Amphoteric oxides (having dual properties)- this is in most cases metal oxides, which have a small electronegativity. Depending on the external conditions, they exhibit either acidic or oxide properties. These oxides are formed by transition metals, which usually exhibit the following oxidation states: ll, lll, lV.
Examples of amphoteric oxides:zinc oxide (ZnO), chromium oxide lll (Cr2O3), aluminum oxide (Al2O3), tin oxide ll (SnO), tin oxide lV (SnO2), lead oxide ll (PbO), lead oxide lV (PbO2), titanium oxide lV (TiO2), manganese oxide lV (MnO2), iron oxide lll (Fe2O3), beryllium oxide (BeO).
Reactions typical of amphoteric oxides:
1.These oxides can react with strong acids. In this case, the salts of these acids are formed. Reactions of this type are manifestations of the properties of the basic type. For example: ZnO (zinc oxide) + H2SO4 (hydrochloric acid) → ZnSO4 (zinc sulphate) + H2O (water).
2. When interacting with strong alkalis, amphoteric oxides and hydroxides exhibit acidic properties. In this case, the duality of properties (i.e., amphotericity) is manifested in the formation of two salts.
In the melt, in the reaction with alkali, an average salt is formed, for example:
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) → Na2ZnO2 (common average salt) + H2O (water).
Al2O3 (alumina) + 2NaOH (sodium hydroxide) = 2NaAlO2 + H2O (water).
2Al (OH) 3 (aluminum hydroxide) + 3SO3 (sulfur oxide) = Al2 (SO4) 3 (aluminum sulfate) + 3H2O (water).
In solution, amphoteric oxides in reaction withalkali form a complex salt, for example: Al2O3 (aluminum oxide) + 2NaOH (sodium hydroxide) + 3H2O (water) + 2Na (Al (OH) 4) (sodium tetrahydroxoaluminate complex salt).
3. Each metal of any amphoteric oxide has its coordination number. For example: for zinc (Zn) - 4, for aluminum (Al) - 4 or 6, for chromium (Cr) - 4 (rare) or 6.
4. Amphoteric oxide does not react with water and does not dissolve in it.
What reactions prove amphoteric metal?
Conventionally, an amphoteric element canexhibit the properties of both metals and non-metals. A similar characteristic is present in the elements of the A-groups: Be (beryllium), Ga (gallium), Ge (germanium), Sn (tin), Pb, Sb (antimony), Bi (bismuth) and some others, -groups are Cr (chrome), Mn (manganese), Fe (iron), Zn (zinc), Cd (cadmium) and others.
Let us prove by the following chemical reactions the amphotericity of the chemical element of zinc (Zn):
1. Zn (OH) 2 (zinc hydroxide) + N2O5 (diazotene pentoxide) = Zn (NO3) 2 (zinc nitrate) + H2O (water).
ZnO (zinc oxide) + 2HNO3 (nitric acid) = Zn (NO3) 2 (zinc nitrate) + H2O (water).
b) Zn (OH) 2 (zinc hydroxide) + Na2O (sodium oxide) = Na2ZnO2 (sodium dioxocincate) + H2O (water).
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) = Na2ZnO2 (sodium dioxocinate) + H2O (water).
In the case when an element with dualproperties in the compound has the following degrees of oxidation, its dual (amphoteric) properties are most noticeable in the intermediate stage of oxidation.
As an example, you can bring chromium (Cr).This element has the following oxidation states: 3+, 2+, 6+. In the case of +3, the basic and acidic properties are approximately the same, while Cr +2 is dominated by the main properties, and Cr +6 is acidic. Here are the reactions proving this statement:
Cr + 2 → CrO (chromium oxide +2), Cr (OH) 2 → CrSO4;
Cr + 3 → Cr2O3 (chromium oxide +3), Cr (OH) 3 (chromium hydroxide) → KCrO2 or chromium sulfate Cr2 (SO4) 3;
Cr + 6 → CrO3 (chromium oxide +6), H2CrO4 → K2CrO4.
In most cases, amphoteric oxideschemical elements with a degree of oxidation of +3 exist in the meta form. As an example, we can give: aluminum metahydroxide (chemical formula AlO (OH) and metahydroxide of iron (chemical formula FeO (OH)).
How do they get amphoteric oxides?
1. The most convenient method for obtaining them is precipitation from an aqueous solution using ammonia hydrate, that is, a weak base. For example:
Al (NO3) 3 (aluminum nitrate) + 3 (H2OxNH3) (aqueous solution of ammonia hydrate) = Al (OH) 3 (amphoteric oxide) + 3NH4NO3 (the reaction is performed at twenty degrees of heat).
Al (NO3) 3 (aluminum nitrate) + 3 (H2OxNH3) (aqueous solution of ammonia hydrate) = AlO (OH) (amphoteric oxide) + 3NH4NO3 + H2O (reaction is carried out at 80 ° C)
In this case, in the exchange reaction of this type in the caseAluminum hydroxide will not precipitate. This is due to the fact that aluminum passes into the anion because of its dual properties: Al (OH) 3 (aluminum hydroxide) + OH- (excess alkali) = [Al (OH) 4] - (anion of aluminum hydroxide).
Examples of reactions of this type:
Al (NO3) 3 (aluminum nitrate) + 4NaOH (excess sodium hydroxide) = 3NaNO3 + Na (Al (OH) 4).
ZnSO4 (zinc sulphate) + 4NaOH (excess sodium hydroxide) = Na2SO4 + Na2 (Zn (OH) 4).
Salts, which are formed in this case, refer tocomplex compounds. They include the following complex anions: (Al (OH) 4) - and more (Zn (OH) 4) 2-. So these salts are called: Na (Al (OH) 4) - sodium tetrahydroxoaluminate, Na2 (Zn (OH) 4) - sodium tetrahydroxocincate. The products of interaction of aluminum or zinc oxides with alkali hard are called differently: NaAlO2 - sodium dioxoaluminate and Na2ZnO2 - sodium dioxocincate.
