What is Weak Acid and Weak Base Equilibrium?
Weak acids and bases only partially ionize in water, establishing dynamic equilibrium between molecular and ionic forms. The equilibrium constant Ka (for acids) or Kb (for bases) quantifies the extent of ionization.
Weak acids: HA ⇌ H⁺ + A⁻, with Ka = [H⁺][A⁻]/[HA]. Weak bases: B + H₂O ⇌ BH⁺ + OH⁻, with Kb = [BH⁺][OH⁻]/[B]. Small Ka/Kb means weak ionization; large values mean stronger ionization.
- 1↓1. Initial stateMolecules (HA) dominate; few ions present
- 2↓2. IonizationSome HA splits: HA → H⁺ + A⁻
- 3↓3. EquilibriumMostly HA left; small amounts of H⁺ and A⁻
- 44. Equilibrium constantKa = [H⁺][A⁻] / [HA] (small value)
Step-by-step worked examples
Acetic acid (CH₃COOH) is weak with Ka = 1.8 × 10⁻⁵. If [CH₃COOH]initial = 0.1 M, estimate [H⁺] at equilibrium.
Setup equilibrium: CH₃COOH ⇌ H⁺ + CH₃COO⁻ Ka = [H⁺][CH₃COO⁻]/[CH₃COOH] = 1.8 × 10⁻⁵ If x = [H⁺]: Ka = x²/(0.1 − x) ≈ x²/0.1 (since Ka is small) x² = 1.8 × 10⁻⁶ x = [H⁺] ≈ 1.34 × 10⁻³ M
Ammonia (NH₃) is a weak base with Kb = 1.8 × 10⁻⁵. Compare to acetic acid's Ka.
Ammonia: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, Kb = 1.8 × 10⁻⁵ Acetic acid: Ka = 1.8 × 10⁻⁵ Same Kb as Ka? Not quite—but both are weak (Ka, Kb < 10⁻³)
For a weak acid, why is [H⁺] less than the initial concentration?
Weak acids only partially ionize. If initial [HA] = 0.1 M, only a fraction (e.g. 1%) ionizes. [H⁺] from ionization << [HA]initial This is why we can use approximation Ka ≈ x²/[HA]initial
Flashcards
Quick quiz
Q1.A weak acid has Ka = 0.001. Does it ionize significantly?
Q2.Hydrochloric acid (HCl) is strong; acetic acid (CH₃COOH) is weak. Why?
Q3.For weak acid HA with Ka = 10⁻⁶, the equilibrium has mostly…
Q4.Ammonia (NH₃) is a weak base. Its conjugate acid (NH₄⁺) is…
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Common mistakes
Weak acids don't produce H⁺. — Correct: Weak acids do produce H⁺, but only partially; most stays as HA molecules.
Ka > Kb always. — Correct: Ka and Kb for a conjugate pair are related by Kw; one large means the other is small.
Diluting a weak acid makes it ionize more (percent ionization goes down). — Correct: Dilution increases percent ionization (more molecules split) but total [H⁺] decreases.
All monoprotic acids have Ka < 1. — Correct: Strong monoprotic acids have Ka >> 1; weak acids have Ka < 1.
FAQ
What is the Ka constant?
Ka is the equilibrium constant for acid ionization: HA ⇌ H⁺ + A⁻. Small Ka = weak acid.
Why doesn't a weak acid fully ionize?
The reverse reaction (H⁺ + A⁻ → HA) is fast; equilibrium favors the molecular form if Ka is small.
How do I calculate [H⁺] for a weak acid?
Set up the equilibrium expression, use Ka = [H⁺][A⁻]/[HA], and solve (often using the quadratic formula or approximations).
Is water a weak acid or base?
Both! Water can act as a weak acid (Ka ≈ 10⁻¹⁶) or weak base (Kb ≈ 10⁻¹⁶).