Amphoteric Oxides

Oxides can be classified as acidic, basic, amphoteric or neutral. An amphoteric oxide is one that can act as either an acid or a base. Generally non-metallic oxides are acidic and metallic oxides are basic. Some non-metallic oxides are neutral and some metallic or semi metallic oxides are amphoteric. There are a number of exceptions to these generalizations.

What are Amphoteric Oxides?

Amphoteric oxides are the oxides that behave as both acidic and basic oxides. Amphoteric Oxides have features of acidic as well as basic oxides that neutralize both acids and bases.”

Amphoteric oxides dissolve in water to form alkaline solutions. Alkaline solutions contain hydroxide ions. Thus aluminium oxide (Al2O3) reacts with hydrochloric acid to form aluminium chloride and water. With sodium hydroxide solution it forms sodium aluminate (NaAlO2) and water. Other common examples of amphoteric oxides include ZnO, SnO and PbO.

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How to Identify Amphoteric Oxides?

Oxides are compounds of metals or nonmetals with oxygen. There are four types of oxide. The difference between these four 0types by their chemical reactions. Amphoteric oxides are classified as metal oxides that react with both acids as well as bases to create salts and water. Amphoteric oxides, among many others, include lead oxide and zinc oxide. Examples include amino acids and proteins that have classes of amine and carboxylic acids and molecules that can be self-ionized, such as water.

Amphoteric oxides are oxygen compounds that show both acidic and basic characteristics. These oxides undergo a neutralisation reaction to form water and salt as they react with acid. This demonstrates the essential properties of the compounds. Similarly, the alkali reacts to the formation of salt and water, demonstrating an acidic property. Example: oxide of aluminium.

All of the oxides can be made by heating the element in oxygen. Reaction of aqueous solutions of the metal trihalides with hydroxide gives the oxides in hydrated form. Going down the group, there is a transition from acidic oxides, through amphoteric to basic owing to increasing metallic character of the elements concerned.

Oxides of the Group 13 elements

Oxides

Properties

B2O3

Weakly acidic

Al2O3

Amphoteric

Ga2O3

Amphoteric

In2O3

Weakly base

Tl2O3

Basic, oxidizing

Amphoteric Oxides Examples

The word amphoteric means both of them acid and base. Amphoteric oxides have both acidic and basic properties. The oxides of aluminium and zinc are examples. They form salts when they react with acids. They also react with alkalis to form complex salts.

Examples:

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

ZnO(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2O(l)

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

Al2O3(s) + 2NaOH(aq) → 2NaAlO2(aq) + H2O(l)

The zincates and aluminates have the ending -ate to show that their ions are compound ions containing oxygen – rather like sulfates, carbonates and nitrates, indicating ions are ZnO22- and aluminate ions are AlO3. Ions are written as Zn(OH)42- and Al(OH)4. Note that sodium zincate and aluminate are soluble in water.

Amphoteric Oxides in Periodic Table

In a given period the oxides progress from strongly basic, through weakly basic, amphoteric and weakly acidic to strongly acidic eg. Na2O, MgO, Al2O3, P4O10, SO3, Cl2O7. Acidity also increases with increasing oxidation state, eg. MnO < Mn2O3 < Mn2O7.

There is a trend in oxide acidity across the periodic table. Generally the trend across a period is basic → amphoteric → acidic. The following table shows the trend in oxide acidity for period 3.

Na2O

MgO

Al2O3

SiO2

P4O18

SO2

Cl2O7

Strongly basic

Basic

Amphoteric

Weakly acidic

Acidic

Acidic

Strongly acidic

The most basic oxides are found near the bottom of Groups I and II. Basicity tends to increase down a periodic group. Thus for Group V the acidity of the oxides are NO2(acidic), P2O3(acidic), As2O3(amphoteric). Sb2O3(amphoteric), Bi2O3 (basic).

In the main groups of elements, basicity of oxides increases with increase in atomic number down the group, eg. BeO < MgO < CaO < SrO < BaO, though the trend is reversed in later transition elements groups.

Frequently Asked Questions on Amphoteric Oxides

How do you identify amphoteric oxides?

Any oxides react, that is, they are amphoteric, with both acids and bases. Amphoteric oxide, for instance, is Al2O3. Amphoteric oxides are found in the lighter elements of Groups 2 and 13, some of the d-block elements, and the heavier elements of Groups 14 and 15.

How are amphoteric oxides formed?

An amphoteric oxide is an oxide that can act either as an acid or as a base to create salt and water in a reaction. The oxidation states open to a chemical species are based on amphoterism. Amphoteric oxides and hydroxides are formed when metals have several oxidation states.

What are amphoteric oxides examples?

Amphoteric oxides are classified as metal oxides that react with both acids as well as bases to create salts and water. Amphoteric oxides, among many others, include lead oxide and zinc oxide. Amphiprotic molecules, which can either donate or accept a proton (H+), are one sort of amphoteric species.

Is HCl amphoteric?

The idea of an acid and a base can be well understood from the Bronsted-Lowry idea of Amphoteric compounds. HCl and Cl form a conjugate acid-base pair in example 1 seen above since they vary by a proton. Similarly, another conjugate acid-base pair also forms NH4+ and NH3 in example 2.

Which is more basic in nature?

The basic existence of amines relies on the supply of a single nitrogen pair for donation. The electron releasing group such as -CH3 increases the amine basicity by increasing the density of electrons over nitrogen, which promotes the donation of lone electron pairs. (CH3)2NH is then the most simple.

Are basic oxides soluble in water?

In water, basic oxides are normally insoluble. Many that dissolve form alkaline solutions in water.

Which element reacts with oxygen to a basic solution?

For alkali metals, oxygen is very reactive. The alkali name is given to alkali metals because the oxides of these metals react to form a simple or alkaline metal hydroxide with water. Lithium produces oxide, sodium produces peroxide, and superoxide is formed by potassium, cesium, and rubidium.

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