In chemistry, hydrogen halides (hydrohalic acids when in the aqueous phase) are diatomic, inorganic compounds that function as Arrhenius acids. The formula is HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, astatine, or tennessine. All known hydrogen halides are gases at standard temperature and pressure.

{| class="wikitable" style="text-align:center;"

! Compound || Chemical formula || Bond length <BR> d(H−X) / pm<BR>(gas phase) || model || Dipole <BR> μ / D || Aqueous phase (acid) || Aqueous Phase pK<sub>a</sub> values

|-

| hydrogen fluoride<br>(fluorane) || HF || 68px || 45px || 1.86 || hydrofluoric acid || 3.1

|-

| hydrogen chloride<br>(chlorane) || HCl || 95px || 70px|| 1.11 || hydrochloric acid || −3.9

|-

| hydrogen bromide<br>(bromane) || HBr || 108px || 72px|| 0.788 || hydrobromic acid || −5.8

|-

| hydrogen iodide<br>(iodane) || HI || 110px || 84px|| 0.382 || hydroiodic acid || −10.4

|-

| hydrogen astatide<br>astatine hydride<br>(astatane) || HAt ||140px ||84px || &minus;0.06 || hydroastatic acid ||

|-

| hydrogen tennesside<br>tennessine hydride<br>(tennessane) || HTs || 160px || 105x105px|| −0.24 ? || hydrotennessic acid ||

|-

|}

Comparison to hydrohalic acids

The hydrogen halides are diatomic molecules with no tendency to ionize in the gas phase (although liquified hydrogen fluoride is a polar solvent somewhat similar to water). Thus, chemists distinguish hydrogen chloride from hydrochloric acid. Hydrogen chloride is a gas 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 normal distillation. Often, the names of the acid and the molecules are not clearly distinguished, and in lab jargon, "HCl" often means hydrochloric acid, not the gaseous hydrogen chloride.

Occurrence and production

Hydrogen fluoride, chloride, and bromide are volcanic gases.

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. Hydrogen fluoride is a byproduct of the production of phosphoric acid. Fluorine, chlorine, and bromine react with hydrogen gas to give HF, HCl, and HBr. These gases can also be produced by treatment of halide salts with sulfuric acid. The least stable hydrogen halide, HI, is produced less directly, by the reaction of iodine with hydrogen sulfide or with hydrazine.

:C<sub>2</sub>H<sub>4</sub> + HCl → CH<sub>3</sub>CH<sub>2</sub>Cl

See also

  • Pseudohalogen
  • Hypohalous acid
  • Group 13 hydrides
  • Group 14 hydrides
  • Group 15 hydrides
  • Group 16 hydrides

References