# Hydroxides

Metal hydroxide deposits: iron (III), copper (II), cobalt (II) and tin (II) hydroxide

Hydroxides are salt-like substances that contain hydroxide ions ([OH] - ) as negative lattice building blocks ( anions ). Soluble hydroxides such as sodium hydroxide and potassium hydroxide form strongly alkaline solutions (lyes) with water , which are known as caustic soda and potassium hydroxide . Less soluble hydroxides, e.g. B. barium hydroxide and calcium hydroxide , form weakly alkaline suspensions with water . The saturated solutions are called barite water and lime water . When these two substances come into contact with carbon dioxide , they become cloudy. In the chemical laboratory , metal hydroxides are usually produced by adding sodium or potassium hydroxide to salt solutions and then filtering off the precipitates, washing them and drying them in the air. In some cases, it is not the pure hydroxides that form, but rather oxide hydroxides, such as iron (III) oxide hydroxide, after the precipitation .

## Reaction equation

${\ displaystyle \ mathrm {metal oxide + water \ longrightarrow metal hydroxide}}$

Example:

${\ displaystyle \ mathrm {Na_ {2} O + H_ {2} O \ longrightarrow 2 \ NaOH}}$
Sodium oxide and water react to form sodium hydroxide .
${\ displaystyle \ mathrm {CaO + H_ {2} O \ longrightarrow Ca (OH) _ {2}}}$
Calcium oxide and water react to form calcium hydroxide .

## Structure of aqueous hydroxide solutions

In aqueous solution , the hydroxide ion is usually surrounded by four to five water molecules . Four water molecules are arranged around the oxygen atom of the OH - in such a way that they can each form a hydrogen bond to it (i.e. they point to the OH - with one of their hydrogen atoms ). These four water molecules are located approximately in a plane with the OH - ion, ie in a different geometry than the (like sp 3 - hybridization expected) approximately tetrahedral arrangement of the electron pairs in the water and H 3 O + . The OH - ion can also form a - albeit weak - hydrogen bond with its proton, so that the complexes [OH - (H 2 O) 4 ] and [OH - (H 2 O) 5 ] occur, depending on whether these trained or not. For this reason, hydroxides are often very voluminous and - unlike crystalline precipitation products - sediment only very slowly.

## Precipitation / formation of hydroxides

Precipitation of manganese (II) hydroxide with partial oxidation to brown manganese (III) or manganese (IV) oxide hydroxide by atmospheric oxygen

Metal hydroxides are formed in a certain pH range, which depends on the solubility product of the hydroxide and the concentration of the cation to be precipitated. For example, the following equation applies to a divalent metal ion:

${\ displaystyle \ mathrm {Me ^ {2 +} + 2 \ OH ^ {-} \ longrightarrow Me (OH) _ {2}}}$
The following illustration shows the precipitation pH range of various hydroxides, with the first pH value marking the beginning of precipitation and the second pH value marking the complete separation:
monovalent
Metal hydroxide pH range
AgOH   8.3-11.3
bivalent
Metal hydroxide pH range
Ca (OH) 2  12.4-13.9
Mg (OH) 2  9.6-11.1
Fe (OH) 2  8.3-9.8
Ni (OH) 2  8.1-9.6
Cd (OH) 2    8.1-9.6
Mn (OH) 2  7.9-9.4
Pb (OH) 2  7.2-8.7
Co (OH) 2    7.2-8.7
Zn (OH) 2  6.6-8.1
Be (OH) 2    5.7-7.2
Cu (OH) 2  5.1-6.6
Sn (OH) 2  2.4-3.9
trivalent
Metal hydroxide pH range
Cr (OH) 3    4.6-5.6
Al (OH) 3  3.8-4.8
Fe (OH) 3  2.2-3.2
Sb (OH) 3    0.9-1.9

Amphoteric hydroxides go back into solution at higher pH values. Example:

${\ displaystyle \ mathrm {Al (OH) _ {3} + OH ^ {-} \ longrightarrow \ [Al (OH) _ {4}] ^ {-}}}$

After precipitation, some metal hydroxides are oxidized by atmospheric oxygen to hydroxides with a higher oxidation number . Thus, manganese (II) hydroxide quickly to manganese (III) - or converted manganese (IV) oxide hydroxide which is in the oximetry utilizes for fixing oxygen. Similarly, freshly precipitated green iron (II) hydroxide is oxidized to brown iron (III) oxide hydroxide by the presence of atmospheric oxygen :

${\ displaystyle \ mathrm {4 \ Fe (OH) _ {2} + O_ {2} \ longrightarrow 4 \ FeO (OH) +2 \ H_ {2} O}}$

## Hydroxides in organic chemistry

In organic chemistry, hydroxide ions are used as nucleophiles . The reaction of suitable bromine or chloroalkanes with sodium hydroxide solution or potassium hydroxide solution yields alkanols and the corresponding alkali metal halide . As a reaction to compete with this substitution reaction , an elimination can also take place, which leads to alkenes .

## Individual evidence

1. a b Jander Blasius: Textbook of analytical and preparative inorganic chemistry , 14th edition, S. Hirzel, Stuttgart-Leipzig 1995.
2. ^ Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry , Springer-Verlag, 1972, pp. 147–148, ISBN 3-211-81060-9 .