There are hydroxides that react with both acids and bases, depending on the conditions. These compounds exhibiting a dual nature are called amphoteric hydroxides. They are formed by a metal cation and a hydroxide ion, like all bases. Only those hydroxides that contain the following metals in their composition have the ability to act as acids and bases: Be, Zn, Al, Pb, Sn, Ga, Cd, Fe, Cr (III), etc. As can be seen from the Periodic system of D. AND. Mendeleev, hydroxides with a dual nature form metals that are closest to non-metals. It is believed that such elements are transitional forms, and the division into metals and non-metals is rather arbitrary.
Amphoteric hydroxides are solid powdery finely crystalline substances, which most often have a white color, do not dissolve in water and poorly conduct current (weak electrolytes). However, some of these bases can dissolve in acids and alkalis. The dissociation of "dual compounds" in aqueous solutions occurs according to the type of acids andgrounds. This is due to the fact that the retention force between metal and oxygen atoms (Me-O) and between oxygen and hydrogen atoms (O-H) is practically equal, i.e. Me - O - N. Therefore, these bonds will break simultaneously, and these substances will dissociate into H + cations and OH- anions.
The amphoteric hydroxide - Be(OH)2 will help confirm the dual nature of these compounds. Consider the interaction of beryllium hydroxide with an acid and a base.
1. Be(OH)2+ 2HCl –BeCl2+2H2O.
2. Be(OH)2 + 2KOH – K2 [Be(OH)4] – potassium tetrahydroxoberyllate.
In the first case, a neutralization reaction takes place, the result of which is the formation of s alt and water. In the second case, the reaction product will be a complex compound. The neutralization reaction is typical for all hydroxides without exception, but the interaction with their own kind is typical only for amphoteric ones. Such dual properties will be exhibited by other amphoteric compounds - oxides and the metals themselves, with which they are formed.
Other chemical properties of such hydroxides will be characteristic of all bases:
1. Thermal decomposition, reaction products - the corresponding oxide and water:
2. Neutralization reaction with acids.
3. Reaction with acid oxides.
You also need to remember that there are substances with which amphoteric hydroxides do notinteract, i.e. there is no chemical reaction, this is:
- non-metals;
- metals;
- insoluble bases;
- amphoteric hydroxides.
- medium s alts.
These compounds are obtained by alkali precipitation of the corresponding s alt solutions:
BeCl2 + 2KOH – Be(OH)2+ 2KCl.
S alts of some elements in the course of this reaction form a hydrate, the properties of which almost completely correspond to those of hydroxides with a dual nature. The very same bases with dual properties are part of the minerals, in the form of which they are found in nature (bauxite, goethite, etc.).
Thus, amphoteric hydroxides are inorganic substances, which, depending on the nature of the substance that reacts with them, can act as bases or as acids. Most often they correspond to amphoteric oxides containing the corresponding metal (ZnO-Zn(OH)2; BeO - Be(OH)2), etc. e.).
