CHEM-GUIDE: Types of oxides

Oxides are binary compounds of oxygen with another element, e.g., CO₂, SO₂, CaO, CO, ZnO, BaO₂, H₂O, etc. These are termed as oxides because here, oxygen is in combination with only one element.

Types of Oxides

Based on their acid-base characteristics oxides are classified as acidic or basic. An oxide that combines with water to give an acid is termed as an acidic oxide. The oxide that gives a base in water is known as a basic oxide.

Acidic oxides

Acidic oxides are the oxides of non-metals. When combined with water, they produce acids, e.g.,

Acidic oxides are, therefore, known as acid anhydrides, e.g., sulphur dioxide is sulphurous anhydride; sulphur trioxide is sulphuric anhydride.

When these oxides combine with bases, they produce salts, e.g.,

Basic oxides

Basic oxides are the oxides of metals. If soluble in water they react with water to produce hydroxides (alkalies) e.g.,

These metallic oxides are therefore, known as basic anhydrides. They react with acids to produce salts, e.g.,

Amphoteric oxides

Amphoteric oxides are metallic oxides, which show both basic as well as acidic properties. When they react with an acid, they produce salt and water, showing basic properties. While reacting with alkalies they form salt and water showing acidic properties, e.g.,

Neutral oxides

These are the oxides, which show neither basic nor acidic properties, that is, they do not form salts when reacted with acids or bases, e.g., carbon monoxide (CO); nitrous oxide (N₂O); nitric oxide (NO), etc., are neutral oxides.

Peroxides and dioxides

A peroxide is a metallic oxide which gives hydrogen peroxide by the action of dilute acids. They contain more oxygen than the corresponding basic oxide, e.g., sodium, calcium and barium peroxides.

Dioxides like PbO₂ and MnO₂ also contain higher percentage of oxygen like peroxides and have similar molecular formulae. These oxides, however, do not give hydrogen peroxide by action with dilute acids. Dioxides on reaction with concentrated HCl yield Cl₂ and on reacting with concentrated H₂SO₄ yield O₂.

Compound oxides

Compound oxides are metallic oxides and they behave as if they are made up of two oxides, lower and higher oxides of the same metal, e.g.,

Red lead: Pb₃O₄ = PbO₂ + 2PbO

Ferro-ferric oxide: Fe₃O₄ = Fe₂O₃ + FeO

On treatment with an acid, compound oxides give a mixture of salts.

Acidic - Basic Nature of Oxides in a Period

The oxides of elements in a period become progressively more acidic as one goes from left to right in a period of the periodic table. For example, in third period, the behavior of oxides changes as follows:

Preparation of Oxides

By direct heating of an element with oxygen

Many metals and non-metals burn rapidly when heated in oxygen or air, producing their oxides, e.g.,

By reaction of oxygen with compounds at higher temperatures

At higher temperatures, oxygen also reacts with many compounds forming oxides, e.g.,

Sulphides are usually oxidized when heated with oxygen.

When heated with oxygen, compounds containing carbon and hydrogen are oxidized.

By thermal decomposition of certain compounds like hydroxides,

carbonates, and nitrates

By oxidation of some metals with nitric acid

By oxidation of some non-metals with nitric acid

Problem

10. Why is it not possible to obtain oxygen directly from water?

Solution

Water as such is a neutral stable molecule. It is difficult to break the covalent O-H bonds easily. Hence, electrical energy through the electrolysis process is applied to separate dioxygen from water. When a small amount of acid is added to water ionization is initiated which helps in electrochemical reactions as follows.