hydrogen fluoride

 

chemical compound, HF, a colorless, fuming liquid or colorless gas that boils at 19.54°C. It is miscible with water and is soluble in benzene, toluene, and concentrated sulfuric acid. Hydrofluoric acid is a water solution of hydrogen fluoride; hydrofluoric acid containing 35.35% hydrogen fluoride by weight is an azeotrope with a constant boiling point of 120°C. Whether gaseous, liquid, or in solution, hydrogen fluoride is a dangerous chemical and must be handled with caution, since it attacks the skin and other tissue. Hydrogen fluoride has a number of properties that distinguish it from the other hydrogen halides. It polymerizes, forming molecules such as H₂F₂ and H₆F₆; this explains in part its relatively high boiling point. It is a relatively weak acid. It attacks glass, reacting with the silica, SiO₂, to form the gas silicon tetrafluoride, SiF₄, and water; this leaves the surface of the glass etched. Major industrial uses of hydrogen fluoride include the synthesis of fluorocarbons (e.g., Freon and teflon) and the production of aluminum fluoride and synthetic cryolite for use in aluminum refining. It is also employed in refining uranium for use as a nuclear fuel, in manufacturing various organic chemicals, in producing stainless steel, and for various other applications. Hydrogen fluoride is produced commercially by heating purified fluorspar (calcium fluoride) with concentrated sulfuric acid to produce the gas, which may be condensed by cooling or dissolved in water. Hydrogen fluoride is available commercially either in an anhydrous (water-free) state or in water solutions of various concentrations. Because it attacks glass, it is usually stored in steel tanks, cylinders, or drums, or, in small amounts, in plastic bottles.

Hydrogen fluoride
Other names	Hydrogen fluoride Fluoric acid Hydrofluoride Hydrofluoric acid Fluorine monohydride
Molecular formula	HF
Molecular mass	20.01 g/mol
Physical state	Liquid
CAS number	7664-39-3
Density	0.922 kg m-3
Solubility (water)	miscible
Melting point	-84 °C (190 K, -118 ºF)
Boiling point	19.54 °C (293 K, 67.2 ºF)
NFPA 704	0 4 2
Disclaimer and references

Hydrogen fluoride is a chemical compound with the formula HF. Together with hydrofluoric acid, it is the principal industrial source of fluorine and hence the precursor to many important compounds including pharmaceuticals and polymers (e.g. Teflon). HF is widely used in the petrochemical industry and a component of many superacids. HF boils just below room temperature whereas the other hydrogen halides condense at much lower temperatures. Aqueous solutions of HF, called hydrofluoric acid, are strongly corrosive.

Structure of HF

HF forms orthorhombic crystals, consisting of zig-zag chains of HF molecules. The HF molecules, with a short H-F bond of 0.95 Å, are linked to neighboring molecules by intermolecular H--F distances of 1.55 Å.^[1]

Liquid HF also consists of chains of HF molecules, but the chains are shorter, consisting on average of only five or six molecules.^[2] The higher boiling point of HF relative to analogous species, such as HCl, is attributed to hydrogen bonding between HF molecules, as indicated by the existence of chains even in the liquid state.

Acidity

Dilute hydrogen fluoride solution is a weak acid due to the extensive intermolecular H-bonds present. The molecules tend to remain in chains rather than ionize to form H⁺ and F⁻ ions.

But in a more concentrated hydrogen fluoride solution, F⁻ ions forms a soluble [HF₂]⁻(aq) complex with HF molecules. HF molecules remaining ionize to compensate the loss of F⁻ ions. More H⁺ ions are thus formed, making concentrated HF an effectively strong acid.

Anhydrous hydrogen fluoride is an extremely strong acid (H₀ ~ -11), comparable in strength to anhydrous sulfuric acid (H₀ ~ -12).

Uses

HF is used for fluorinating polymers giving fluorocarbons, petroleum refining, glassmaking, aluminium manufacturing, titanium pickling, quartz purification, and metal finishing. It is also used to synthesize UF₆, which is key to separating uranium isotopes.

Hydrogen fluoride can be found in consumer products for removing rust, cleaning brass, and glass etching, although use in consumer products is discouraged ^{[citation needed]} due to HF's corrosiveness and toxicity. Hydrogen fluoride is typically marketed in three common forms: anhydrous HF, aqueous 70% HF, aqueous 49% HF. HF is manufactured by the reaction of calcium fluoride (fluorspar) and sulfuric acid:

CaF₂ + H₂SO₄ → CaSO₄ + 2 HF

The vapors from this reaction are a mixture of hydrogen fluoride, sulfuric acid, and a few minor byproducts, from which hydrogen fluoride can be isolated by distillation.

Health effects

Main article: Hydrofluoric acid

HF immediately converts to hydrofluoric acid upon contact with moisture, including tissue. Hydrofluoric acid is highly corrosive and toxic.

References

1. ^ Johnson, M. W.; Sándor, E.; Arzi, E. "The Crystal Structure of Deuterium Fluoride" Acta Crystallographica 1975, B31, pages 1998-2003.doi:10.1107/S0567740875006711
2. ^ Sylvia E. McLain, S. E.; Benmore, C. J.; Siewenie, J. E.; Urquidi, J.; Turner, J. F. C. "On the Structure of Liquid Hydrogen Fluoride" Angewandte Chemie, International Edition, 2004, volume 43, pages 1952-55 doi:10.1002/anie.200353289

External links

• Links to external chemical sources

http://www.chemical-supermarket.com/Hydrofluoric_Acid%2C_49%25%2C_250ml%2C_Semi_Grade-p32.html

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