There are hydroxides that interact with both acids and bases, depending on the conditions. These compounds, showing a dual nature, are called amphoteric hydroxides. They are formed by a metal cation and a hydroxide ion, like all bases. The ability to act as acid and base is possessed only by those hydroxides that contain such metals in their composition: Be, Zn, Al, Pb, Sn, Ga, Cd, Fe, Cr (ะะะ), etc. As can be seen from Periodic System 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, powdered, fine crystalline substances that are most often white in color, do not dissolve in water, and poorly conduct current (weak electrolytes). However, some of these bases may dissolve in acids and alkalis. Dissociation of โdual compoundsโ in aqueous solutions occurs according to the type of acids and bases. This is due to the fact that the holding force between the metal and oxygen atoms (Me โ O) and between the oxygen and hydrogen atoms (O โ H) is practically equal, i.e. Me - O - N. Therefore, these bonds will be broken simultaneously, and these substances dissociate into H + cations and OH - anions.
Amphoteric hydroxide - Be (OH) 2 will help confirm the dual nature of these compounds. Consider the interaction of beryllium hydroxide with acid and base.
1. Be (OH) 2 + 2HCl โBeCl 2 + 2H 2 O.
2. Be (OH) 2 + 2KOH - K 2 [Be (OH) 4 ] - potassium tetrahydroxoberylate.
In the first case, a neutralization reaction takes place, the result of which is the formation of salt and water. In the second case, the reaction product is a complex compound. The neutralization reaction is typical for all hydroxides without exception, but interaction with similar ones is characteristic only for amphoteric ones. Other amphoteric compounds โ oxides and the metals themselves with which they are formed โ will also exhibit such dual properties.
Other chemical properties of such hydroxides will be characteristic of all bases:
1. Thermal decomposition, reaction products - the corresponding oxide and water: Be (OH) 2 โBeO + H 2 O.
2. The neutralization reaction with acids.
3. Reacting with acid oxides.
You also need to remember that there are substances with which amphoteric hydroxides do not interact, i.e. the chemical reaction does not go, it:
- non-metals;
- metals;
- insoluble bases;
- amphoteric hydroxides.
- medium salts.
These compounds are prepared by alkali precipitation of the appropriate salt solutions:
BeCl 2 + 2KOH - Be (OH) 2 + 2KCl.
The salts of some elements during this reaction form a hydrate, the properties of which almost completely correspond to those of hydroxides with a dual nature. The bases themselves 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.).