Oxides

Definition and Classification of Oxides

• Oxide:
  A compound of oxygen with another element.
  General formula: MₓOᵧ where M is the other element.
  Examples: CO₂, Na₂O, Al₂O₃.
• Main types of oxides based on their chemical behaviour:
  1. Acidic oxides – usually non-metal oxides that react with bases/alkalis to form a salt and water.
  2. Basic oxides – usually metal oxides that react with acids to form a salt and water.
  3. Amphoteric oxides – oxides that can react with both acids and bases to form a salt and water.

Acidic Oxides

• Definition:
  Non-metal oxides that dissolve in water to form acidic solutions or react with bases/alkalis to form salts and water.
• General reaction with alkalis:
  Acidic oxide + alkali → salt + waterExample:
  SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l)
• Examples:
  • Sulfur dioxide (SO₂) – dissolves in water to form sulfurous acid (H₂SO₃).
    SO₂(g) + H₂O(l) → H₂SO₃(aq)
  • Carbon dioxide (CO₂) – dissolves in water to form carbonic acid (H₂CO₃).
    CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)
• Properties:
  • Generally gases at room temperature.
  • Form acids when dissolved in water.
  • Do not react with acids.

Basic Oxides

• Definition:
  Metal oxides that react with acids to form salts and water.
• General reaction with acids:
  Basic oxide + acid → salt + waterExample:
  CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
• Examples:
  • Copper(II) oxide (CuO) – black solid.
  • Calcium oxide (CaO) – white solid, reacts with water to form calcium hydroxide.
• Properties:
  • Usually solid at room temperature.
  • Insoluble in water (except alkali metal oxides like Na₂O, K₂O, which form strong alkalis).
  • Do not react with alkalis.

Amphoteric Oxides

• Definition:
  Oxides that react with both acids and bases to produce a salt and water.
• General reactions:
  With acids:
  Amphoteric oxide + acid → salt + waterWith bases:
  Amphoteric oxide + base → complex salt + water
• Examples:
  • Aluminium oxide (Al₂O₃)
    With acid:
    Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
    With base:
    Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄
  • Zinc oxide (ZnO)
    With acid:
    ZnO(s) + 2H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l)
    With base:
    ZnO(s) + 2NaOH(aq) + H₂O(l) → Na₂Zn(OH)₄
• Properties:
  • Can behave as acidic or basic depending on the reactant.
  • Usually found in elements close to the border between metals and non-metals in the periodic table (metalloids and some metals).

Relationship Between Oxide Type and Metallic / Non-metallic Character

• Metals:
  • Tend to form basic oxides (e.g., MgO, CaO).
  • Some form amphoteric oxides (e.g., Al₂O₃, ZnO).
• Non-metals:
  • Tend to form acidic oxides (e.g., SO₂, CO₂).
• Reason:
  • Metallic character increases → oxide becomes more basic.
  • Non-metallic character increases → oxide becomes more acidic.

Practical Significance and Examples in Real Life

• Acidic oxides:
  • CO₂ – contributes to ocean acidification.
  • SO₂ – causes acid rain when dissolved in atmospheric moisture.
• Basic oxides:
  • CaO – used in cement manufacture, neutralises acidic soil.
• Amphoteric oxides:
  • Al₂O₃ – used in extraction of aluminium by electrolysis.
  • ZnO – used in rubber manufacturing and sunblock creams