Classification of Oxides: Acidic, Basic, Amphoteric & Neutral Oxides (2023)

Oxides are binary oxygen compounds having lower electronegative components among the oxygen compounds. The only element that has a higher electronegativity than oxygen is fluorine. All binary oxygen compounds, with the exception of fluorine, are considered oxides. The oxygen in the most stable oxides is in the -2 oxidation state. Na₂O, CaO, A₂O₃, CO₂, N₂O₃, Cl₂O, and XeO₂ are examples of oxides. Oxide refers to a vast group of chemical compounds in which oxygen is coupled with another element. Except for the lighter inert gases (helium [He], neon [Ne], argon [Ar], and krypton [Kr]), oxygen (O) forms binary oxides with all of the elements. In oxygen-containing compounds, both metals and non-metals can achieve their highest oxidation states (i.e., donate their maximum number of accessible valence electrons).

Key Takeaways: Oxides, Acidic Oxide, Basic Oxide, Amphoteric Oxide, Carbon Dioxide, Metals, Non-Metals, Neutral Oxide, Chemical Reaction, Anion, Cation, Carbon Monoxide

Definition of Oxides

An oxide is an oxygen ion with an oxidation state of -2 or O^2-. An oxide is a chemical compound that has the anion O^2- as its cation. Some people use the term more loosely to refer to any chemical in which oxygen is the anion. Metal oxides (such as Ag₂O and Fe₂O₃) are the most common type of oxide, accounting for the majority of the Earth's crust's mass. Metals react with oxygen from the air or water to generate these oxides. Metal oxides are solids at normal temperatures, but they can also form gaseous oxides. Under normal temperature and pressure, water is an oxide that is a liquid. Nitrogen dioxide (NO₂), sulphur dioxide (SO₂), carbon monoxide (CO), and carbon dioxide (CO₂) are some of the oxides found in the air.

Oxides are characterised as acidic, basic, amphoteric, or neutral based on their acid-base characteristics:

• Acidic oxide is an oxide that forms an acid when it reacts with water.
• A basic oxide can be defined as an oxide that produces a base in water.
• An oxide that can react chemically as either an acid or a base is known as an amphoteric oxide.
• It is conceivable, however, for oxide to be neither acidic nor basic, but rather neutral.

Classification of Oxides

The three types of oxides can be distinguished by their various qualities. Anhydride (meaning "without water") refers to substances that absorb H₂O to generate an acid or a basic when water is added.

Classification of Oxides

Acidic Oxides

Acidic oxides are the oxides of non-metals falling in Groups 14-17 and theseacid anhydridesform acids with water. Acid anhydrides are acidic oxides that produce salts when associated with bases (e.g., sulphur dioxide is sulphurous anhydride and sulphur trioxide is sulfuric anhydride)

• Sulfurous Acid: SO₂+H₂O→H₂SO₃
• Sulfuric Acid: SO₃+H₂O→H₂SO₄
• Carbonic Acid: CO₂+H₂O→H₂CO₃

Basic Oxides

Basically, Group 1andGroup 2elements form bases called base anhydrides or basic oxides e.g.,

K₂O(s) +H₂O (l) →2KOH (aq)

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

• CaO+H₂O→Ca (OH)₂
• MgO+ H₂O →Mg (OH)₂
• Na₂O+ H₂O →2NaOH

These metallic oxides are known as basic anhydrides. They react with acids in producing salts, e.g.,

• MgO+2HCl→MgCl₂+H2O
• Na₂O+H₂SO₄→Na₂SO₄+H₂O

Read More: Metals and Non-Metals- Formula and Equations

Amphoteric Oxides

An oxide that can react chemically as either an acid or a base is known as an amphoteric oxide. When HSO₄^- interacts with water, for example, it produces both hydroxide and hydronium ions:

• HSO₄^- +H₂O→SO₂^4-+H₃O⁺
• HSO₄^- +H₂O→H₂SO₄+OH⁻

Both basic and acidic characteristics are found in amphoteric oxides. Salt and water are produced when they react with an acid, demonstrating basic characteristics. When alkalies react with them, they produce salt and water, both of which have acidic qualities.

• Example 1: ZnO displays basic behaviour withHCl

ZnO+2HCl→ZnCl₂ (zinc chloride) +H₂O (basic nature)

and acidic behaviour withNaOH

ZnO+2NaOH→Na₂ZnO₂ (sodium zincate) +H₂O (acidic nature)

• Example 2: Al₂O₃show basic behaviour withH₂SO₄

Al₂O₃+3H₂SO₄→Al₂ r(SO₄)3+³H₂O (basic nature)

and acidic behaviour withNaOH

Al₂O₃+2NaOH→2NaAlO₂+H₂O (acidic nature)

Read More: Amphoteric Oxides

Neutral Oxides

Neutral oxides such as carbon monoxide (CO), nitrous oxide (N₂O), nitric oxide (NO), and others, have neither basic nor acidic characteristics and hence do not produce salts when reacting with acids or bases.

Read More: Hydrogen Peroxide

Periodic Table of Elements Depicting Nature of Oxides

Compound Oxides

Compound oxides are metallic oxides that act as if they are made up of two oxides of the same metal, one with lower oxidation and the other with higher oxidation. For example,

• Red lead:Pb₃O₄=PbO₂+ 2PbO
• Ferro-ferric oxide:Fe₃O₄ = Fe₂O₃+FeO

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

Fe₃O₄ (Ferro-ferric oxide) +8HCl→2FeCl₃ (ferric chloride) +FeCl₂ (ferrous chloride) + 4H₂O

Preparation of Oxides

Oxides can be made in a variety of ways. Here are a few examples.

By heating an element directly with oxygen: When heated in oxygen or air, many metals and non-metals burn quickly, forming oxides. For example,

• 2Mg+O₂ (Applying Heat)→ 2MgO
• 2Ca+O₂(Applying Heat)→ 2CaO
• S+O₂(Applying Heat)→ SO₂
• P4+5O₂(Applying Heat)→ 2P₂O₅

At higher temperatures, oxygen reacts with compounds: Oxygen reacts with a variety of substances at higher temperatures, generating oxides. A few examples:

Sulphides are frequently oxidised when heated with oxygen.

• 2PbS+3O₂→Δ2PbO+2SO₂
• 2ZnS+3O₂→Δ2ZnO+2SO₂

Compounds containing carbon and hydrogen are oxidised when heated with oxygen.

C₂H₅OH+3O₂→2CO₂+3H₂O

Certain compounds, such as hydroxides, carbonates, and nitrates, are thermally decomposed.

• CaCO₃→ΔCaO+CO₂
• 2Cu (NO₃)₂→Δ2CuO+4NO₂+O₂
• Cu (OH)₂→ΔCuO+H₂O

Though oxidation of some metals with nitric acid

• 2Cu+8HNO₃(Applying Heat)→ 2CuO+8NO₂+4H₂O+O₂
• Sn+4HNO₃(Applying Heat)→ SnO₂+4NO₂+2H₂O

Through oxidation of some non-metals with nitric acid

C+4HNO₃→CO₂+4NO₂+2H₂O

Read More: Difference Between Acid and Base

Things to Remember

• There are 4 types of oxides on the basis of their chemical behaviour: Acidic Oxides, Basic Oxides, Amphoteric Oxides, and Neutral Oxides.
• A chemical molecule that has at least one oxygen atom and one other element is known as an oxide. Oxide, which is denoted by O^2-, is a dianion of oxygen. At least one dianion of oxygen is present in all Oxides compounds. Oxides are binary compounds that are made up of oxygen and another element.
• When a non-metal element reacts with oxygen, acidic oxides occur. They generate salt and water when they react with bases. SO₂ and SiO₂ are two common examples.
• In case a metal reacts with oxygen, it forms a basic oxide. They generate salt and water when they react with acids. NaOH, KOH, and Ca (OH)₂ are examples.
• Some oxides are claimed to be neutral because they do not react with acids or bases. N₂O, NO, and CO are a few examples.
• Amphoteric oxides are a strange class of oxides that can act as both an acid and a base depending on whether the other reactant is an acid or a base. Zinc oxide and aluminium oxide are the two most common amphoteric oxides.

Sample Questions

Ques. Explain the formation of oxides. (3 Marks)

Ans. The majority of elements form oxides. Noble gases can produce oxides on rare occasions. Noble metals don't react well with oxygen, but they do generate oxides in the lab. Oxides are formed naturally through either oxygen oxidation or hydrolysis. Oxides are easily formed when materials burn in an oxygen-rich environment (such as metals in the thermite reaction). Metals (particularly alkali metals) react with water to form hydroxides. A combination of oxides and hydroxides coats the majority of metal surfaces. This coating often passivates the metal, preventing it from corroding further due to oxygen or water exposure. When iron is exposed to dry air, it forms iron(II) oxide, but when oxygen and water are present, hydrated ferric oxides (rust), Fe₂O₃x-(OH)2x, develop.

Ques. State the nomenclature of oxides. (3 Marks)

Ans. A substance that contains the oxide anion is simply referred to as an oxide. CO and CO₂ are both carbon oxides, for example. Copper (II) oxide and copper (I) oxide, respectively, are CuO and Cu₂O. Alternatively, the name could be based on the cation-oxygen atom ratio. For naming, Greek numeral prefixes are employed. Water, sometimes known as H₂O, is dihydrogen monoxide. CO2 is short for carbon dioxide. CO stands for carbon dioxide. The -a suffix can also be used to name metal oxides. Alumina, chromia, and magnesia are represented as Al₂O₃, Cr₂O₃, and MgO, respectively. Oxides are given unique names based on the comparison of lower and higher oxygen oxidation states. O₂^2- denotes peroxide, while O₂^- denotes superoxide. Hydrogen peroxide, for instance, is H₂O₂.

Ques. Describe the structure of oxides. (2 Marks)

Ans. Metal oxides frequently create structures that resemble polymers, in which the oxide connects three or six metal atoms. In most cases, polymeric metal oxides are insoluble in water. Some oxides have a molecular structure. All of the simple nitrogen oxides, as well as carbon monoxide and carbon dioxide, fall into this category.

Ques. What is a neutral oxide? (2 Marks)

Ans. A neutral oxide is one that has no tendency to produce salts when it comes into contact with acids or bases. Their solutions are unambiguous. Nitrous oxide and carbon monoxide are some examples of neutral oxides.

Ques. State a few characteristics of Acidic Oxide with examples. (5 Marks)

Ques. State a few characteristics of Basic Oxide with examples. (5 Marks)

Ques. State a few characteristics of Amphoteric Oxide with examples (3 Marks)

Ques. Explain the classification of oxides. (3 Marks)

Ques. Why is it difficult to obtain oxygen directly from water? (5 Marks)

Ques. BaO₂ is which of the following: hydroxide, peroxide, or superoxide? (2 Marks)

Ans. Peroxide is BaO₂. The oxygen atoms have an oxidation state of -1 because barium has an oxidation state of +2. As a result, the chemical is referred to as barium peroxide rather than peroxide.

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