Amphoteric Oxides

Oxides are metal or nonmetal substances that include oxygen. There are four different kinds of oxides. Metal oxides, amphoteric oxides are metal oxides that react with acids and bases to produce water and salts. Zinc oxide and lead oxide are two examples of amphoteric oxides. Proteins and amino acids, which belong to the carboxylic acid class, amine, and molecules that can be self-ionized, such as water, are examples.

What are Amphoteric Oxides?

Amphoteric oxides are oxides, which behave as both basic and acidic oxides. Amphoteric Oxides contain the features of acidic and basic oxides as well that neutralise both acids and bases. This is the simple amphoteric oxides meaning.

Amphoteric oxides dissolve in water to produce alkaline solutions. Alkaline solutions have hydroxide ions. Thus aluminium oxide (which is Al2O3) reacts with hydrochloric acid to form water and aluminium chloride. Aluminium oxide is an amphoteric oxide. With sodium hydroxide solution, it produces water and sodium aluminate (NaAlO2). The other common examples of amphoteric oxides include SnO, ZnO, and PbO. Al2O3 is an amphoteric oxide. The amphoteric oxide formula is Al2O3.

Identification of Amphoteric Oxides

Amphoteric oxides are oxygen compounds, which show both basic and acidic characteristics. These oxides undergo a neutralisation reaction to produce water and salt as they react with acid. This describes the important properties of the compounds. In the same way, the alkali reacts to the formation of both salt and water, demonstrating an acidic property. An example is an oxide of aluminium.

All of the oxides can be formed by heating the element in oxygen. The reaction of aqueous solutions of the metal trihalides with hydroxide forms the oxides in hydrated form. Going down the group, there is a transition from the acidic oxides, through amphoteric to basic, owing to the increasing metallic character of the elements that are concerned. The amphoteric oxide formula is Al2O3.

Oxides of the Group 13 Elements

Amphoteric Oxides Examples

The term amphoteric means both base and acid. Amphoteric oxides hold both acidic and basic properties. The oxides of zinc and aluminium are examples. When they react with acids, they form salts. Also, they react with alkalis to produce complex salts.

Amphoteric Oxides Examples

Let us look at some of the amphoteric oxides examples or amphoteric oxides list below:

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

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

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

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

These are a few amphoteric oxides listed. The aluminates and zincates have the ending -ate to represent that their ions are the compound ions containing oxygen – rather like carbonates, nitrates and sulphates, indicating ions are ZnO22- and the aluminate ions are AlO2–. Ions can be written as Al(OH)4– and Zn(OH)42-. It should be noted that aluminate and sodium zincate are soluble in water.

Amphoteric Oxides in Periodic Table

In the given period, the oxides progress from strongly basic, through the weakly basic oxides, amphoteric and weakly acidic to strongly acidic oxides.

Some examples are MgO, Na2O, P4O10, Al2O3, Cl2O7 and SO3. Also, acidity increases with an increasing oxidation state. For example, MnO < Mn2O3 < Mn2O7.

There exists a trend in oxide acidity across the periodic table. In general, the trend across a period - basic → amphoteric → acidic. The below table represents the trend in oxide acidity for period 3.

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

In the primary groups of amphoteric elements, the basicity of oxides increases with an increase in atomic number down the group, for example, BeO < MgO < CaO < SrO < BaO (BeO is amphoteric), though the trend is reversed in the later transition amphoteric elements groups.

Some Amphoteric Oxides and their Chemical Basis

The chemical reason is simple. For the substance to behave as a base in solution, it should break the H-OH bond somehow to release OH- ions while reacting with the proton produced on the dissociation.

For example, potassium oxide (K2O) produces 2K+ ions (which can not hydrolyze water molecules due to their low positive charge) and an O2- ion, which can break the H-OH bond by attracting the H atom of H2O and releasing OH- ions in solution. Thus, it is a basic oxide.

Now since the potassium contains only a +1 charge, it is not strongly attracted to the OH- of water, and hence we only see the reaction with water.

But if we take an example of the amphoteric oxide, say aluminium oxide, we observe that the aluminium carries a very high positive charge of +3 that makes it attracted to the hydroxyl group of water. Therefore, it can react with water to release protons instead of the Hydroxyl groups. The same is with the case of Chromium, Iron including all the other metals which form amphoteric oxides. If the metal carries a high positive charge, it will act as an acid and react with the hydroxyl group of water in a solution.

Therefore, basically, Al2O3 can accept OH- due to the presence of H+ and Al3+ because of the presence of O2-. Hence, it can act as both acid and base. If the solution is given as alkaline, the Al3+ reacts with the OH- (or with the lone pair if we go by the Lewis concept of acids and bases), acting as an acidic oxide. If the solution is acidic, the O2- will react with the protons in the solution and also act as a basic oxide.

Hence, we call Al2O3 an amphoteric oxide. Few other examples are given as Fe3O4, Fe2O3, PbO, ZnO, Cr2O3, PbO2, etc. The other interesting and important point is that most (but not all) amphoteric oxides belong to the d-block of the periodic table. Al2O3 is an amphoteric oxide.

Reason Behind Aluminium is Amphoteric

Aluminium contains an electronegativity of 1.5. At the point where the oxides of amphoteric elements begin to become acidic.

Oxides of elements of the electronegativities down to 1.5 readily give off their oxygens in the water to grab the hydrogen's water, forming hydroxide ions. Often, this is due to the bonding being ionic; thus, the bond between the oxygen and element breaks easily in water.

However, if it is beyond 1.5, the element will hold on to the oxygen strongly and will instead form acids by covalently bonding with more oxygens to form the acid anions. In the oxygen atoms, the negative charge is going to exist.

Al2O3, being exactly 1.5, is equally likely either to give off or grab the oxygens because of the de-facto ionic/covalent nature of its bonds. Whereas, in acidic conditions, protons present in the solution will protonate the oxygen forming water and the corresponding salt of Al.

However, in certain basic conditions, the hydroxide ions promote the formation of more hydroxide ions from Al's oxygens by forming water and a hydrated aluminate complex. Here, the reaction is way more complicated compared to a regular acid-base reaction.

Are Metal Oxides said to be Amphoteric Oxides?

No. Amphoterism is described as a characteristic expression of the metal's non-metallic nature, as it pertains to its capacity to show non-metallic properties in some way. To explain, in other words, the amphoterism of a few oxides is likely related to the ability of the metal itself to polarise the oxide ions that it is bonded to so that a significant percentage of the covalent character can be introduced into the bond.

This is why the typical amphoteric metal oxides (as well as hydroxides) are the ones, which come from a "borderline" metal:

• Al, - forms Al₂O₃ and Al(OH)₃;

• Zn, - forms ZnO and Zn(OH)₂;

• Be, - forms BeO and Be(OH)₂;

• Sn, - forms SnO and Sn(OH)₂;

• Ga, - forms Ga₂O₃ and Ga(OH)₃.

In other words, metal oxides of the metals like Group 1 and 2 metals (except for Be) do not exhibit amphoterism.

Furthermore, the oxidation state plays an essential role, too: although there exists no clear trend, as the rule of thumb, the amphoteric behaviour of a metal cation increases with an increasing oxidation state, which is very much so in, say, the transition metal oxides.

To keep it differently, like the transition metals exhibit metal cations with various charges and the acidity of a cation increases as its charge increases, several transition metals do form oxides, which are either acidic, basic, or amphoteric.

Some Common Uses of Amphoteric Oxide Salts

The following are some of the most common applications for amphoteric oxide salts:

• Zinc Oxide (ZnO) can be used as an additive in a number of products and materials, including plastics, rubbers, glass, ceramics, lubricants, cement, ointments, paints, sealants, adhesives, foods, pigments, fire retardants, batteries, and first-aid tapes are all ferrites (compounds containing Fe2O3).

• Aluminium Oxide (Al2O3) is also used in the production of aluminium. In addition, being fairly chemically white and inert, this oxide is a favoured filler for plastics. It is a common ingredient in sunscreen and sometimes present in cosmetics such as lipstick, nail polish and blush. Sometimes, aluminium oxide is used as an ingredient of several glass formations. It can be used as a catalyst for the dehydration of alcohol to alkenes and the Claus process. Aluminium oxide is called amphoteric oxide.

• Lead Oxide (PbO) is extensively used in making glass. Depending on the type of glass, the benefit of using PbO in glass may be decreasing the viscosity of the glass, increasing the refractive index of the glass, increasing the ability of the glass and increasing the electrical resistivity of the glass to absorb X-rays. Adding PbO to industrial ceramics (and glass) renders them electrically and magnetically inert (raises the Curie temperature) and is commonly used for these applications.

Conclusion

Oxides that may act as both basic and acidic substances are known as amphoteric oxides. Amphoteric oxides have properties that are similar to both acidic and basic oxides, and they can neutralise both acids and bases. Because they react with acids and bases, as well as water and salts, to generate water and salt solutions, amphoteric oxides are classified as metal oxides. Amphoteric oxide salts have a number of applications, some of which are listed below: Plastics, rubbers, glass, ceramics, and lubricants are just some of the products and materials available.