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Weak Acid and Base Equilibria

Weak acids and bases do not fully dissociate in water. Understanding their equilibria is crucial for predicting pH and reaction behavior.

Weak Acid Equilibria​

  • General Form: HA ⇌ H⁺ + A⁻
  • Equilibrium Constant (Ka):
    Ka = [H⁺][A⁻]/[HA]
  • Degree of Ionization: Small fraction of HA dissociates; depends on Ka and concentration.

Weak Base Equilibria​

  • General Form: B + H₂O ⇌ BH⁺ + OH⁻
  • Equilibrium Constant (Kb):
    Kb = [BH⁺][OH⁻]/[B]
  • Relationship to Ka: Ka × Kb = Kw (for conjugate acid-base pairs)

pH Calculations​

  • For weak acids: pH = –log([H⁺]) with [H⁺] from Ka expression.
  • For weak bases: pOH = –log([OH⁻]) with [OH⁻] from Kb expression.
  • Use ICE tables to solve for equilibrium concentrations when needed.

Key Points​

  • Weak acids/bases establish dynamic equilibria.
  • The extent of dissociation depends on Ka, Kb, and initial concentration.
  • Important in buffer solutions and titration curves.

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