Preparation of Chromium

Preparation of Chromium. Perform the experiment in the presence of your instructor, in a fume cupboard Wear eye protection or a mask ) Roast chromium(III) oxide in an iron crucible. Triturate potassium dichromate crystals in a mortar, melt the powder in a porcelain crucible with the flame of a gas burner, pour the substance onto a glazed tile plate, and after it solidifies again triturate it into a powder. Weigh 12 g of roasted chromium(III) oxide, 12 g of the fused potassium dichromate, and 10 g of an aluminium powder, and thoroughly mix all the substances in a mortar. Spill 10 g of a calcium fluoride powder onto the bottom of a chamotte crucible, and then the prepared mixture. (If there is no chamotte crucible in your laboratory, make a box from asbestos paper in the form of a crucible and fasten it with wire.) Tamp the substance with a pestle and make a hollow in the middle using a test tube (see Fig. 120). 

Prepare an incendiary mixture by shaking 5 g of an aluminium powder and 15 g of barium peroxide in a jar. (2 o not triturate the mixture in a mortar ) Spill the incendiary mixture into the hollow made in the reaction mass and insert a magnesium ribbon cleaned of oxide. Place the crucible on a pan and spill dry sand over it from all sides. Ignite the magnesium ribbon with a burning splinter insert- 

When the reaction ends, let the crucible cool, break it, extract the chromium bead, and weigh it. Determine the yield in per cent. Explain the course of the reaction. 

Properties of Chromium. Crush the chromium bead into small pieces in an iron mortar. Note the colour of the metal in a fresh fracture, and test its hardness. To do this, scratch a glass plate with a piece of chromium. How does a magnet act on chromium  

Put several pieces of chromium into test tubes and test its reaction with water, alkalies, and also with dilute and with concentrated solutions of hydrochloric, sulphuric, and nitric acids (in the cold and with heating). Write the equations of the reactions. What position does chromium occupy in the electrochemical series of metals  

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Chemically, carbon dioxide is not very reactive, and it is often used as an inactive gas to replace air when the latter might interact with a substance, for example in the preparation of chromium II) salts (p. 383). Very reactive metals, for example the alkali metals and magnesium can, however, continue to bum in carbon dioxide if heated sufficiently, for example... 

Although it has been known since 19051 that very pure chromium metal reacts with acids, under oxygen-free conditions, to produce large quantities of chromium (II), this approach to the preparation of chromium(II) compounds has not been developed. Rather, syntheses generally involved (1) reduction of chro-mium(III), either by electrolytic means or by chemical agents (for example Zn/Hg), or (2) metathetical procedures. Both methods are inefficient and often lead to impure products. Recently2-8 extensive use of reactions between electrolytic chromium and various acids has led to the synthesis of a wide variety of chromium (II) complexes which would be considerably more difficult to prepare by other methods.9-11... 

Because of the sensitivity of chromium(II) to air oxidation, synthetic work on these systems requires the use either of vacuum-line techniques12 or of a nitrogen-filled box,6 or both. This synthesis describes the preparation of chromium(II) complexes using a combination of a closed ground-glass filter stick 18 and a nitrogen-filled box.6 The filter stick enables preparations, filtrations, and recrystallizations to be carried out... 

Comments on the Procedure if Other Methods for Preparation of Chromium(II) Chloride Are Used... 

Chromium hexacarbonyl is used as an additive to gasoline to increase the octane number as a catalyst in isomerization and polymerization reactions and in the preparation of chromium mirror or plate. 

Chromyl chloride is used in many organic synthetic reactions including oxidation and chlorination. It also is used as a catalyst in olefin polymerization in the preparation of chromium complexes and as a solvent for chromic anhydride. 

Preparation of Chromium(II) Chloride. (Perform one of the following two experiments.) 1. Put several pieces of metallic chromium into a test tube, pour in 2-3 ml of a 20% hydrochloric acid solution, and insert a stopper with a gas-discharge tube. Connect to the latter a rubber tube with a slot closed by a glass rod (a Bunsen valve. Fig. 125). What is the role of the slot in the rubber tube Identify the substance in the solution. Write the equation of the reaction. 

Preparation of Chromium Potassium Alum. Pour 25 ml of water into a 50-ml beaker and dissolve 2.5 g of potassium dichromate in it. Add a concentrated sulphuric acid solution to the mixture (one-and-a-half the stoichiometric amount). First cool the mixture to room temperature, and then put it into water with ice and add ethanol dropwise from a dropping funnel until the solution acquires a violet colour (sulphur dioxide can also be used as the reducing agent). Keep the temperature below 40 °C (why ). After adding the ethanol, let the solution stand to your next lesson. Write the equation of the reaction. 

Preparation of Chromium(II) Acetate. Assemble an apparatus as shown in Fig. 126. Fill wash bottles 2 and 3 with an alkaline solution of pyrogallol, and columns 4, 5, and 6 with solid granulated sodium hydroxide. 

Preparation of Chromium(III) Nitride. Work in a fume cupboard Assemble an apparatus for preparing nitrides (see Fig. 84). Put 0.5-1 g of anhydrous chromium(III) chloride into a boat. Put the latter into a tubular furnace. Displace the air from the apparatus with a stream of dry ammonia and then heat the furnace to 600 °C. Continue the heating in an ammonia stream for one hour, next switch off the furnace and cool the apparatus without stopping the stream of gas. Extract the boat and weigh the product. Write the equation of the reaction. Calculate the yield in per cent. 

Preparation of Chromium(VI) Oxysulphate. (Perform the experiment in a fume cupboard ) a. Preparation of Chromyl Chloride. Assemble an apparatus as shown in Fig. 127a. Dry 12.5 g of sodium chloride and 20 g of potassium dichromate in a drying cabinet at 100-120 °C. Rapidly mix them in a mortar and put the mixture into flask 1. Add 22 ml of a 98% sulphuric acid solution to the reaction... 

Methods for the preparation of chromium(II) solutions and the isolation of solids foi synthetic work are set out below. 

The easy formation of hydroxo- or oxo-bridged Cr111 polymers in basic aqueous solution, the comparative lability of the Cr—N bond, and the precautions needed to obtain chromium(II) complexes compared with cobalt(II) complexes have meant that the preparative chemistry of chromium(III) is more difficult than that of cobalt(III). A greater variety of non-aqueous solvents is now in use, and there is greater knowledge of chromium(II) chemistry to be exploited in the preparation of chromium(III) complexes generally, but few new methods of preparation of amine complexes have been devised since the early work. 

The physical properties of samples of Cr(OMe)3 prepared by a variety of methods have been investigated.736 Good crystals could not be obtained but limited crystallography suggested a layered lattice with chromium occupying octahedral sites. A useful summary of earliei preparations and attempted preparations of chromium methoxides can be found in this paper. 

Equations (85) and (86) express the general method592,618 for the preparation of chromium(IV) dialkylamides. The chromium(III) dialkylamide (Section 35.4.2.7) is extracted into pentane from the residue after removal of the solvent careM evaporation of the pentane gives Cr(NR2)3 which disproportionates on further heating in vacuum because of the volatility of CrIV(NR2)4- The method has been successful with R = Et, Pr and R2 = MeBu, C5Hi0, but failed with higher homologues because of the low volatility of the Crrv species, and with... 

Compensation behavior found for the decomposition of hydrogen peroxide on preparations of chromium (III) oxide, which had previously been annealed to various temperatures, was attributed to variations in the energy states of the active centers (here e 0.165). Compensation behavior has also been observed (284) in the decomposition of hydrogen peroxide on cobalt-iron spinels the kinetic characteristics of reactions on these catalysts were ascribed to the electronic structures of the solids concerned. 

The preparation of chromium(III) and cobalt(III) complexes of tetradentate formazans in which the coordination sphere of the metal is completed by a variety of neutral ligands has been reported116 and these (185) were claimed to be very stable towards acids. By analogy with copper complexes, however, some doubt must exist regarding the structures assigned to those complexes derived from l,5-bis(2-carboxyphenyl)formazans. Various dyestuff and pigment applications have... 

The simplest procedure, dissolution of metallic iron in the aqueous mineral acid, suffers from the risk of accidental oxidation. The following relatively simple procedure overcomes this difficulty. This method, with minor modifications, has also been used successfully by the author for the preparation of chromium-(II) halides. 

Chromium Complexes. Because of their high stability chromium complexes of tri-dentate azo dyes are the most important class of metal-complex dyes. This is due to the reluctance of hexacoordinated chromium(m) complexes to exchange ligands, which, however, complicates the preparation of chromium complex dyes from hexaaqua chromium(m) salts, and makes it possible to prepare triaqua 1 1 chromium complex dyes. Generally, 1 1 chromium complexes can be made in... 

Imidodiphosphoric acid tetramide, formation of, from phosphoryl triamide, 6 110, llln. Iodination apparatus, for preparation of chromium(III) iodide, 6 128... 

As discussed above, the ligands that have been typically utilized for the preparation of chromium nitrides are multidentate. Consequently, ligand exchange reactions of such complexes are difficult and rare. Wieghardt and co-workers have reported such a process, however, for the synthesis of a nitrido chromium cyanide complex 43 (Eq. (13)) [18]. Thus, treatment of CrN(salen) 42 with excess sodium cyanide and tetramethyl ammonium chloride results in the formation of a six-coordinate penta-cyano chromium nitride [21]. 

Cummins has reported that treatment of CrVI complex 47 with 48 at elevated temperature affords the corresponding nitride 49 (Eq. (15)) [23]. This method offers a useful alternative to the other existing methodologies for the preparation of chromium nitrides. The thermodynamic stability of anthracene contributes significantly to the driving force for this process. Cummins has also reported the preparation of chromium nitride 51 from NO and a Cr111 triamide 50, and (THF)2V(Mes)3 (Eq. (16)) [24],... 

For most methoxycarbene complexes a one pot modification of the above method is utilized. This involves direct alkylation of the initially formed lithium acylate carbene complex with trifluoromethanesulfonate or with methyl fluorosulfonate. The method is successfully employed for preparation of chromium as well as molybdenum and tungsten monocyclic (equation 65), bicyclic (equation 66) and tricyclic carbene complexes (equation 67)... 

Lalancette et al. described the preparation of chromium(VI) oxide intercalated in graphite and its use as a selective oxidant for primary alcohols to aldehydes (Table 17). Secondary and tertiary alcohols are inert under the reaction conditions, but 1,2-diols are cleaved. However, Ebert and coworkers demonstrated that Lalancette probably used CriOg rather than chromium(VI) oxide intercalated gnq ite for these oxidations. This reagent does appear to show the properties described by Lalancette et al. Indeed, when Kagan and coworkers prepared fully characterize chromium(VI)-inteieiated gnqihite it appeared to lack any oxidative properties. ... 

A list of methods for the preparation of chromium dioxide is given by Eidley, Chem. News, 1924, 129, 35. 

Procedures for the preparation of chromium(II) acetate have been presented in three of the preceding volumes of Inorganic Syntheses.In Volume VI it is pointed out that the methods described earlier require complicated apparatus, whereas a glove box can be used more easily to provide an inert atmosphere. Complete displacement of air from a dry-box, however, often is difficult. A technique has been developed which involves a less cumbersome closed system used in conjunction with a dry-bag for transfers. ... 

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