amphoteric oxides (having dual properties) - in most cases, metal oxides, which have a small electronegativity.Depending on external conditions exhibit any acidic oxide or properties.These are formed of transition metal oxides, which typically 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 form the salts of these same acids.Reactions of this type are a manifestation of the properties of the basic type.For example: ZnO (zinc oxide) + H2SO4 (hydrochloric acid) → ZnSO4 (zinc sulfate) + H2O (water).
2. When interacting with strong alkalis amphoteric oxides, hydroxides exhibit acidic properties.This duality of properties (ie amphoteric) is manifested in the formation of the two salts.
In the melt by reaction with an alkali salt produced average common example:
ZnO (zinc oxide) + 2NaOH (sodium hydroxide) → Na2ZnO2 (normal 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).The solution
amphoteric oxides by reaction with an alkali to form a complex salt, for example: Al2O3 (aluminum oxide) + 2NaOH (Sodium hydroxide) + 3H2O (water) + 2Na (Al (OH) 4) (complex tetragidroksoalyuminat sodium salt).
3. Each metal of any amphoteric oxide has a coordination number.For example to zinc (Zn) - 4, aluminum (Al) - 4 or 6, for the 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?
Relatively speaking, an amphoteric element can exhibit properties of both metals and non-metals.This feature is present in the elements of A-groups: Be (beryllium), Ga (gallium), Ge (germanium), Sn (tin), Pb, Sb (antimony), Bi (bismuth), and some others, as well as many of the elements B-groups - a Cr (chromium), Mn (manganese), Fe (iron), Zn (zinc), Cd (cadmium) and others.
prove the following chemical reactions amphoteric chemical element zinc (Zn):
1. Zn (OH) 2 (zinc hydroxide) + N2O5 (dinitrogen 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 (dioksotsinkat sodium) + H2O (water).
ZnO (zinc oxide) + 2NaOH (Sodium hydroxide) = Na2ZnO2 (dioksotsinkat sodium) + H2O (water).
In that case, if the element with the dual properties of the compound has the following oxidation, its dual (amphoteric) properties most noticeably appears in an intermediate oxidation step.
As example is chromium (Cr).This element has the following oxidation states: 3+ 2+ 6+.In the case of three basic and acidic properties are expressed approximately equally in while Cr +2 basic properties prevail, and in Cr +6 - acid.That response proves this:
Cr + 2 → CrO (chromium oxide +2), Cr (OH) 2 → CrSO4;
Cr + 3 → Cr2O3 (chromium oxide +3), Cr (OH) 3 (chromium hydroxide) → KCrO2 or chromium sulphate Cr2 (SO4) 3;
Cr + 6 → CrO3 (chromium oxide +6), H2CrO4 → K2CrO4.
Most amphoteric oxides of chemical elements in oxidation state +3 exist in the meta form.As an example, we can bring, aluminum metahydroxide (chem. Formula AlO (OH) and metahydroxide iron (chemical. Formula is FeO (OH)).
How to get amphoteric oxides?
1. The most convenient method for their preparation is precipitation from watersolution using ammonium hydroxide, that is a weak base. For example:
Al (NO3) 3 (aluminum nitrate) + 3 (H2OxNH3) (aqueous ammonia hydrate) = Al (OH) 3 (amphoteric oxide) + 3NH4NO3 (reaction performed attwenty degrees of heat).
Al (NO3) 3 (aluminum nitrate) + 3 (H2OxNH3) (aqueous solution of ammonium hydroxide) = AlO (OH) (amphoteric oxide) + 3NH4NO3 + H2O (reaction performed at 80 ° C)
Whenin this exchange reaction of this type in the case of excess alkali aluminum hydroxide will not precipitate. This is due to the fact that the 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 this type of reaction:
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 that are thus formed, are complex.They include complex anions: (Al (OH) 4) - and (Zn (OH) 4) 2-.That's called these salts: Na (Al (OH) 4) - tetragidroksoalyuminat sodium, Na2 (Zn (OH) 4) - sodium tetragidroksotsinkat.The reaction products of aluminum or zinc oxides with alkali firm called differently: NaAlO2 - sodium dioksoalyuminat and Na2ZnO2 - sodium dioksotsinkat.
