Hydrogen fluoride

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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/Gas
CAS number	7664-39-3
Density	0.922 kg m-3
Solubility (water)	miscible
Melting point	-84 °C (190 K, -118 °F)
Boiling point	19.54 °C (293 K, 67.2 °F)
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.

Contents 1 Structure 2 Acidity 3 Uses 4 Health effects 5 References

[edit] Structure

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.

[edit] Acidity

Dilute aqueous HF solutions are weakly acidic in contrast to corresponding solutions of the other hydrogen halides. A qualitative explanation for this behavior is related to the tendency of HF to hydrogen-bond and form ion-pair clusters such as F^-·H₃O⁺.^[3][4]

In concentrated hydrogen fluoride solution, F⁻ ions forms a [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).

[edit] 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.

[edit] Health effects

Main article: Hydrofluoric acid

Upon contact with moisture, including tissue, hydrogen fluoride immediately converts to hydrofluoric acid, which is highly corrosive and toxic, and requires immediate medical attention.

[edit] References

1. ^ Johnson, M. W.; Sándor, E.; Arzi, E. (1975). "The Crystal Structure of Deuterium Fluoride". Acta Crystallographica B31: pages 1998–2003. doi:10.1107/S0567740875006711. 
2. ^ Mclain, Sylvia E. (2004). "On the Structure of Liquid Hydrogen Fluoride". Angewandte Chemie, International Edition 43: 1952–55. doi:10.1002/anie.200353289. 
3. ^ Giguere, Paul A. (1980). "The nature of hydrofluoric acid. A spectroscopic study of the proton-transfer complex H₃O⁺...F⁻". J. Am. Chem. Soc. 102: 5473. doi:10.1021/ja00537a008. 
4. ^ Radu Iftimie, Vibin Thomas, Sylvain Plessis, Patrick Marchand, and Patrick Ayotte (2008). "Spectral Signatures and Molecular Origin of Acid Dissociation Intermediates". J. Am. Chem. Soc. 130: 5901. doi:10.1021/ja077846o.