Chromium sulfate refers to inorganic compounds with the chemical formula CrSO4·n H2O. Several related hydrated salts are known; the pentahydrate is a blue solid that dissolves in water. Solutions of chromium are oxidized by air to Cr species. Solutions of Cr are used as specialized reducing agents of value in organic synthesis; the salt is produced by treating chromium metal with aqueous sulfuric acid: Cr + H2SO4 + 5 H2O → CrSO4·5 H2O + H2It can be produced through the reaction of sulfate salts and chromium acetate or, for in situ use, the reduction of chromium sulfate with zinc. In aqueous solutions chromium sulfate forms metal aquo complexes with six water ligands; the structures of the crystalline salts are similar to the corresponding hydrates of copper sulfate: pentahydrate, trihydrate and anhydrous derivatives of chromous sulfate are known. In all of these compounds, the Cr centre adopts octahedral coordination geometry, being coordinated to six oxygen centers provided by a combination of water and sulfate ligands
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most simplest shapes found in crystals and minerals. There are three main varieties of these crystals: Primitive cubic Body-centered cubic, Face-centered cubic Each is subdivided into other variants listed below. Note that although the unit cell in these crystals is conventionally taken to be a cube, the primitive unit cell is not; the three Bravais lattices in the cubic crystal system are: The primitive cubic system consists of one lattice point on each corner of the cube. Each atom at a lattice point is shared between eight adjacent cubes, the unit cell therefore contains in total one atom; the body-centered cubic system has one lattice point in the center of the unit cell in addition to the eight corner points. It has a net total of 2 lattice points per unit cell; the face-centered cubic system has lattice points on the faces of the cube, that each gives one half contribution, in addition to the corner lattice points, giving a total of 4 lattice points per unit cell.
Each sphere in a cF lattice has coordination number 12. Coordination number is the number of nearest neighbours of a central atom in the structure; the face-centered cubic system is related to the hexagonal close packed system, where two systems differ only in the relative placements of their hexagonal layers. The plane of a face-centered cubic system is a hexagonal grid. Attempting to create a C-centered cubic crystal system would result in a simple tetragonal Bravais lattice; the isometric crystal system class names, point groups, examples, International Tables for Crystallography space group number, space groups are listed in the table below. There are a total 36 cubic space groups. Other terms for hexoctahedral are: normal class, ditesseral central class, galena type. A simple cubic unit cell has a single cubic void in the center. A body-centered cubic unit cell has six octahedral voids located at the center of each face of the unit cell, twelve further ones located at the midpoint of each edge of the same cell, for a total of six net octahedral voids.
Additionally, there are 24 tetrahedral voids located in a square spacing around each octahedral void, for a total of twelve net tetrahedral voids. These tetrahedral voids are not local maxima and are not technically voids, but they do appear in multi-atom unit cells. A face-centered cubic unit cell has eight tetrahedral voids located midway between each corner and the center of the unit cell, for a total of eight net tetrahedral voids. Additionally, there are twelve octahedral voids located at the midpoints of the edges of the unit cell as well as one octahedral hole in the center of the cell, for a total of four net octahedral voids. One important characteristic of a crystalline structure is its atomic packing factor; this is calculated by assuming that all the atoms are identical spheres, with a radius large enough that each sphere abuts on the next. The atomic packing factor is the proportion of space filled by these spheres. Assuming one atom per lattice point, in a primitive cubic lattice with cube side length a, the sphere radius would be a⁄2 and the atomic packing factor turns out to be about 0.524.
In a bcc lattice, the atomic packing factor is 0.680, in fcc it is 0.740. The fcc value is the highest theoretically possible value for any lattice, although there are other lattices which achieve the same value, such as hexagonal close packed and one version of tetrahedral bcc; as a rule, since atoms in a solid attract each other, the more packed arrangements of atoms tend to be more common. Accordingly, the primitive cubic structure, with low atomic packing factor, is rare in nature, but is found in polonium; the bcc and fcc, with their higher densities, are both quite common in nature. Examples of bcc include iron, chromium and niobium. Examples of fcc include aluminium, copper and silver. Compounds that consist of more than one element have crystal structures based on a cubic crystal system; some of the more common ones are listed here. The space group of the caesium chloride structure is called Pm3m, or "221"; the Strukturbericht designation is "B2". One structure is the "interpenetrating primitive cubic" structure called the "caesium chloride" structure.
Each of the two atom types forms a separate primitive cubic lattice, with an atom of one type at the center of each cube of the other type. Altogether, the arrangement of atoms is the same as body-centered cubic, but with alternating types of atoms at the different lattice sites. Alternately, one could view this lattice as a simple cubic structure with a secondary atom in its cubic void. In addition to caesium chloride itself, the structure appears in certain other alkali halides when prepared at low temperatures or high pressures; this structure is more to be formed from two elements whose ions are of the same size. The coordination
Chromyl chloride is a chemical compound with the formula CrO2Cl2. This compound is a hygroscopic dark red liquid; the molecule is tetrahedral, like most encountered chromium derivative chromate, 2−. In terms of physical properties and structure, it resembles SO2Cl2. Chromyl chloride can be prepared by mixing potassium chromate or potassium dichromate with sodium chloride and treating this mix with concentrated sulfuric acid, followed by gentle distillation. K2Cr2O7 + 4NaCl + 6H2SO4 → 2CrO2Cl2 + 2KHSO4 + 4NaHSO4 +3H2O It can be prepared directly by exposing chromium trioxide to anhydrous hydrogen chloride gas. CrO3 + 2HCl ⇌ CrO2Cl2 + H2O CrO2Cl2 is electrophilic and an aggressive oxidizing agent, e.g. causing spontaneous combustion when dripped onto amorphous sulfur. Its electrophilicity is demonstrated by its reversible hydrolysis to chromic acid and hydrochloric acid: CrO2Cl2 + 2H2O ⇌ H2CrO4 + 2HClIts high reactivity toward water is further indicated by the fact that CrO2Cl2 fumes in moist air.
The chromyl chloride test entails heating a sample suspected of containing chloride with potassium dichromate and concentrated sulfuric acid. If chloride is present, chromyl chloride is formed and red fumes of CrO2Cl2 are evident. If there is no chloride present, no red fumes are produced. No analogous compounds are formed with fluorides, bromides and cyanides, so this test is therefore specific for chlorides; the test is related to the synthesis shown above, exposure of CrO42− to HCl Depending on solvent, CrO2Cl2 oxidizes terminal alkenes to aldehydes. Internal alkenes give related derivatives, it will attack benzylic methyl groups to give aldehydes via the Étard reaction. Apart from this it can be used for testing the absence of nitrate ions. CrO2Cl2 is such an aggressive reagent. In light of its high reactivity toward water, CrO2Cl2 can be expected to decompose upon exposure to alcohols, similar to the behavior of other electrophilic chlorides such as VOCl3, TiCl4, SO2Cl2. Typical for other electrophilic chlorides, chlorocarbons are excellent solvents dichloromethane As a further practical complication, chromyl chloride attacks most greases.
CrO2Cl2 reacts with water to release hydrochloric acid and hexavalent chromium Acute: HCl can be acutely lethal. Exposure to chromyl chloride vapour irritates the respiratory system and irritates the eyes, the liquid burns the skin and eyes. Ingestion would cause severe internal damage. Chronic: CrVI can produce chromosomal aberrations and is a human carcinogen via inhalation. Frequent exposure of the skin to chromyl chloride may result in ulceration. Thus, CrO2Cl2 should be handled in a well ventilated area. CrO2Cl2 is so aggressive that its storage can be problematic as it attacks rubber and most plastics as well as greases. F. Freeman "Chromyl Chloride" in Encyclopedia of Reagents for Organic Synthesis 2004, J. Wiley & Sons, New York. Doi:10.1002/047084289. CDC - NIOSH Pocket Guide to Chemical Hazards - Chromyl Chloride
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter; the smallest group of particles in the material that constitutes this repeating pattern is the unit cell of the structure. The unit cell reflects the symmetry and structure of the entire crystal, built up by repetitive translation of the unit cell along its principal axes; the translation vectors define the nodes of the Bravais lattice. The lengths of the principal axes, or edges, of the unit cell and the angles between them are the lattice constants called lattice parameters or cell parameters; the symmetry properties of the crystal are described by the concept of space groups. All possible symmetric arrangements of particles in three-dimensional space may be described by the 230 space groups.
The crystal structure and symmetry play a critical role in determining many physical properties, such as cleavage, electronic band structure, optical transparency. Crystal structure is described in terms of the geometry of arrangement of particles in the unit cell; the unit cell is defined as the smallest repeating unit having the full symmetry of the crystal structure. The geometry of the unit cell is defined as a parallelepiped, providing six lattice parameters taken as the lengths of the cell edges and the angles between them; the positions of particles inside the unit cell are described by the fractional coordinates along the cell edges, measured from a reference point. It is only necessary to report the coordinates of a smallest asymmetric subset of particles; this group of particles may be chosen so that it occupies the smallest physical space, which means that not all particles need to be physically located inside the boundaries given by the lattice parameters. All other particles of the unit cell are generated by the symmetry operations that characterize the symmetry of the unit cell.
The collection of symmetry operations of the unit cell is expressed formally as the space group of the crystal structure. Vectors and planes in a crystal lattice are described by the three-value Miller index notation; this syntax uses the indices ℓ, m, n as directional orthogonal parameters, which are separated by 90°. By definition, the syntax denotes a plane that intercepts the three points a1/ℓ, a2/m, a3/n, or some multiple thereof; that is, the Miller indices are proportional to the inverses of the intercepts of the plane with the unit cell. If one or more of the indices is zero, it means. A plane containing a coordinate axis is translated so that it no longer contains that axis before its Miller indices are determined; the Miller indices for a plane are integers with no common factors. Negative indices are indicated with horizontal bars, as in. In an orthogonal coordinate system for a cubic cell, the Miller indices of a plane are the Cartesian components of a vector normal to the plane. Considering only planes intersecting one or more lattice points, the distance d between adjacent lattice planes is related to the reciprocal lattice vector orthogonal to the planes by the formula d = 2 π | g ℓ m n | The crystallographic directions are geometric lines linking nodes of a crystal.
The crystallographic planes are geometric planes linking nodes. Some directions and planes have a higher density of nodes; these high density planes have an influence on the behavior of the crystal as follows: Optical properties: Refractive index is directly related to density. Adsorption and reactivity: Physical adsorption and chemical reactions occur at or near surface atoms or molecules; these phenomena are thus sensitive to the density of nodes. Surface tension: The condensation of a material means that the atoms, ions or molecules are more stable if they are surrounded by other similar species; the surface tension of an interface thus varies according to the density on the surface. Microstructural defects: Pores and crystallites tend to have straight grain boundaries following higher density planes. Cleavage: This occurs preferentially parallel to higher density planes. Plastic deformation: Dislocation glide occurs preferentially parallel to higher density planes; the perturbation carried by the dislocation is along a dense direction.
The shift of one node in a more dense direction requires a lesser distortion of the crystal lattice. Some directions and planes are defined by symmetry of the crystal system. In monoclinic, rhombohedral and trigonal/hexagonal systems there is one unique axis which has higher rotational symmetry than the other two axes; the basal plane is the plane perpendicular to the principal axis in these crystal systems. For triclinic and cubic crystal systems the axis designation is arbitrary and there is no principal axis. For the special case of simple cubic crystals, the lattice vectors are orthogonal and of equal length. So, in this common case, the Miller indices and both denote normals/directions in Cartesian coordinates. For cubic crystals with lattice constant a, the spacing d between adjacent l
Chromium carbide is a ceramic compound that exists in several different chemical compositions: Cr3C2, Cr7C3,and Cr23C6. At standard conditions it exists as a gray solid, it is hard and corrosion resistant. It is a refractory compound, which means that it retains its strength at high temperatures as well; these properties make it useful as an additive to metal alloys. When chromium carbide crystals are integrated into the surface of a metal it improves the wear resistance and corrosion resistance of the metal, maintains these properties at elevated temperatures; the hardest and most used composition for this purpose is Cr3C2. Related minerals include tongbaite and isovite, 23C6, both rare, yet another chromium-rich carbide mineral is yarlongite, Cr4Fe4NiC4. There are three different crystal structures for chromium carbide corresponding to the three different chemical compositions. Cr23C6 has a cubic crystal structure and a Vickers hardness of 976 kg/mm2. Cr7C3 has a hexagonal crystal structure and a microhardness of 1336 kg/mm2.
Cr3C2 is the most durable of the three compositions, has an orthorhombic crystal structure with a microhardness of 2280 kg/mm2. For this reason Cr3C2 is the primary form of chromium carbide used in surface treatment. Synthesis of chromium carbide can be achieved through mechanical alloying. In this type of process metallic chromium and pure carbon in the form of graphite are loaded into a ball mill and ground into a fine powder. After the components have been ground they are pressed into a pellet and subjected to hot isostatic pressing. Hot isostatic pressing utilizes an inert gas argon, in a sealed oven; this pressurized gas applies pressure to the sample from all directions. The heat and pressure cause the graphite and metallic chromium to react with one another and form chromium carbide. Decreasing the percentage of carbon content in the initial mixture results in an increase in the yield of the Cr7C3, Cr23C6 forms of chromium carbide. Another method for the synthesis of chromium carbide utilizes chromium oxide, pure aluminum, graphite in a self-propagating exothermic reaction that proceeds as follows: 3Cr2O3 + 6Al + 4C → 2Cr3C2 + 3Al2O3In this method the reactants are ground and blended in a ball mill.
The blended powder is pressed into a pellet and placed under an inert atmosphere of argon. The sample is heated. A heated wire, a spark, a laser, or an oven may provide the heat; the exothermic reaction is initiated, the resulting heat propagates the reaction throughout the rest of the sample. Chromium carbide is useful in the surface treatment of metal components. Chromium carbide is used to coat the surface of another metal in a technique known as thermal spraying. Cr3C2 powder is mixed with solid nickel-chromium; this mixture is heated to high temperatures and sprayed onto the object being coated where it forms a protective layer. This layer is its own metal matrix composite, consisting of hard ceramic Cr3C2 particles embedded in a nickel-chromium matrix; the matrix itself contributes to the corrosion resistance of the coating because both nickel and chromium are corrosion resistant in their metallic form. After over spraying the coating, the coated part must run through a diffusion heat treatment to reach the best results in matter of coupling strength to the basemetal and in matter of hardness.
Another technique utilizes chromium carbide in the form of overlay plates. These are prefabricated chromium carbide coated steel plates, which are meant to be welded onto existing structures or machinery in order to improve performance. Chromium carbide is used as an additive in cutting tools made out of cemented carbides, in order to improve toughness by preventing the growth of large grains; the primary constituent in most hard cutting tools is tungsten carbide. The tungsten carbide is combined with other carbides such as titanium carbide, niobium carbide, chromium carbide and sintered together with a cobalt matrix. Cr3C2 prevents large grains from forming in the composite, which results in a fine-grained structure of superior toughness. National Pollutant Inventory - Chromium compounds fact sheet
Chromium acetate hydrate known as chromous acetate, is the coordination compound with the formula Cr242. This formula is abbreviated Cr242; this red-coloured compound features a quadruple bond. The preparation of chromous acetate once was a standard test of the synthetic skills of students due to its sensitivity to air and the dramatic colour changes that accompany its oxidation, it exists as the anhydrous forms. Cr242 is a reddish diamagnetic powder. Consistent with the fact that it is nonionic, Cr242 exhibits poor solubility in methanol; the Cr242 molecule contains two atoms of chromium, two ligated molecules of water, four acetate bridging ligands. The coordination environment around each chromium atom consists of four oxygen atoms in a square, one water molecule, the other chromium atom, giving each chromium centre an octahedral geometry; the chromium atoms are joined together by a quadruple bond, the molecule has D4h symmetry. The same basic structure is adopted by Rh242 and Cu242, although these species do not have such short M–M contacts.
The quadruple bond between the two chromium atoms arises from the overlap of four d-orbitals on each metal with the same orbitals on the other metal: the dz2 orbitals overlap to give a sigma bonding component, the dxz and dyz orbitals overlap to give two pi bonding components, the dxy orbitals give a delta bond. This quadruple bond is confirmed by the low magnetic moment and short intermolecular distance between the two atoms of 236.2 ± 0.1 pm. The Cr–Cr distances are shorter, 184 pm being the record, when the axial ligand is absent or the carboxylate is replaced with isoelectronic nitrogenous ligands. Eugène-Melchior Péligot first reported a chromium acetate in 1844, his material was the dimeric Cr242. The unusual structure, as well as that of copper acetate, was uncovered in 1951; the preparation begins with reduction of an aqueous solution of a Cr compound using zinc. The resulting blue solution is treated with sodium acetate, which results in the rapid precipitation of chromous acetate as a bright red powder.
2 Cr3+ + Zn → 2 Cr2+ + Zn2+ 2 Cr2+ + 4 OAc− + 2 H2O → Cr242The synthesis of Cr242 has been traditionally used to test the synthetic skills and patience of inorganic laboratory students in universities because the accidental introduction of a small amount of air into the apparatus is indicated by the discoloration of the otherwise bright red product. The anhydrous form of chromium acetate, related chromium carboxylates, can be prepared from chromocene: 4 RCO2H + 2 Cr2 → Cr24 + 4 C5H6This method provides anhydrous derivatives in a straightforward manner; because it is so prepared, Cr242 is a starting material for other chromium compounds. Many analogues have been prepared using other carboxylic acids in place of acetate and using different bases in place of the water. Chromium acetate has few practical applications, it has been used to dehalogenate organic compounds such as chlorohydrins. The reactions appear to proceed via 1e− steps, rearrangement products are sometimes observed; because the compound is a good reducing agent, it will reduce the O2 found in air and can be used as an oxygen scrubber.
Chromium acetate Chromium acetate hydroxide Rice, Steven F.. "Electronic Absorption Spectrum of Chromous Acetate Dihydrate and Related Binuclear Chromous Carboxylates". Inorg. Chem. 19: 3425–3431. Doi:10.1021/ic50213a042. Http://www.molecules.org/coordcpds.html#Cr2OAc4H2O http://alpha.chem.umb.edu/chemistry/ch370/documents/CH371chromiumacetate06.pdf http://wwwchem.uwimona.edu.jm/courses/chromium.pdf
Chromium chloride describes any of several compounds of with the formula CrCl3 · xH2O, where x can be 0, 5, 6. The anhydrous compound with the formula CrCl3 is a violet solid; the most common form of the trichloride is the dark green "hexahydrate", CrCl3 · 6H2O. Chromium chloride finds uses as precursors to dyes for wool. Anhydrous chromium chloride adopts the YCl3 structure, with Cr3+ occupying one thirds of the octahedral interstices in alternating layers of a pseudo-cubic close packed lattice of Cl− ions; the absence of cations in alternate layers leads to weak bonding between adjacent layers. For this reason, crystals of CrCl3 cleave along the planes between layers, which results in the flaky appearance of samples of chromium chloride. Chromium chlorides display the somewhat unusual property of existing in a number of distinct chemical forms, which differ in terms of the number of chloride anions that are coordinated to Cr and the water of crystallization; the different forms exist both as solids, in aqueous solutions.
Several members are known of the series of z+. The main hexahydrate can be more described as Cl · 2H2O, it consists of the cation trans-+ and additional molecules of water and a chloride anion in the lattice. Two other hydrates are known, pale green Cl2 · H2O and violet Cl3. Similar behaviour occurs with other chromium compounds. Anhydrous chromium chloride may be prepared by chlorination of chromium metal directly, or indirectly by carbothermic chlorination of chromium oxide at 650–800 °C Cr2O3 + 3 C + 3 Cl2 → 2 CrCl3 + 3 CODehydration with trimethylsilyl chloride in THF gives the solvate: CrCl3 · 6H2O + 12 Me3SiCl → CrCl33 + 6 2O + 12 HClIt may be prepared by treating the hexahydrate with thionyl chloride: CrCl3 · 6H2O + 6 SOCl2 → CrCl3 + 6 SO2 + 12 HClThe hydrated chlorides are prepared by treatment of chromate with hydrochloric acid and methanol. In laboratory the hydrates are prepared by dissolving the chromium metal or chromium oxide in hydrochloric acid. Slow reaction rates are common with chromium complexes.
The low reactivity of the d3 Cr3+ ion can be explained using crystal field theory. One way of opening CrCl3 up to substitution in solution is to reduce a trace amount to CrCl2, for example using zinc in hydrochloric acid; this chromium compound undergoes substitution and it can exchange electrons with CrCl3 via a chloride bridge, allowing all of the CrCl3 to react quickly. With the presence of some chromium, solid CrCl3 dissolves in water. Ligand substitution reactions of solutions of + are accelerated by chromium catalysts. With molten alkali metal chlorides such as potassium chloride, CrCl3 gives salts of the type M3CrCl6 and K3Cr2Cl9, octahedral but where the two chromiums are linked via three chloride bridges. CrCl3 is a Lewis acid, classified as "hard" according to the Hard-Soft Acid-Base theory, it forms a variety of adducts of the type z. For example, it reacts with pyridine to form an adduct: CrCl3 + 3 C5H5N → CrCl33Treatment with trimethylsilylchloride in THF gives the anhydrous THF complex: CrCl3.6 + 12 3SiCl + 3 THF → CrCl33 + 6 2O + 12 HCl Chromium chloride is used as the precursor to many organochromium compounds, for example bischromium, an analogue of ferrocene: Phosphine complexes derived from CrCl3 catalyse the trimerization of ethylene to 1-hexene.
One niche use of CrCl3 in organic synthesis is for the in situ preparation of chromium chloride, a reagent for the reduction of alkyl halides and for the synthesis of -alkenyl halides. The reaction is performed using two moles of CrCl3 per mole of lithium aluminium hydride, although if aqueous acidic conditions are appropriate zinc and hydrochloric acid may be sufficient. Chromium chloride has been used as a Lewis acid in organic reactions, for example to catalyse the nitroso Diels-Alder reaction. A number of chromium-containing dyes are used commercially for wool. Typical dyes are triarylmethanes consisting of ortho-hydroxylbenzoic acid derivatives. Although trivalent chromium is far less poisonous than hexavalent, chromium salts are considered toxic. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990; the Merck Index, 7th edition, Merck & Co, New Jersey, USA, 1960. J. March, Advanced Organic Chemistry, 4th ed. p. 723, New York, 1992. K. Takai, in Handbook of Reagents for Organic Synthesis, Volume 1: Reagents and Catalysts for C-C Bond Formation, pp. 206–211, New York, 1999.
International Chemical Safety Card 1316 International Chemical Safety Card 1532 National Pollutant Inventory – Chromium compounds fact sheet NIOSH Pocket Guide to Chemical Hazards IARC Monograph "Chromium and Chromium compounds"