Hydrogen halide
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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, or astatine): hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI).
General
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.
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 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.
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.
Hydrogen halides react differently than their haloacid form. For example, while hydrofluoric acid will react with glass/silicon dioxide, extremely pure and dry hydrogen fluoride will not react with perfectly dry silicon dioxide. The presence of water is thus necessary for the reaction of HF and SiO2 to take place.
Reaction of hydrochloride acid with transition metals will only yield only the hydrated form of their respective chlorides. Likewise, the presence of water may prevent the addition of HX to some unsaturated organic compounds.
Preparation
Hydrogen halides cannot be obtained from their respective hydrohalic acids, as they form an azeotrope with water. Likewise, the presence of water traces in a conc. hydrogen halide product will alter the properties of said product. In general, they can be obtained by reacting their corresponding elements, but this can be dangerous, so usually other routes are preferred, such as distillation from a mixture of MX and conc. H2SO4.
