Beryllium fluoride

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Beryllium fluoride
BeF2str.PNG
Beryllium fluoride.JPG
Identifiers
CAS number 7787-49-7 YesY
PubChem 24589
ChemSpider 22992 YesY
ChEBI CHEBI:49499 YesY
RTECS number DS2800000
Jmol-3D images Image 1
Properties
Molecular formula BeF2
Molar mass 47.01 g/mol
hygroscopic
Appearance colorless lumps
Density 1.986 g/cm3
Melting point 554 °C (1,029 °F; 827 K)
Boiling point 1,169 °C (2,136 °F; 1,442 K)[1]
Solubility in water very soluble
Solubility sparingly soluble in alcohol
Structure
Crystal structure Trigonal, α-quartz
Space group P3121 (No. 152), Pearson symbol hP9[2]
Lattice constant a = 473.29 pm, c = 517.88 pm
Molecular shape Linear
Thermochemistry
Specific
heat capacity
C
1.102 J/K or 59 J/mol K
Std molar
entropy
So298
45 J/mol K
Std enthalpy of
formation
ΔfHo298
-1028.2 kJ/g or -1010 kJ/mol
Gibbs free energy ΔG -941 kJ/mol
Hazards
EU Index 004-002-00-2
EU classification Carc. Cat. 2
Highly toxic (T+)
Irritant (Xi)
Dangerous for the environment (N)
R-phrases R49, R25, R26, R36/37/38, R43, R48/23, R51/53
S-phrases S53, S45, S61
Flash point Non-flammable
LD50 98 mg/kg (oral, rat)
Related compounds
Other anions Beryllium chloride
Beryllium bromide
Beryllium iodide
Other cations Magnesium fluoride
Calcium fluoride
Strontium fluoride
Barium fluoride
Radium fluoride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Beryllium fluoride is the inorganic compound with the formula BeF2. This white solid is the principal precursor for the manufacture of beryllium metal. Its structure resembles that of quartz, but BeF2 is highly soluble in water.

Properties[edit]

Beryllium fluoride has unique optical properties. In the form of fluoroberyllate glass it has the lowest refractive index for a solid at room temperature of 1.275. It dispersive power is the lowest for a solid at 0.0093, and the non linear coefficient is also the lowest at 2 × 10−14.

Structure and bonding[edit]

Structure of gaseous BeF2.

The structure of solid BeF2 resembles that of cristobalite. Be2+ centers are four coordinate and tetrahedral and the fluoride centers are two-coordinate.[3] Analogous to SiO2,BeF2 can also adopt a number of related structures. An analogy also exists between BeF2 and AlF3: both adopt extended structures at mild temperature.

Gaseous and liquid BeF2[edit]

Like the isoelectronic molecules CO2 and SiO2, it is a linear molecule, gaseous BeF2 adopts a linear structure. The Be-F distance of 177 pm.[4]

BeF2 reaches a vapor pressure of 10 Pa at 686 °C, 100 Pa at 767 °C, 1 kilopascal at 869 °C, 10 kPa at 999 °C, and 100 kPa at 1172 °C.[5]

Molten BeF2 resembles water in some ways. Both liquid consist of triatomic molecules with strong intermolecular interactions via Be—F—Be bonds. As in water, the density of BeF2 decreases near its melting point. Liquid (molten) beryllium fluoride also has a fluctuating tetrahedral structure.[6]

Production[edit]

The processing of beryllium ores generates impure Be(OH)2. This material reacts with ammonium bifluoride to give ammonium tetrafluoroberyllate:

Be(OH)2 + 2 (NH4)HF2 → (NH4)2BeF4 + 2 H2O

Tetrafluoroberyllate is a robust ion, which allows its purification by precipitation of various impurities as their hydroxides. Heating purified (NH4)2BeF4 gives the desired product:

(NH4)2BeF4 → 2 NH3 + 2 HF + BeF2

In general the reactivity of BeF2 ions with fluoride are quite analogous to the reactions of SiO2 with oxides.[7]

Applications[edit]

Reduction of BeF2 at 1300 °C with magnesium in a graphite crucible provides the most practical route to metallic beryllium:[4]

BeF2 + Mg → Be + MgF2

The chloride is not a useful precursor because of its volatility.

Niche uses[edit]

Beryllium fluoride is used in biochemistry, particularly protein crystallography as a mimic of phosphate. Thus, ADP and beryllium fluoride together tend to bind to ATP sites and inhibit protein action, making it possible to crystallise proteins in the bound state.[8][9]

Beryllium fluoride forms a basic constituent of the preferred fluoride salt mixture used in liquid-fluoride nuclear reactors. Typically beryllium fluoride is mixed with lithium fluoride to form a base solvent (FLiBe), into which fluorides of uranium and thorium are introduced. Beryllium fluoride is exceptionally chemically stable and LiF/BeF2 mixtures (FLiBe) have low melting points (360 C - 459 C) and the best neutronic properties of fluoride salt combinations appropriate for reactor use. MSRE used two different mixtures in the two cooling circuits.

Safety[edit]

All beryllium compounds are highly toxic. Beryllium fluoride is very soluble in water and is thus absorbed easily; as mentioned above, it inhibits ATP uptake. The LD50 in mice is about 100 mg/kg by ingestion and 1.8 mg/kg by intravenous injection.

References[edit]

  1. ^ Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3. 
  2. ^ Wright, Albert F.; Fitch, Andrew N.; Wright, Adrian C. (1988). "The preparation and structure of the α- and β-quartz polymorphs of beryllium fluoride". Journal of Solid State Chemistry 73 (2): 298. Bibcode:1988JSSCh..73..298W. doi:10.1016/0022-4596(88)90113-2. 
  3. ^ Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ISBN 0-19-855370-6
  4. ^ a b Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  5. ^ Vapor pressure, physics.nyu.edu, p. 6-63, from Ohe, S. (1976) Computer Aided Data Book of Vapor Pressure, Data Book Publishing Co., Tokyo.
  6. ^ Agarwal, M.; Chakravarty C (2007). "Waterlike Structural and Excess Entropy Anomalies in Liquid Beryllium Fluoride". J. Phys. Chem. B 111 (46): 13294. doi:10.1021/jp0753272. PMID 17963376. 
  7. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 
  8. ^ Reiko Kagawa, Martin G Montgomery, Kerstin Braig, Andrew G W Leslie and John E Walker (2004). "The structure of bovine F1-ATPase inhibited by ADP and beryllium fluoride". The EMBO Journal 23 (5): 2734–2744. doi:10.1038/sj.emboj.7600293. PMC 514953. PMID 15229653. 
  9. ^ Bigay J, Deterre P, Pfister C, Chabre M (1987). "Fluoride complexes of aluminium or beryllium act on G-proteins as reversibly bound analogues of the gamma phosphate of GTP.". The EMBO Journal 6 (10): 2907–2913. PMC 553725. PMID 2826123. 

External links[edit]