Hydrogen fluoride

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.

Liquid HF also consists of chains of HF molecules, but the chains are shorter, consisting on average of only five or six molecules. 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 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.

Health effects
Upon contact with moisture, including tissue, hydrogen fluoride immediately converts to hydrofluoric acid, which is highly corrosive and toxic.