Uranium hexachloride

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Uranium hexachloride
Uranium Hexachloride.png
Identifiers
CAS number 13763-23-0
Properties
Molecular formula UCl6
Molar mass 450.745 g/mol
Appearance dark green crystalline solid
Density 3600 kg/m3
Melting point 177 °C
Boiling point 75 °C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Uranium hexachloride is an inorganic chemical compound of uranium in the +6 oxidation state.[1][2]The chemical compound Uranium hexachloride (UCl6) is a metal halide composed of Uranium and Chlorine. It is a multi-luminescent dark green crystalline solid with a vapor pressure between 1-3 mmHg at 373.15K[3]UCl6 is stable in a vacuum, dry air, nitrogen and helium at room temperature. It is soluble in carbon tetrachloride (CCl4). Compared to the other uranium halides, little is known about UCl6.


Structure and Bonding[edit]

Uranium hexachloride has an octahedral geometry with a point group of Oh. Its lattice (dimensions: 10.95 ± 0.02Å x 6.03 ± 0.01Å) is hexagon in shape with three molecules per cell; the average theoretical U-Cl bond is 2.472Å long (X-ray diffraction experimental U-Cl length is 2.42Å)[4] and the distance between two adjacent chlorine atoms is 3.65Å .

Chemical Properties[edit]

Uranium hexachloride is a highly hygroscopic compound and decomposes readily when exposed to ordinary atmospheric conditions.[5] therefore it should be handled in either a vacuum apparatus or in a dry box.

Thermal decomposition[edit]

UCl6 is stable up to temperatures between 120oC and 150oC. The decomposition of UCl6 results in a solid phase transition from one crystal form of UCl6 to another more stable form.[6] However the decomposition of gaseous UCl6 produces (UCl5). The Activation energy for this reaction is about 40kcal per mole.

2UCl6 (g) → 2UCl5 (s) + Cl2 (g)

Solubility[edit]

UCl6 is not a very soluble compound. It dissolves in CCl4 to give a brown solution. It is slightly soluble in isobutyl bromide and in fluorocarbon (C7F16).[7]

Solvents Temperature (oC) Grams of UCl6/100g of solution
CCl4 -18 2.64
CCl4 0 4.9
CCl4 20 7.8
6.6% Cl2 : 93.4% CCl4 -20 2.4
12.5% Cl2 : 87.5% CCl4 -20 2.23
12.5% Cl2 : 87.5% CCl4 0 3.98
Liquid Cl2 -33 2.20
CH3Cl -24 1.16
Benzene 80 Insoluble
Freon 113 45 1.83

Reaction with Hydrogen Fluoride[edit]

When UCl6 is reacted with purified anhydrous liquid Hydrogen Fluoride (HF) at room temperature produces UF5. [8]

2UCl6+ 10HF → 2UF5 + 10HCl + Cl2

Synthesis[edit]

Uranium hexachloride can be synthesized from the reaction of uranium trioxide (UO3) with a mixture of liquid CCl4 and hot chlorine (Cl2). However the yield can be increased if the reaction carried out in the presence of UCl5.[9] The UO3 is converted to UCl5, which in turn reacts with the excess Cl2 to form UCl6. It requires a substantial amount of heat for the reaction to take place; the temperature range is from 65oC to 170oC depending on the amount of reactants (ideal temp: 100oC - 125oC) The reaction is carried out in a closed gas-tight vessel (for example a glovebox) that can withstand the pressure the builds up.

Step 1: 2UO3 + 5Cl2 → 2UCl5 + 3O2

Step 2: 2UCl5 + Cl2 → 2UCl6

Overall reaction: 2UO3 + 6Cl2 → 2UCl6 + 3O2

This metal hexahalide can also be synthesized by blowing Cl2 gas over sublimed UCl4 at 350oC. [10]

Step 1: 2UCl4 + Cl2 → 2UCl5

Step 2: 2UCl5 + Cl2 → 2UCl6

Overall Reaction: UCl4 + Cl2 → UCl6

References[edit]

  1. ^ Zachariasen, W. H. (1948). "Crystal chemical studies of the 5f-series of elements. V. The crystal structure of uranium hexachloride". Acta Crystallographica 1 (6): 285. doi:10.1107/S0365110X48000788. 
  2. ^ Taylor, J. C.; Wilson, P. W. (1974). "Neutron and X-ray powder diffraction studies of the structure of uranium hexachloride". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry 30 (6): 1481. doi:10.1107/S0567740874005115. 
  3. ^ Van Dyke, R. E.; Evers, E. C. (1955). "Preparation of Uranium Hexachloride". google patent: 2. 
  4. ^ Batista, E. R.; Martin, R. L.; Hay, P. J. (2004). "Density Functional Investigations of the Properties and Thermodynamics of UFn and UCln (n=1,...,6)". J. Chem.Phys. 121 (22): 8. doi:10.1063/1.1811607. 
  5. ^ Lipkin, D.; Wessman, S. (1955). "Process and Apparatus for protecting Uranium hexachloride from Deterioration and Contamination". google patent: 2. 
  6. ^ Katz,J.J; Rabinowitch,E. (1951). The Chemistry of Uranium. Ann Arbor: The McGraw-Hill Book Company. 
  7. ^ Katz,J.J; Rabinowitch,E. (1951). The Chemistry of Uranium. Ann Arbor: The McGraw-Hill Book Company. 
  8. ^ Katz,J.J; Rabinowitch,E. (1951). The Chemistry of Uranium. Ann Arbor: The McGraw-Hill Book Company. 
  9. ^ Van Dyke, R. E.; Evers, E. C. (1955). "Preparation of Uranium Hexachloride". google patent: 2. 
  10. ^ Thornton, G.; Edelstein, R.G.; Rösch, N.; Woodwark, D.R.; Edgell (1979). "The Electronic Structure of UCl6: Photoelectron Spectra and Scattered Wave Xα Calculations". J. Chem.Phys. 70 (11): 6. doi:10.1063/1.437313.