Amphoteric Oxides

The term “Amphoteric” is derived from the Greek word amphoteroi (ἀμφότεροι), which means “both”. Amphoteric oxides are oxides of elements (usually metals) that show amphoteric behaviour.

Amphoterism or amphoteric behaviour is the property of a compound to act as both an acid and as a base. Depending upon reactants and other reaction conditions, the compound behaves either like acid or as a base.

Example of amphoteric oxides: Aluminium Oxide Al₂O₃ is an amphoteric oxide because it can neutralise both HCl and NaOH.

Other elements which form amphoteric oxides are gallium, indium, scandium, titanium, zirconium, vanadium, chromium, tin, iron, cobalt, copper, zinc, lead, silver, gold, germanium, antimony, bismuth, beryllium and tellurium.

Synthesis

Since, amphoteric oxides are oxides of certain elements, therefore they are a direct result of burning those elements in air. For example Zinc(II) oxide is produced as a result of direct combustion of Zn in plenty of supply of air:

2Zn(s)  +   O₂(g) -> 2ZnO(s)

Similarly,

4Al(s)  +  3O₂(g) ->  2Al₂O₃(s)

Lead (II) oxide is produced by heating Lead (II) Nitrate:

2Pb(NO₃)₂ -> \(\Delta\) 2PbO(s)   +   4NO₂  +  O₂(g)

Tin(II) oxide also known as Stannous Oxide could be produced by heating Tin(II) Oxide in an inert environment. Tin(II) Oxide is formed when Tin salts react with alkali such as Sodium Hydroxide.

SnCl₂  +  2NaOH -> Sn(OH)₂   +  2NaCl

Sn(OH)₂ -> \(\Delta\) SnO + H₂O

Amphoteric Oxides in Periodic Table

• In a given period of the periodic table, the oxides progress from basic to amphoteric and finally to acidic. For example consider 3^rd period elements: Na₂O and MgO are basic , Al₂O₃ is amphoteric, SiO₂ is weakly acidic, P₂O₅ , P₂O₃, SO₂, SO₃ , Cl₂O and Cl₂O₇ are acidic.
• Down the groups, basicity increases with increasing atomic number.
  Consider group 15 oxides:
  NO₂ and P₂O₃ are acidic, As₂O₃ and Sb₂O₃ are amphoteric , Bi₂O₃ is basic.
• Acidity also increases with an increase in the oxidation number of the same elemental oxide. Example: Acidity series for manganese oxides is
  Mn₂O₇ > Mn₂O₃ > MnO
  Similarly, for Arsenic and Antimony,
  As₂O₅ > As₂O₃
  and
  Sb₂O₅ > Sb₂O₃

Physical Properties of Amphoteric Oxides

Amphoteric oxides usually have very high melting and boiling points. They have giant covalent structures which require a lot of energy to break.

Amphoteric oxides are insoluble in water.

Chemical Properties of Amphoteric Oxides

Amphoteric oxides react with both acids and bases.

Basic Nature

The basic nature of amphoteric oxides is made clear by their reaction with acids. Amphoteric oxides react with common inorganic acids to form metal salt and water.

ZnO(s) + 2HNO₃(aq) -> Zn(NO₃)₂(aq) + H₂O(l)

Al₂O₃(s) + 6HCl(aq) -> 2AlCl₃(aq) + 3H₂O(l)

SnO(s) + 2HCl(aq) -> SnCl₂(aq) + H₂O(l)

PbO(s) + 2HNO₃(aq) -> Pb(NO₃)₂(aq) + H₂O(l)

As₂O₃(s) + 6HCl(aq) -> 2AsCl(aq) + 3H₂O(l)

Acidic Nature

The acidic nature of amphoteric oxides is made clear by their reaction with bases. Amphoteric oxides react with common inorganic bases to form complex metal salts and water.

ZnO(s) + 2NaOH(aq) -> Na₂ZnO₂(aq) + H₂O(l)

Note that Na₂ZnO₂ is compounded with water in reality.

So the more accurate equation is:

ZnO(s) + 2NaOH(aq) + H₂O(l)  ->  Na₂[Zn(OH)₄](aq)

where Na₂[Zn(OH)₄] is hydrated sodium Zincate and the anion is [Zn(OH)₄]⁺² .

Al₂O₃(s) + 2NaOH(aq) ->  2NaAlO₂(aq) + H₂O(l)

or

Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) ->  2Na[Al(OH)₄](aq)

where Na[Al(OH)₄] is hydrated Sodium Aluminate and [Al(OH)₄]⁺¹ is the anion.

PbO(s) + 2NaOH(aq) + H₂O(l) -> Na₂[Pb(OH)₄](aq)

SnO(s) + 4NaOH(aq) + H₂O(l)  <=> Na₄[Sn(OH)₆](aq)

Identification of Amphoteric Oxides

An amphoteric oxide can neutralise both an acid and a base. So, in order to identify if a compound is amphoteric, its reaction with an acid such as HCl and reaction with a base such as NaOH need to be observed.

After confirming that the given substance is amphoteric, we need to find out if the given substance is an oxide or not. Tests for other anions like Sulphate, Nitrate, Nitrite, Chloride etc., are done. If all tests show negative results, the given compound is an oxide.

Uses and Applications

Common uses of some amphoteric oxides are:

• Lead(II) Oxide: Adding PbO to glass increases its electrical resistance, refractive index and ability to absorb X-Rays. At the same time, it decreases the viscosity of glass. Hence PbO is essential for the glassware industry.
  PbO is also used in ceramic industries to make ceramic articles that are magnetically and electrically inert.
• Zinc Oxide: Zinc Oxide is used extensively in the cement and cosmetic industries. It is also used to make dry cells and other types of permanent cells.
• Aluminium Oxide: Aluminium oxide ores such as bauxite are present in the Earth’s crust. First, these ores are concentrated to form pure Aluminium Oxide. This Aluminium Oxide is then used in the Hall-Héroult process to obtain pure aluminium metal.

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