the organic solvent and color it blue. This reaction is specific for chromium and very sensitive. It can detect chromium in metals and alloys. The first step is to dissolve the metal. During 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. In order to be able to carry out the detection reaction, we first neutralize it with caustic soda. A gray-green chromium (III) hydroxide precipitates, which will dissolve in an excess of NaOH and form green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution turns 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.

Add a 2M NaOH solution to 3-4 drops of a solution of chromium chloride CrCl 3 until the initially precipitated precipitate is dissolved. Note the color of the resulting sodium chromite. Heat the resulting solution in a water bath. What happens then?
Add an equal volume of 8M NaOH solution and 3-4 drops of 3% H 2 O 2 solution to 2-3 drops of CrCl 3 solution. Heat the reaction mixture in a water bath. What happens then? What precipitate is formed if the resulting colored solution is neutralized, add CH 3 COOH 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 mixture for a few minutes in a water bath. Note the color change in the solution. What caused it?
Add 2-3 drops of H 2 O 2 solution to 3-4 drops of K 2 Cr 2 O 7 solution acidified with nitric acid and mix. The appearing blue coloration of the solution is due to the appearance of perchromic acid H 2 CrO 6:

Cr 2 O 7 2- + 4H 2 O 2 + 2H + \u003d 2H 2 CrO 6 + 3H 2 O

Pay attention to the fast decomposition of H 2 CrO 6:

2H 2 CrO 6 + 8H + \u003d 2Cr 3+ + 3O 2 + 6H 2 O
blue green

Perchromic acid is significantly more stable in organic solvents.

Add 5 drops of isoamyl alcohol, 2-3 drops of H 2 O 2 solution to 3-4 drops of a solution of K 2 Cr 2 O 7 acidified with nitric acid and shake the reaction mixture. The organic solvent layer 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.
During the interaction of CrO 4 2- and Ba 2+ ions, a yellow precipitate of barium chromate BaCrO 4 precipitates.
Silver nitrate forms with ions CrO 4 2- a brick-red silver chromate precipitate.
Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution in one of them, in the second - the same volume of K 2 CrO 4 solution, and in the third - three drops of both solutions. Then add three drops of potassium iodide solution to each tube. Explain your result. Acidify the solution in the second tube. What happens then? 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. Heating 2 mg of glycerin with a small amount of (NH 4) 2 Cr 2 O 7, followed by the addition of alcohol, after filtration, a bright green solution is obtained, which, when acid is added, turns yellow, and turns green in a neutral or alkaline medium.
Place in the center of a can with a termite "ruby mixture" - thoroughly crushed and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). To keep the jar from cooling down longer, it is necessary to bury it under the upper edge in sand, and after setting fire to the termite and the start of the reaction, cover it with an iron sheet and cover it with sand. Dig out the jar in a day. As a result, a ruby-red powder is formed.
10 g of potassium dichromate 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 push out the stick and set fire to it from the end, then a "snake" will begin to creep out, first black, and after cooling - green. A rod 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, an amorphous brown precipitate of the composition 4CuCrO 4 * 3NH 3 * 5H 2 O will precipitate, which dissolves in hydrochloric acid with the formation of a yellow solution, and in an 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 under strong light.
The chromium oxide remaining after the "volcano" or "pharaoh's snake" experiments can be regenerated. To do this, it is necessary to melt 8 g Cr 2 O 3 and 2 g Na 2 CO 3 and 2.5 g KNO 3 and process the cooled alloy with boiling water. A soluble chromate is obtained, which can 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 \u003d 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 \u003d 6NaBr + 2Na 2 CrO 4 + 4H 2 O
d) 2Na 2 CrO 4 + 2HCl \u003d 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 \u003d 2Cr (OH) 3
b) Cr (OH) 3 + 3HCl \u003d CrCl 3 + 3H 2 O
c) 2CrCl 3 + 2KMnO 4 + 3H 2 O \u003d K 2 Cr 2 O 7 + 2Mn (OH) 2 + 6HCl
d) K 2 Cr 2 O 7 + 2KOH \u003d 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
Cr 2+

a) CrO + 2HCl \u003d CrCl 2 + H 2 O
b) CrO + H 2 O \u003d Cr (OH) 2
c) Cr (OH) 2 + 1 / 2O 2 + H 2 O \u003d 2Cr (OH) 3
d) Cr (OH) 3 + 3HNO 3 \u003d 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 \u003d 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 XVIII-XIX centuries, a technology was developed for obtaining many painting materials. Louis Nicolas Vauquelin in 1797, who discovered a previously unknown element chromium in Siberian red ore, prepared a new, remarkably stable paint - chrome green. Its chromophore is hydrous chromium (III) oxide. It was launched under the name "emerald green" in 1837. Later L. Vauquelen proposed several new paints: barite, zinc and chrome yellow. Over time, they were supplanted by the more persistent yellow, orange cadmium-based pigments.

Chrome green is the strongest and most lightfast paint, resistant to atmospheric gases. Chromium greens ground in oil has great covering 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 reflowing 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 chrome. Black oxide of cobalt, applied to the surface of the porcelain, is fused with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. Such blue porcelain tableware, decarred with cobalt, is well known to all. Chromium (III) oxide does not react chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze with a "dull" layer.

In addition to chrome green, artists use paints obtained from wolkonskoite. 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, 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). The variability of the composition of the mineral, found in the Urals, in the Perm and Kirov regions, determines its varied color - from the color of a darkened winter fir to the bright green color of a marsh frog.

Pablo Picasso asked the geologists of our country to study the reserves of wolkonskoite, which gives the paint a uniquely fresh tone. Currently, a method has been developed for producing artificial wolkonskoite. It is interesting to note that according to modern research, Russian icon painters used paints from this material in the Middle Ages, long before its "official" discovery. Guinier greens (created in 1837) were also popular among artists, the chromoform of which is chromium oxide hydrate Cr 2 O 3 * (2-3) H 2 O, where part of the water is chemically bound, and part is adsorbed. This pigment gives the paint an emerald hue.

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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 sets new challenges for it. Chemical elements, discovered several hundred years ago and studied in all manifestations, are being used in more technological directions in the modern world. This trend extends to compounds existing in nature and 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, characteristics. One of the most common types of compounds is oxide (oxide, oxide). It includes sand, water, carbon dioxide, that is, the fundamental substances for the existence of mankind and the entire biosphere of the Earth. Oxides are substances that contain oxygen atoms with an oxidation state of -2, while the bond between the elements is binary. Their formation occurs as a result of a chemical reaction, the conditions of which differ 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 an 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 into the oxidative reaction.

According to the accepted classification, oxides are basic and acidic. An amphoteric species also stands out, which exhibits the properties of a basic oxide. Acid oxides are compounds of non-metals or elements with high valence, acids are their hydrates. The basic oxides include all substances with an oxygen + metal bond; bases are their hydrates.

Chromium

In the 18th century, chemist I.G. Lehmann discovered an unknown mineral that was named Siberian red lead. Professor of the Paris Mineralogical School Vauquelin carried out a series of chemical reactions with the obtained 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 from the wide range of colors, which is characterized by compounds of the element. The metal is quite inert, it does not occur in its pure form in natural conditions.

The main minerals containing chromium are chromite (FeCr 2 O 4), melanochroite, vokelenite, ditzeite, tarapakaite. 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 valency +2, +3, +6, with the most stable compounds of the trivalent metal. Reactions are possible in which the oxidation state is +1, +5, +4. Chromium is chemically inactive; the metal surface is covered with a film (passivation effect), which prevents reactions with oxygen and water under normal conditions. Chromium oxide, which forms on the surface, prevents the metal from interacting with acids and halogens in the absence of catalysts. Compounds with simple substances (not metals) are possible at temperatures from 300 ° C (chlorine, bromine, sulfur).

When interacting with complex substances, additional conditions are required, for example, the reaction does not occur with an alkali solution, the process proceeds very slowly with its melts. Chromium reacts with acids in the presence of high temperature as a catalyst. Chromium oxide can be obtained from various minerals by exposure to temperature. Concentrated acids are used depending on the future oxidation state of the element. In this case, the valency of chromium in the compound varies from +2 to +6 (the highest chromium oxide).

Application

Due to their unique anticorrosive 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 fragility, which reduces the possibilities of processing alloys. The most common use of metal is to make coatings (chrome plating). The protective film can be 0.005 mm thick, but it will reliably protect the metal product from corrosion and external influences. Chromium compounds are used for the manufacture of refractory structures in the metallurgical industry (melting furnaces). Anti-corrosion decorative coatings (cermets), special alloy steel, electrodes for welding machines, alloys based on silicon, 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 ° C).

Divalent compounds

Chromium oxide (2) 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), not toxic. In the process of heating to 100 ° 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.

Receiving, application

This substance is considered to be a lower oxide. Has a fairly narrow scope. In the chemical industry, chromium oxide 2 is used to purify hydrocarbons from oxygen, which it attracts during oxidation at temperatures above 100 ° C. You can get bivalent chromium oxide 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 with oxygen or nitric acid.

Trivalent compounds

For chromium oxides, the oxidation state +3 is the most stable form of the substance. Cr 2 O 3 (chrome green, sesquioxide, eskolaid) is chemically inert, insoluble in water, has a high melting point (over 2000 o C). Chromium oxide 3 is a green refractory powder, very hard, has amphoteric properties. The substance is soluble in concentrated acids, the reaction with alkalis occurs as a result of fusion. Can be reduced to pure metal by interaction with a strong reducing agent.

Receiving and using

Due to its high hardness (comparable to corundum), the most common use of the substance in abrasive and polishing materials. Chromium oxide (formula Cr 2 O 3) has a green color, therefore it is used as a pigment in the manufacture of glasses, paints, 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, several types of chemical reactions are used:

Oxidation of chromium oxide.
Heating (calcining) dichromate or ammonium chromate.
Decomposition of trivalent chromium hydroxide or hexavalent oxide.
Calcination of chromate or dichromate of mercury.

Hexavalent compounds

The highest chromium oxide formula is CrO 3. A substance of violet or dark red color, 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 - well soluble in water, under normal conditions interacts with air (spreads), melting point - 196 o C. The substance has pronounced acidic characteristics. In a chemical reaction with water, dichromic or chromic acid is formed; without additional catalysts, it interacts with alkalis (yellow chromates). For halogens (iodine, sulfur, phosphorus) it is a strong oxidizing agent. As a result of heating above 250 ° C, free oxygen and trivalent chromium oxide are formed.

How is it obtained and where it is used

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 application:

Obtaining a pure metal - chromium.
In the process of chrome plating of surfaces, including the electrolytic method.
Oxidation of alcohols (organic compounds) in the chemical industry.
In rocketry it is used as a fuel igniter.
In chemical laboratories, it cleans dishes from organic compounds.
Used in the pyrotechnic industry.