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
Chromium (II) hydroxide Cr(OH) 2 is obtained in the form of a yellow precipitate by treating solutions of chromium (II) salts with alkalis in the absence of oxygen:
CrСl 2 +2NaOH=Cr(OH) 2 ¯+2NaCl
Cr(OH) 2 has typical basic properties and is a strong reducing agent:
2Cr(OH) 2 +H 2 O+1/2O 2 =2Cr(OH) 3 ¯
Aqueous solutions of chromium (II) salts are obtained without access to air by dissolving chromium metal in dilute acids in a hydrogen atmosphere or by reducing trivalent chromium salts with zinc in an acidic environment. Anhydrous salts of chromium (II) are white, and aqueous solutions and crystalline hydrates are blue.
In their chemical properties, chromium (II) salts are similar to divalent iron salts, but differ from the latter in more pronounced reducing properties, i.e. are more easily oxidized than the corresponding ferrous iron compounds. This is why it is very difficult to obtain and store divalent chromium compounds.
Chromium (III) hydroxide Cr(OH) 3 is a gelatinous precipitate of gray-green color, it is obtained by the action of alkalis on solutions of chromium (III) salts:
Cr 2 (SO 4) 3 +6NaOH=2Cr(OH) 3 ¯+3Na 2 SO 4
Chromium (III) hydroxide has amphoteric properties, dissolving both in acids to form chromium (III) salts:
2Cr(OH) 3 +3H 2 SO 4 =Cr 2 (SO 4) 3 +6H 2 O and in alkalis with the formation of hydroxychromites: Cr(OH) 3 +NaOH=Na 3
When Cr(OH) 3 is fused with alkalis, metachromites and orthochromites are formed:
Cr(OH) 3 +NaOH=NaCrO 2 +2H 2 O Cr(OH) 3 +3NaOH=Na 3 CrO 3 +3H 2 O
When chromium (III) hydroxide is calcined, chromium (III) oxide is formed:
2Cr(OH) 3 =Cr 2 O 3 +3H 2 O
Salts of trivalent chromium, both in the solid state and in aqueous solutions, are colored. For example, anhydrous chromium (III) sulfate Cr 2 (SO 4) 3 is violet-red in color; aqueous solutions of chromium (III) sulfate, depending on conditions, can change color from purple to green. This is explained by the fact that in aqueous solutions the Cr 3+ cation exists only in the form of a hydrated 3+ ion due to the tendency of trivalent chromium to form complex compounds. The purple color of aqueous solutions of chromium (III) salts is due precisely to the 3+ cation. When heated, chromium(III) complex salts can
partially lose water, forming salts of various colors, even green.
Trivalent chromium salts are similar to aluminum salts in composition, crystal lattice structure, and solubility; Thus, for chromium (III), as well as for aluminum, the formation of chromium-potassium alum KCr(SO 4) 2 12H 2 O is typical; they are used for tanning leather and as a mordant in textiles.
Chromium salts (III) Cr 2 (SO 4) 3, CrCl 3, etc. stable when stored in air, but subject to hydrolysis in solutions:
Cr 3+ +3Сl - +НОН «Cr(ОН) 2+ +3Сl - +Н +
Hydrolysis occurs at stage I, but there are salts that are completely hydrolyzed:
Cr 2 S 3 +H 2 O=Cr(OH) 3 ¯+H 2 S
In redox reactions in an alkaline environment, chromium (III) salts behave as reducing agents:
It should be noted that in the series of chromium hydroxides of various oxidation states Cr(OH) 2 - Cr(OH) 3 - H 2 CrO 4, the basic properties are naturally weakened and the acidic properties are strengthened. This change in properties is due to an increase in the degree of oxidation and a decrease in the ionic radii of chromium. In the same series, the oxidizing properties are consistently enhanced. Cr(II) compounds are strong reducing agents and are easily oxidized, turning into chromium(III) compounds. Chromium(VI) compounds are strong oxidizing agents and are easily reduced to chromium(III) compounds. Compounds with an intermediate oxidation state, i.e. chromium (III) compounds can, when interacting with strong reducing agents, exhibit oxidizing properties, turning into chromium (II) compounds, and when interacting with strong oxidizing agents, exhibit reducing properties, turning into chromium (VI) compounds.
Chromium hydride
CrH(g). The thermodynamic properties of gaseous chromium hydride in the standard state at temperatures of 100 - 6000 K are given in table. CrH.
In addition to the band 3600 – 3700Å, another weaker band of CrH was detected in the ultraviolet region of the spectrum [55KLE/LIL, 73SMI]. The band lies in the region of 3290 Å and has edges of a complex structure. The band has not yet been analyzed.
The most studied is the infrared system of CrH bands. The system corresponds to the transition A 6 Σ + - X 6 Σ + , the edge of the 0-0 band is located at 8611 Å. This system was studied in [55KLE/LIL, 59KLE/UHL, 67O’C, 93RAM/JAR2, 95RAM/BER2, 2001BAU/RAM, 2005SHI/BRU, 2006CHO/MER, 2007CHE/STE, 2007CHE/BAK]. In [55KLE/LIL], an analysis of the oscillatory structure along the edges was performed. In [59KLE/UHL], the rotational structure of the 0-0 and 0-1 bands was analyzed and the type of transition 6 Σ - 6 Σ was established. In [ 67O’C ], rotational analysis of the 1-0 and 1-1 bands, as well as rotational analysis of the 0-0 band of CrD, were performed. In [93RAM/JAR2], in higher resolution spectra obtained using a Fourier spectrometer, the positions of the lines of the 0-0 band were refined, and more accurate values of the rotational constants and fine structure constants of the upper and lower states were obtained. Analysis of perturbations in the A 6 Σ + state showed that the perturbing state is a 4 Σ + with energy T 00 = 11186 cm -1 and rotational constant B 0 = 6.10 cm -1 . In [95RAM/BER2] and [2001BAU/RAM] the rotational structure of the bands 0-1, 0-0, 1-0 and 1-2 of the CrD molecule [95RAM/BER2] and 1-0 and 1 was obtained and analyzed using a Fourier spectrometer -1 molecule CrH [ 2001BAU/RAM ]. In [2005SHI/BRU], the lifetimes of the v = 0 and 1 levels of the A 6 Σ + state were determined by the method of resonant two-photon ionization, and the wave numbers of the lines of the 0-0 band of the 50 CrH isotopomer were measured. In [2006CHO/MER], the wave numbers of the first lines (N ≤ 7) of the 1-0 CrH band were measured in the laser excitation spectrum. The observed perturbations of the rotational levels of the state A 6 Σ + (v=1) are attributed to the states a 4 Σ + (v=1) and B 6 Π(v=0). In [2007CHE/STE], the shifts and splitting in a constant electric field of the first few lines of the 0-0 CrD band were measured in the laser excitation spectra, and the dipole moment in the states X 6 Σ + (v=0) and A 6 Σ + (v=0) was determined ). In [2007CHE/BAK], the Zeeman splitting of the first rotational lines of the 0-0 and 1-0 CrH bands was studied in laser excitation spectra. The infrared CrH system has been identified in the spectra of the sun [ 80ENG/WOH ], S-type stars [ 80LIN/OLO ] and brown dwarfs [ 99KIR/ALL ].
Vibrational transitions in the ground electronic state of CrH and CrD were observed in [79VAN/DEV, 91LIP/BAC, 2003WAN/AND2]. In [79VAN/DEV], absorption frequencies of 1548 and 1112 cm -1 in the Ar matrix at 4 K are assigned to CrH and CrD molecules. In [91LIP/BAC], the rotational lines of vibrational transitions 1-0 and 2-1 of the CrH molecule were measured using laser magnetic resonance, and the vibrational constants of the ground state were obtained. In [2003WAN/AND2], CrH and CrD molecules, taking into account the data from [91LIP/BAC], are assigned absorption frequencies in the Ar matrix of 1603.3 and 1158.7 cm‑1.
Rotational transitions in the ground state of CrH and CrD were observed in [91COR/BRO, 93BRO/BEA, 2004HAL/ZIU, 2006HAR/BRO]. In [91COR/BRO], about 500 laser magnetic resonances associated with 5 lower rotational transitions were measured, and a set of parameters was obtained that describe the rotational energy, fine and hyperfine splitting of rotational levels in the vibrational level v=0 of the ground state. The work [93BRO/BEA] presents the refined frequencies of the 6 components of the rotational transition N = 1←0. In [2004HAL/ZIU], the components of the N = 1←0 CrH transition and the components of the N = 2←1 CrD transition were measured directly in the submillimeter absorption spectrum. The components of the N = 1←0 CrH transition were remeasured (with a better signal-to-noise ratio) in [2006HAR/BRO]. The data from these measurements were processed in [2006HAR/BRO] together with the measurement data from [91COR/BRO] and [91LIP/BAC], and the currently best set of constants, including equilibrium ones, for the ground state of CrH was obtained.
The EPR spectrum of a CrH molecule in an Ar matrix was studied in [79VAN/DEV, 85VAN/BAU]. It was established that the molecule has a ground state of 6 Σ.
The photoelectron spectrum of the CrH - and CrD - anions was obtained in [87MIL/FEI]. According to the authors' interpretation, the spectrum shows transitions from the ground and excited states of the anion to the ground and A 6 Σ + states of the neutral molecule. Several peaks in the spectrum were not assigned. The vibrational frequency in the ground state of CrD was determined to be ~ 1240 cm -1.
Quantum mechanical calculations of CrH were performed in [81DAS, 82GRO/WAH, 83WAL/BAU, 86CHO/LAN, 93DAI/BAL, 96FUJ/IWA, 97BAR/ADA, 2001BAU/RAM, 2003ROO, 2004GHI/ROO, 2006FUR/PER, 2006K OS /MAT, 2007JEN/ROO, 2008GOE/MAS ]. The energies of excited electronic states were calculated in [93DAI/BAL, 2001BAU/RAM, 2003ROO, 2004GHI/ROO, 2006KOS/MAT, 2008GOE/MAS].
The energies of excited states are given according to experimental data [93RAM/JAR2] ( a 4 Σ +), [ 2001BAU/RAM ] ( A 6 Σ +), [ 2006CHO/MER ] ( B 6 Π), [ 84HUGH/GER ] ( D(6 Π)) and estimated from the calculation results [93DAI/BAL, 2006KOS/MAT] ( b 4 Π, c 4 Δ), [ 93DAI/BAL, 2003ROO, 2004GHI/ROO, 2006KOS/MAT ] ( C 6 Δ).
The vibrational and rotational constants of the excited states of CrH were not used in the calculations of thermodynamic functions and are given in Table Cr.D1 for reference. For state A 6 Σ + experimental constants are given [2001BAU/RAM], rotational constant a 4 Σ + is given according to [93RAM/JAR2]. For other states the values of w e and r e averaged according to the calculation results [93DAI/BAL] ( B 6 Π, C 6Δ, b 4 Π, c 4 Δ), [2003ROO] ( C 6 Δ), [ 2004GHI/ROO ] ( B 6 Π, C 6Δ, D(6 Π)), [ 2006KOS/MAT ] ( B 6 Π, C 6 Δ).
The statistical weights of the synthetic states are estimated using the Cr + H - ion model. They combine statistical term weights of the Cr + ion with estimated energies in the ligand field below 40,000 cm -1 . The energies of terms in the ligand field were estimated based on the assumption that the relative positions of terms of one configuration are the same in the ligand field and in the free ion. The shift of configurations of a free ion in the ligand field was determined based on the interpretation (within the framework of the ionic model) of the experimentally observed and calculated electronic states of the molecule. Thus, the ground state X 6 Σ + is put in correspondence with the term 6 S of the configuration 3d 5, and the states A 6 Σ +, B 6 Π, C 6 Δ and 4 Σ +, 4 Π, 4 Δ – with the components of the splitting of the terms 6 D and 4 D configuration 4s 1 3d 4. The D(6 Π) state is assigned to the 4p 1 3d 4 configuration. The energies of terms in a free ion are given in [71MOO]. The splitting of terms in the ligand field was not taken into account.
Thermodynamic functions CrH(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 eleven 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 X 6 Σ + state and its derivatives were calculated using the equations K ‑1 × mol ‑1
H o (298.15 K)- H o (0) = 8.670 ± 0.021 kJ× mol ‑1
The main errors in the calculated thermodynamic functions of CrH(g) are due to the calculation method. Errors in the values of Φº(T) at T = 298.15, 1000, 3000 and 6000 K are estimated at 0.07, 0.2, 0.7 and 1.7 J × K ‑1 × mol ‑1, respectively.
Thermodynamic functions of CrH(g) have not been previously published.
Thermochemical values for CrH(g).
The equilibrium constant of the reaction CrH(g)=Cr(g)+H(g) was calculated from the accepted value of the dissociation energy
D° 0 (CrH) = 184 ± 10 kJ× mol ‑1 = 15380 ± 840 cm -1.
The accepted value is based on the results of measurements of the energies of two gas heterolytic reactions, namely: CrH = Cr - + H + (1), ΔE(1) = 1420 ± 13 kJ × mol -1, ion cyclotron resonance method [85SAL/LAN] and CrH = Cr + + H - (2), ΔE(2) = 767.1 ± 6.8 kJ× mol -1, determination of threshold energies for reactions of interaction of Cr + with a number of amines [93CHE/CLE]. The combination of these values with the values accepted in this publication EA(H) = -72.770 ± 0.002 kJ× mol -1, IP(H) = 1312.049 ± 0.001 kJ× mol -1, IP(Cr) = 652.869 ± 0.004 kJ× mol - 1, as well as with the value EA(Cr) = -64.3 ± 1.2 kJ× mol -1 given in [85HOT/LIN] leads to the values D° 0 (CrН) = 172.3 ± 13 and D° 0 (CrН) = 187.0 ± 7 kJ × mol -1 for works [85SAL/LAN, 93CHE/CLE], respectively. The obtained values are in reasonable agreement; the weighted average is 184 ± 6 kJ× mol‑1. This meaning is adopted in this publication. The error is slightly increased due to the difficulties of reliably attributing the results of cited works to a specific temperature. An attempt to detect the CrH molecule under equilibrium conditions (Knudsen mass spectrometry, [81KAN/MOO]) was unsuccessful; ratio given in [81KAN/MOO] D° 0 (CrН) ≤ 188 kJ× mol‑1 does not contradict the recommendation.
The accepted value corresponds to the following values:
Δf Hº(CrH, g, 0 K) = 426.388 ± 10.2 kJ mol -1 and
Δf Hº(CrH, g, 298.15 K) = 426.774 ± 10.2 kJ mol -1 .
- Designation - Cr (Chromium);
- Period - IV;
- Group - 6 (VIb);
- Atomic mass - 51.9961;
- Atomic number - 24;
- Atomic radius = 130 pm;
- Covalent radius = 118 pm;
- Electron distribution - 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 ;
- melting temperature = 1857°C;
- boiling point = 2672°C;
- Electronegativity (according to Pauling/according to Alpred and Rochow) = 1.66/1.56;
- Oxidation state: +6, +3, +2, 0;
- Density (no.) = 7.19 g/cm3;
- Molar volume = 7.23 cm 3 /mol.
Chromium (color, paint) was first found at the Berezovsky gold deposit (Middle Urals), the first mentions date back to 1763; in his work “The First Foundations of Metallurgy” M.V. Lomonosov calls it “red lead ore”.
Rice. Structure of the chromium atom.
The electronic configuration of the chromium atom is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 (see Electronic structure of atoms). In the formation of chemical bonds with other elements, 1 electron located on the outer 4s level + 5 electrons of the 3d sublevel (6 electrons in total) can participate, therefore, in compounds, chromium can take oxidation states from +6 to +1 (the most common are +6 , +3, +2). Chromium is a chemically inactive metal; it reacts with simple substances only at high temperatures.
Physical properties of chromium:
- bluish-white metal;
- very hard metal (in the presence of impurities);
- fragile when n. y.;
- plastic (in its pure form).
Chemical properties of chromium
- at t=300°C reacts with oxygen:
4Cr + 3O 2 = 2Cr 2 O 3; - at t>300°C reacts with halogens, forming mixtures of halides;
- at t>400°C reacts with sulfur to form sulfides:
Cr + S = CrS; - at t=1000°C finely ground chromium reacts with nitrogen, forming chromium nitride (a semiconductor with high chemical stability):
2Cr + N 2 = 2CrN; - reacts with dilute hydrochloric and sulfuric acids to release hydrogen:
Cr + 2HCl = CrCl 2 + H 2;
Cr + H 2 SO 4 = CrSO 4 + H 2; - warm concentrated nitric and sulfuric acids dissolve chromium.
With concentrated sulfuric and nitric acid at no. chromium does not react, and chromium also does not dissolve in aqua regia; it is noteworthy that pure chromium does not react even with dilute sulfuric acid; the reason for this phenomenon has not yet been established. During long-term storage in concentrated nitric acid, chromium becomes covered with a very dense oxide film (passivates) and stops reacting with dilute acids.
Chromium compounds
It was already said above that the “favorite” oxidation states of chromium are +2 (CrO, Cr(OH) 2), +3 (Cr 2 O 3, Cr(OH) 3), +6 (CrO 3, H 2 CrO 4 ).
Chrome is chromophore, i.e., an element that gives color to the substance in which it is contained. For example, in the oxidation state +3, chromium gives a purple-red or green color (ruby, spinel, emerald, garnet); in the oxidation state +6 - yellow-orange color (crocoite).
In addition to chromium, chromophores also include iron, nickel, titanium, vanadium, manganese, cobalt, copper - all these are d-elements.
The color of common compounds that include chromium:
- chromium in oxidation state +2:
- chromium oxide CrO - red;
- chromium fluoride CrF 2 - blue-green;
- chromium chloride CrCl 2 - has no color;
- chromium bromide CrBr 2 - has no color;
- Chromium iodide CrI 2 - red-brown.
- chromium in oxidation state +3:
- Cr 2 O 3 - green;
- CrF 3 - light green;
- CrCl 3 - violet-red;
- CrBr 3 - dark green;
- CrI 3 - black.
- chromium in oxidation state +6:
- CrO 3 - red;
- potassium chromate K 2 CrO 4 - lemon yellow;
- ammonium chromate (NH 4) 2 CrO 4 - golden yellow;
- calcium chromate CaCrO 4 - yellow;
- Lead chromate PbCrO 4 - light brown-yellow.
Chromium oxides:
- Cr +2 O - basic oxide;
- Cr 2 +3 O 3 - amphoteric oxide;
- Cr +6 O 3 - acidic oxide.
Chromium hydroxides:
- ".
Application of chromium
- as a alloying additive in the smelting of heat-resistant and corrosion-resistant alloys;
- for chrome plating of metal products in order to give them high corrosion resistance, abrasion resistance and a beautiful appearance;
- chromium-30 and chromium-90 alloys are used in plasma torch nozzles and in the aviation industry.
Chromium (II) oxide CrO- pyrophoric black powder (pyrophoricity - the ability to ignite in air in a finely divided state). It is obtained by oxidizing chromium amalgam with atmospheric oxygen. Dissolves in dilute hydrochloric acid:
In air, when heated above 100° C, chromium (II) oxide turns into chromium (III) oxide.
Chromium (II) salts. In their chemical properties, Cr 2+ salts are similar to Fe 2+ salts. By treating their solutions with alkalis in the absence of oxygen, a yellow precipitate can be obtained chromium(II) hydroxide:
which has typical basic properties. Is a reducing agent. When Cr(OH) 2 is calcined in the absence of oxygen, chromium (II) oxide CrO is formed. When calcined in air it turns into Cr 2 O 3.
All chromium (II) compounds are quite unstable and are easily oxidized by atmospheric oxygen into chromium (III) compounds:
Chromium (III) salts. Trivalent chromium salts are similar to aluminum salts in composition, crystal lattice structure and solubility. In aqueous solutions, the Cr 3+ cation occurs only in the form of a hydrated ion [Cr(H 2 O) 6 ] 3+, which gives the solution a purple color (for simplicity, we write Cr 3+).
When alkalis act on chromium (III) salts, a gelatinous precipitate forms chromium (III) hydroxide - Cr(OH) 3 Green colour:
Chromium(III) hydroxide has amphoteric properties, dissolving both in acids to form chromium (III) salts:
and in alkalis with the formation of tetrahydroxychromites, i.e. salts in which Cr 3+ is part of the anion:
As a result of calcination of Cr(OH) 3, one can obtain chromium (III) oxide Cr 2 O 3 :
Chromium (III) oxide Cr 2 O 3- refractory green powder. It is close to corundum in hardness, which is why it is included in polishing agents. It is obtained by combining elements at high temperature.
Cr 2 O 3 is green crystals, practically insoluble in water. Cr 2 O 3 can also be obtained by calcination of potassium and ammonium dichromates:
When Cr 2 O 3 is fused with alkalis, soda and acid salts, Cr 3+ compounds are obtained that are soluble in water:
Chromium(VI) oxide- acid oxide, anhydride chromic H 2 CrO 4 and dichromic H 2 Cr 2 O 7 acids.
It is obtained by reacting concentrated sulfuric acid with a saturated solution of sodium or potassium dichromate:
CrO 3 is acidic in nature: it dissolves easily in water, forming chromic acids. With excess water it forms chromic acid H 2 CrO 4:
At a high concentration of CrO 3, dichromic acid H 2 Cr 2 O 7 is formed:
which, when diluted, turns into chromic acid:
Chromic acids exist only in aqueous solution. However, their salts are very stable.
CrO 3 is bright red crystals, easily soluble in water. Strong oxidizing agent: oxidizes iodine, sulfur, phosphorus, coal, turning into Cr 2 O 3. For example:
When heated to 250° C, it decomposes:
It reacts with alkalis to form yellow chromatesСrO 4 2-:
In an acidic environment, the CrO 4 2- ion turns into the Cr 2 O 7 2- ion.
In an alkaline environment, this reaction proceeds in the opposite direction:
IN acidic environment The dichromate ion is reduced to Cr 3+:
If we compare chromium hydroxides with different oxidation states
Cr 2+ (OH) 2, Cr 3+ (OH) 3 and H 2 Cr 6+ O 4, it is easy to conclude that As the degree of oxidation increases, the basic properties of hydroxides weaken and the acidic properties increase.
Cr(OH) 2 exhibits basic properties, Cr(OH) 3 - amphoteric, and H 2 CrO 4 - acidic.
Chromates and dichromates (VI). The most important chromium compounds in the highest oxidation state 6+ are potassium chromate (VI) K 2 CrO 4 and potassium dichromate (VI) K 2 Cr 2 O 7 .
Chromic acids form two series of salts: chromates, the so-called salts of chromic acid, and dichromates, the so-called salts of dichromic acid. Chromates are colored yellow (the color of the chromate ion CrO 4 2-), dichromates are colored orange (the color of the dichromate ion Cr 2 O 7 2-).
Dichromates Na 2 Cr 2 O 7 × 2H 2 O and K 2 Cr 2 O 7 are called chrome peaks. They are used as oxidizing agents in the leather (tanning of leather), paint and varnish, match and textile industries. Chromium mixture - the name given to a 3% solution of potassium dichromate in concentrated sulfuric acid - is used in chemical laboratories for washing glassware.
Salts of chromic acids in an acidic environment are strong oxidizing agents:
Chromium (III) compounds play the role of reducing agents in an alkaline environment. Under the influence of various oxidizing agents - Cl 2, Br 2, H 2 O 2, KmnO 4, etc. - they turn into chromium (IV) compounds - chromates:
Here the Cr(III) compound is depicted in the form of Na, since it exists in the form of Na + and - ions in an excess of alkali solution.
Strong oxidizing agents, such as KMnO 4, (NH 4) 2 S 2 O 8 in an acidic environment convert Cr (III) compounds into dichromates:
Thus, the oxidizing properties consistently increase with a change in oxidation states in the series: Cr 2+ ® Cr 3+ ® Cr 6+. Cr(II) compounds are strong reducing agents and are easily oxidized, turning into chromium compounds. (III). Chromium (VI) compounds are strong oxidizing agents and are easily reduced to chromium (III) compounds. Compounds with an intermediate oxidation state, i.e. chromium (III) compounds, can, when interacting with strong reducing agents, exhibit oxidizing properties, turning into chromium (II) compounds, and when interacting with strong oxidizing agents (for example, bromine, KMnO 4) exhibit reducing properties properties, turning into chromium (VI) compounds.
Chromium (III) salts are very diverse in color: purple, blue, green, brown, orange, red and black. All chromic acids and their salts, as well as chromium (VI) oxide, are toxic: they affect the skin, respiratory tract, and cause inflammation of the eyes, therefore, when working with them, all precautions must be taken.