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Inchemistry,hydrogen halides (hydrohalic acids when in theaqueous phase) arediatomic,inorganic compounds that function asArrhenius acids. The formula isHX where X is one of thehalogens:fluorine,chlorine,bromine,iodine,astatine, ortennessine.[1] All known hydrogen halides are gases atstandard temperature and pressure.[2]
| Compound | Chemical formula | Bond length d(H−X) / pm (gas phase) | model | Dipole μ /D | Aqueous phase (acid) | Aqueous Phase pKa values |
|---|---|---|---|---|---|---|
| hydrogen fluoride (fluorane) | HF |  |  | 1.86 | hydrofluoric acid | 3.1 |
| hydrogen chloride (chlorane) | HCl |  |  | 1.11 | hydrochloric acid | −3.9 |
| hydrogen bromide (bromane) | HBr |  |  | 0.788 | hydrobromic acid | −5.8 |
| hydrogen iodide (iodane) | HI |  |  | 0.382 | hydroiodic acid | −10.4[3] |
| hydrogen astatide astatine hydride (astatane) | HAt |  |  | −0.06 | hydroastatic acid | ? |
| hydrogen tennesside tennessine hydride (tennessane) | HTs |  |  | −0.24 ? | hydrotennessic acid | ?[4] |
The hydrogen halides are diatomic molecules with no tendency toionize in thegas phase (although liquified hydrogen fluoride is apolar solvent somewhat similar towater). Thus, chemists distinguish hydrogen chloride from hydrochloric acid. Hydrogen chloride is agas at room temperature that reacts with water to give hydrochloric acid; once the acid has formed, the hydrogen chloride can be regenerated, but only with difficulty and not by normaldistillation. Often, the names of the acid and the molecules are not clearly distinguished, and in labjargon, "HCl" often means hydrochloric acid, not the gaseous hydrogen chloride.
Hydrogen fluoride, chloride, and bromide arevolcanic gases.[5][6]
The hydrogen halides can be produced by many routes industrially and in the laboratory. Focusing on the most abundant compound, hydrogen chloride is mainly produced as a side product in production of chlorocarbons.[7] Hydrogen fluoride is a byproduct of the production ofphosphoric acid. Fluorine, chlorine, and bromine react with hydrogen gas to give HF, HCl, and HBr. These gases can also be produced by treatment of halidesalts withsulfuric acid. The least stable hydrogen halide, HI, is produced less directly, by the reaction of iodine withhydrogen sulfide or withhydrazine.[1]: 809–815
The hydrogen halides are colourless gases atstandard conditions for temperature and pressure (STP) except for hydrogen fluoride, which boils at 19 °C. Alone of the hydrogen halides, hydrogen fluoride exhibitshydrogen bonding between molecules, and therefore has the highestmelting andboiling points of the HX series. From HCl to HI the boiling point rises. This trend is attributed to the increasing strength of intermolecularvan der Waals forces, which correlates with numbers of electrons in the molecules. Concentrated hydrohalic acid solutions produce visible white fumes. This mist arises from the formation of tiny droplets of their concentratedaqueous solutions of the hydrohalic acid.
Upon dissolution in water, which is highlyexothermic, the hydrogen halides give the corresponding acids. These acids are very strong, reflecting their tendency toionize inaqueous solution, yieldinghydronium ions (H3O+). With the exception of hydrofluoric acid, the hydrogen halides arestrong acids, with acid strength increasing down the group.Hydrofluoric acid is complicated because its strength depends on the concentration, owing to the effects ofhomoconjugation. However, as solutions in non-aqueous solvents, such asacetonitrile, the hydrogen halides are only moderately acidic.
Similarly, the hydrogen halides react withammonia (and other bases), forming ammonium halides:
In organic chemistry, thehydrohalogenation reaction is used to prepare halocarbons. For example,chloroethane is produced byhydrochlorination ofethylene:[8]
