11.03 Acid and Alkali Solutions: The Importance of Hydrogen and Hydroxide Ions

1. Introduction to Acids and Alkalis

• Acids: Substances containing hydrogen (H) in their formulas.
  • Examples: Hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃).
• Alkalis (Bases): Substances containing hydroxide ions (OH⁻) in their formulas.
  • Examples: Sodium hydroxide (NaOH), potassium hydroxide (KOH).

2. Ionization in Water

Pure Water (H₂O)

• Autoionization: Water dissociates into hydrogen ions (H⁺) and hydroxide ions (OH⁻) in equal amounts.
• Ion Concentration:
  • [H⁺] = [OH⁻] = 1 × 10⁻⁷ M at 25°C.
• Electrical Conductivity: Pure water is a poor conductor of electricity due to low ion concentration.

Acid Solutions

• Ion Production: Acids release hydrogen ions (H⁺) in water.
• Ion Concentration: [H⁺] > [OH⁻].
• Electrical Conductivity: High, due to the increased ion concentration.

Alkali Solutions

• Ion Production: Alkalis release hydroxide ions (OH⁻) in water.
• Ion Concentration: [OH⁻] > [H⁺].
• Electrical Conductivity: High, due to the increased ion concentration.

3. The pH Scale

• Definition: Measures acidity or alkalinity based on H⁺ ion concentration.
• Range: 0 to 14.
  • pH < 7: Acidic ([H⁺] > [OH⁻]).
  • pH = 7: Neutral ([H⁺] = [OH⁻]).
  • pH > 7: Alkaline ([OH⁻] > [H⁺]).
• Importance: Determines the strength of acids/alkalis and their electrical conductivity.

4. Conductivity of Solutions

• Electrical Conductivity:
  • Acid and alkali solutions conduct electricity better than pure water due to free ions (H⁺ or OH⁻).
• Role of Ions: Ion concentration and mobility affect conductivity.

5. Indicators and Their Responses

• Litmus Paper:
  • Red Litmus: Turns red in acidic solutions (presence of H⁺ ions).
  • Blue Litmus: Turns blue in alkaline solutions (presence of OH⁻ ions).
• Function: Identifies the nature of a solution based on ion presence.

6. Key Ions in Acid and Alkali Solutions

Acid Solutions

• Contain hydrogen ions (H⁺).
  • Examples:
    • Hydrochloric Acid (HCl): H⁺ and Cl⁻.
    • Sulfuric Acid (H₂SO₄): 2 H⁺ and SO₄²⁻.

Alkali Solutions

• Contain hydroxide ions (OH⁻).
  • Examples:
    • Sodium Hydroxide (NaOH): Na⁺ and OH⁻.
    • Potassium Hydroxide (KOH): K⁺ and OH⁻.

7. Important Table: Ions Present in Acids and Alkalis

8. Additional Concepts

• Neutral Solutions: [

[H⁺] = [OH⁻], typically at pH 7.

• Applications: Acids and alkalis are essential in industrial processes, biological systems, and environmental chemistry.