Amphoteric oxides (having dual properties) are in most cases metal oxides that are slightly electronegative. Depending on 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 characteristic of amphoteric oxides:
1. These oxides can react with strong acids. In this case, salts of the same acids are formed. Reactions of this type are a manifestation of the properties of the main type. For example: ZnO (zinc oxide) + H2SO4 (hydrochloric acid) → ZnSO4 (zinc sulfate) + 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, during reaction with alkali, an ordinary ordinary salt is formed, for example:
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) → Na2ZnO2 (ordinary middle salt) + H2O (water).
Al2O3 (aluminum oxide) + 2NaOH (sodium hydroxide) = 2NaAlO2 + H2O (water).
2Al (OH) 3 (aluminum hydroxide) + 3SO3 (sulfur oxide) = Al2 (SO4) 3 (aluminum sulfate) + 3H2O (water).
In a solution, amphoteric oxides, when reacted with alkali, 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 own coordination number. For example: for zinc (Zn) - 4, for aluminum (Al) - 4 or 6, for chromium (Cr) - 4 (rarely) or 6.
4. Amphoteric oxide does not react with water and does not dissolve in it.
What reactions prove amphoteric metal?
Relatively speaking, an amphoteric element can exhibit the properties of both metals and non-metals. A similar characteristic feature 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, as well as many elements of B -groups are Cr (chromium), Mn (manganese), Fe (iron), Zn (zinc), Cd (cadmium) and others.
Let us prove the following chemical reactions amphotericity of the chemical element of zinc (Zn):
1. Zn (OH) 2 (zinc hydroxide) + N2O5 (diazotent 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 dioxozincate) + H2O (water).
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) = Na2ZnO2 (sodium dioxozincate) + H2O (water).
In the event that an element with dual properties in a compound has the following oxidation states, its dual (amphoteric) properties are most noticeable in the intermediate stage of oxidation.
An example is chromium (Cr). This element has the following oxidation states: 3+, 2+, 6+. In the case of +3, the basic and acidic properties are expressed approximately to the same extent, while in Cr +2 the basic properties prevail, while in Cr +6 the acidic properties. 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 oxides of chemical elements with an oxidation state of +3 exist in meta-form. As an example, we can cite: aluminum metahydroxide (chemical formula AlO (OH) and iron metahydroxide (chemical formula FeO (OH)).
How to get amphoteric oxides?
1. The most convenient method for their preparation is precipitation from an aqueous solution using ammonia hydrate, that is, a weak base. For instance:
Al (NO3) 3 (aluminum nitrate) + 3 (H2OxNH3) (aqueous ammonia hydrate) = Al (OH) 3 (amphoteric oxide) + 3NH4NO3 (the reaction is carried out at twenty degrees of heat).
Al (NO3) 3 (aluminum nitrate) + 3 (H2OxNH3) (aqueous ammonia hydrate) = AlO (OH) (amphoteric oxide) + 3NH4NO3 + H2O (reaction is carried out at 80 ° C)
Moreover, in an exchange reaction of this type, in the case of an excess of alkalis , aluminum hydroxide will not precipitate. This is due to the fact that aluminum passes into the anion due to its dual properties: Al (OH) 3 (aluminum hydroxide) + OH− (excess alkali) = [Al (OH) 4] - (aluminum hydroxide anion).
Examples of reactions of this type:
Al (NO3) 3 (aluminum nitrate) + 4NaOH (excess sodium hydroxide) = 3NaNO3 + Na (Al (OH) 4).
ZnSO4 (zinc sulfate) + 4NaOH (excess sodium hydroxide) = Na2SO4 + Na2 (Zn (OH) 4).
Salts, which are formed in this case, are complex compounds. They include the following complex anions: (Al (OH) 4) - and also (Zn (OH) 4) 2−. So these salts are called: Na (Al (OH) 4) - sodium tetrahydroxoaluminate, Na2 (Zn (OH) 4) - sodium tetrahydroxozincate. The products of the interaction of aluminum or zinc oxides with solid alkali are called differently: NaAlO2 - sodium dioxoaluminate and Na2ZnO2 - sodium dioxozincate.