chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, hydrogen halides (hydrohalic acids when in the

aqueous An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in wat ...

phase) are

diatomic Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear mol ...

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

s that function as Arrhenius acids. The formula is HX where X is one of the halogens:

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...

bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...

, iodine,

astatine Astatine is a chemical element; it has Symbol (chemistry), symbol At and atomic number 85. It is the abundance of elements in Earth's crust, rarest naturally occurring element in the Earth's crust, occurring only as the Decay chain, decay product ...

, or tennessine. All known hydrogen halides are gases at standard temperature and pressure.The Acidity of the Hydrogen Halides. (2020, August 21). Retrieved May 5, 2021, from https://chem.libretexts.org/@go/page/3699

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. The former is a gas at room temperature that reacts with water to give the acid. Once the acid has formed, the diatomic molecule can be regenerated only with difficulty, but not by normal distillation. Commonly the names of the acid and the molecules are not clearly distinguished such that in lab jargon, "HCl" often means hydrochloric acid, not the gaseous hydrogen chloride.

Occurrence

Hydrogen chloride, in the form of hydrochloric acid, is a major component of gastric acid. Hydrogen fluoride, chloride and bromide are also

volcanic gas Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (Vesicular texture, vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from ...

es.

Synthesis

The direct reaction of hydrogen with fluorine and chlorine gives hydrogen fluoride and hydrogen chloride, respectively. Industrially these gases are, however, produced by treatment of halide salts with

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

. Hydrogen bromide arises when hydrogen and bromine are combined at high temperatures in the presence of a platinum catalyst. The least stable hydrogen halide, HI, is produced less directly, by the reaction of iodine with

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

or with hydrazine.

Physical properties

The hydrogen halides are colourless gases at standard conditions for temperature and pressure (STP) except for hydrogen fluoride, which boils at 19 °C. Alone of the hydrogen halides, hydrogen fluoride exhibits

hydrogen bonding In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

between molecules, and therefore has the highest melting and boiling points of the HX series. From HCl to HI the boiling point rises. This trend is attributed to the increasing strength of intermolecular van 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 concentrated aqueous solutions of the hydrohalic acid.

Reactions

Upon dissolution in water, which is highly exothermic, the hydrogen halides give the corresponding acids. These acids are very strong, reflecting their tendency to ionize in aqueous solution yielding hydronium ions (H₃O⁺). With the exception of hydrofluoric acid, the hydrogen halides are strong acids, with acid strength increasing down the group. Hydrofluoric acid is complicated because its strength depends on the concentration owing to the effects of homoconjugation. As solutions in non-aqueous solvents, such as acetonitrile, the hydrogen halides are only modestly acidic however. Similarly, the hydrogen halides react with

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

(and other bases), forming ammonium halides: :HX + NH₃ → NH₄X In organic chemistry, the hydrohalogenation reaction is used to prepare halocarbons. For example, chloroethane is produced by hydrochlorination of ethylene:M. Rossberg et al. "Chlorinated Hydrocarbons" in ''Ullmann’s Encyclopedia of Industrial Chemistry'', 2006, Wiley-VCH, Weinheim. :C₂H₄ + HCl → CH₃CH₂Cl

References