hydrogen fluoride

Hydrogen fluoride
Identifiers
CAS number	7664-39-3 Y
PubChem	16211014
RTECS number	MW7875000
Properties
Molecular formula	HF
Molar mass	20.00634 g/mol
Appearance	colorless gas
Density	1.15 g/l, gas (25 °C)
Melting point	−83 °C, 190 K, -117 °F
Boiling point	19.5 °C, 293 K, 67 °F
Solubility in water	miscible
Refractive index (nD)	1.00001
Thermochemistry
Std enthalpy of formation ΔfHo298	−13.66 kJ/g (gas) −14.99 kJ/g (liquid)
Standard molar entropy So298	8.687 J/g K (gas)
Hazards
NFPA 704	0 4 1
Related compounds
Other anions	Hydrogen chloride Hydrogen bromide Hydrogen iodide
Other cations	Sodium fluoride
Related compounds	Hydrofluoric acid
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Hydrogen fluoride is a chemical compound with the formula HF. It is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is 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. Unlike the other hydrogen halides, HF is lighter than air and its odour is particularly penetrating, which can damage the lungs. Aqueous solutions of HF, called hydrofluoric acid, are strongly corrosive.

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.

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 remain ionized 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).

Production and uses

Hydrogen fluoride is produced as a side product of the extraction of the fertilizer precursor phosphoric acid from various minerals. This theme is developed in the article on hydrofluoric acid.

The anhydrous compound hydrogen fluoride is more commonly used than its aqueous solution, hydrofluoric acid. HF serves as a catalyst in alkylation processes in oil refineries. A component of high-octane gasoline called "alkylate" is generated in FCC units that combine C3 and C4 olefins and isobutane to generate gasoline.^[5]

HF is a reactive solvent in the electrochemical fluorination of organic compounds. In this approach, HF is oxidized in the presence of a hydrocarbon and the fluorine replaces C–H bonds with C–F bonds. Perfluorinated carboxylic acids and sulfonic acids are produced in this way.^[5]

Hydrogen fluoride is an important catalyst used in the majority of the installed Linear alkyl benzene production in the world. The process involves dehydrogenation of n-paraffins to olefins, and subsequent reaction with benzene using HF as catalyst.

Elemental fluorine, F₂, is prepared by electrolysis of a solution of HF and potassium bifluoride. Several million kilograms of F₂ are produced annually.^[6]

Health effects

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

References