are acids resulting from the chemical reaction of hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
with one of the halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
, and iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
), which are found in Group 17 (VII, VIIA) of the periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
Astatine is a radioactive chemical element with the symbol At and atomic number 85. It occurs on the Earth only as the result of decay of heavier elements, and decays away rapidly, so much less is known about this element than its upper neighbors in the periodic table...
is not included in the list because it is very rare, unstable and not found as the acid in substantial quantities. Hydrogen halides can be abbreviated as HX where H represents a hydrogen atom and X represents a halogen (fluorine, chlorine, bromine or iodine).
|| d(H−X) / pm
| μ / D
The debye is a CGS unit of electric dipole momentElectric dipole moment is defined as charge times displacement: Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10-10 statcoulomb10-10 statcoulomb is approximately 0.2083...
| hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas 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 . HF is widely used in the...
| hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
| hydrogen bromide
Hydrogen bromide is the diatomic molecule HBr. HBr is a gas at standard conditions. Hydrobromic acid forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions with the addition of a dehydration agent, but not by distillation. Hydrogen bromide and...
| hydrogen iodide
Hydrogen iodide is a diatomic molecule. Aqueous solutions of HI are known as iohydroic acid or hydroiodic acid, a strong acid. Gas and aqueous solution are interconvertible...
| hydrogen astatide
They are acids because of their ability to deprotonate in aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
thus yielding hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
). With the exception of HF, the halogen halides are strong acids, with acid strength increasing down the group. This results from strong hydrogen bonding in HF, which must be overcome before dissociation is possible, and the very small F−
ions which impose order on nearby water molecules and hydronium ions increasing free energy (making dissociation unfavorable). Below HCl however, there is no hydrogen bonding, the anions get larger, and bond enthalpy decreases meaning dissociation happens with increasing ease.
Alone of the hydrogen halides, hydrogen fluoride exhibts 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 is due to the increasing numbers of electrons in the molecules that in turn leads to an increase in the frequency and strength of Van der Waals forces.
The anhydrous hydrogen halides are all colourless gases at STP except for hydrogen fluoride, which boils at 19 °C due to strong hydrogen bonding. Concentrated hydrohalic acid solutions produce visible white fumes . This occurs because the hydrohalic acids react with water vapor to form tiny droplets of their concentrated aqueous solutions and dissociate, as follows:
- HX (g) + (n+1) H2O (g) → H3O+ (aq) + X− (aq) + n H2O (l)
is large or small depends on the humidity, with n
approaching infinity as the humidity approaches 100%.
This phenomenon should not be confused with the dense white fumes formed when the hydrogen halides react with concentrated ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, forming ammonium halides:
- HX (g) + NH3 (g) → NH4X (s)