In
chemistry,
hydroxide is the name for the
diatomic anion OH
−, consisting of
oxygen and
hydrogen atoms, usually derived from the
dissociation of a
base. It is one of the simplest diatomic ions known.
Inorganic compounds that contain the
hydroxyl group are referred to as hydroxides. Common hydroxides include:
Hydroxide as a base
Most compounds containing hydroxide are bases.
An
Arrhenius base is a substance that produces hydroxide ions when dissolved in
aqueous solution. One example would be
ammonia, NH
3:
NH
3(g) + H
2O(l)
NH4+(aq) + OH
−(aq)
Thus, hydroxide ions are heavily involved in
acid-base reactions as well as the special double displacement reaction
Salts containing hydroxide are called
base salts. Base salts will dissociate into a cation and one or more hydroxide ions in
water, making the solution basic. Base salts will undergo
neutralisation reactions with
acids. In general
acid-alkali reactions can be simplified to
by omitting
spectator ions.
Solubility
Most inorganic hydroxide salts are
insoluble in water, except for those with cations from
Group I,
Ba2+,
Sr2+,
Ca2+ (slightly) or
Tl+.
Applications
Hydroxides and hydroxide ions are relatively common. Many useful chemicals and chemical processes involve hydroxides or hydroxide ions.
Sodium hydroxide (lye) is used in industry as a strong base,
potassium hydroxide is used in agriculture, and
iron hydroxide minerals such as
goethite and
limonite have been used as low grade
brown iron ore. The
aluminium ore
bauxite is composed largely of aluminium hydroxides.
Ligand
The hydroxide ion is a kind of
ligand. It donates
lone pairs of electrons, behaving as a
Lewis base. Examples of complexes containing such a ligand include the aluminate ion [Al(OH)
4]
− and aurate ion [Au(OH)
4]
−.
Hydrated hydroxide ion H3O2−
The mono-hydrated hydroxide ion,H
3O
2−, the bihydroxide ion, has been found in the crystals of a small number of compounds. First found in 1979,in the salt Na
2[N(C
2H
5)
3CH
3][Cr(PhC(S)=N-(O))
3].½NaH
3O
2.18H
2O The H
3O
2− ion in this compound is centrosymmetric and has a very short
hydrogen bond, (114.5 pm), which is similar to the length (114 pm) in the
bifluoride, HF
2− ion.
See also
