Lecture Notes: Acids and Bases

Introduction

• Importance of practice problems in chemistry.
• Goal: Understand concepts before diving into calculations.

Definitions of Acids and Bases

Bronsted-Lowry Theory

• Acid: Proton (H⁺) donor.
• Base: Proton acceptor.
• Example Reaction:
  • HA + H₂O → A⁻ + H₃O⁺
  • HA is the acid (donates H⁺), A⁻ is the conjugate base.
  • H₂O is the base (accepts H⁺), H₃O⁺ is the conjugate acid.

Lewis Theory

• Acid: Electron pair acceptor.
• Base: Electron pair donor.

Arrhenius Theory

• Acid: Produces H⁺ in aqueous solution.
• Base: Produces OH⁻ in aqueous solution.

Strong Acids and Bases

Strong Acids

• Definition: Completely dissociate in solution.
• Examples:
  • Hydrochloric acid (HCl)
  • Hydroiodic acid (HI)
  • Hydrobromic acid (HBr)
  • Nitric acid (HNO₃)
  • Perchloric acid (HClO₄)
  • Sulfuric acid (H₂SO₄, diprotic)

Strong Bases

• Definition: Completely dissociate in solution.
• Examples:
  • Group 1 hydroxides (e.g., NaOH, KOH)
  • Some Group 2 hydroxides (e.g., Ca(OH)₂, Sr(OH)₂, Ba(OH)₂)

Relationship of Strength

• Stronger acid → Weaker conjugate base
• Stronger base → Weaker conjugate acid

pH Scale

• Logarithmic scale from 0 to 14.
• Acidic: pH 0-7, higher H⁺ concentration.
• Basic: pH 7-14, higher OH⁻ concentration.
• Neutral: Equal H⁺ and OH⁻ concentration.
• Formula:
  • pH = -log[H⁺]
  • pOH = -log[OH⁻]
  • pH + pOH = 14

Water Auto-Ionization

• Kw = [H⁺][OH⁻] = 1 x 10⁻¹⁴
• Water is amphoteric: acts as acid and base.

Weak Acids and Bases

Weak Acids

• Example: Acetic acid (CH₃COOH)
• Ka: Acid ionization constant.
  • Larger Ka → Stronger acid.
  • pKa = -log(Ka)

Weak Bases

• Example: Ammonia (NH₃)
• Kb: Base ionization constant.
  • Larger Kb → Stronger base.
  • pKb = -log(Kb)

Relationships

• Kw = Ka * Kb
• pKa + pKb = 14*

Buffers

• Mixture of weak acid and its conjugate base.
• Resist pH changes.
• Ideal buffer pKa is ±1 of the desired pH.

Summary

• Covered definitions and theories of acids and bases.
• Discussed strong and weak acids and bases.
• Explored pH scale, water auto-ionization, and buffer systems.
• Next step: Practice problems on calculating pH of strong acids and bases.