Acid strength describes how much an acid donates H⁺ to water at equilibrium. The acid ionization constant Ka measures this tendency for the reaction HA + H₂O ⇌ H₃O⁺ + A⁻. A larger Ka means more products form, so the acid is stronger.
Chemists often use pKa because it turns very large or very small Ka values into easier numbers.
Key Facts
- For HA + H₂O ⇌ H₃O⁺ + A⁻, Ka = [H₃O⁺][A⁻]/[HA]
- For B + H₂O ⇌ BH⁺ + OH⁻, Kb = [BH⁺][OH⁻]/[B]
- pKa = -log(Ka) and pKb = -log(Kb)
- Larger Ka means stronger acid, while smaller pKa means stronger acid
- For a conjugate acid-base pair at 25 °C, Ka Kb = Kw = 1.0 x 10^-14
- At 25 °C, pKa + pKb = 14.00 for a conjugate acid-base pair
Vocabulary
- Ka
- Ka is the acid ionization constant that measures how far an acid dissociates in water.
- Kb
- Kb is the base ionization constant that measures how strongly a base accepts H⁺ from water.
- pKa
- pKa is the negative logarithm of Ka, so lower pKa values indicate stronger acids.
- Conjugate base
- A conjugate base is the particle left after an acid donates H⁺.
- Hydronium
- Hydronium, H₃O⁺, is the ion formed when water accepts H⁺ from an acid.
Common Mistakes to Avoid
- Treating a larger pKa as a stronger acid is wrong because pKa = -log(Ka), so acid strength increases as pKa decreases.
- Using Ka Kb = Kw for two unrelated substances is wrong because the equation only applies to a conjugate acid-base pair.
- Putting water in the Ka expression for dilute aqueous solutions is wrong because liquid water is a pure liquid and is not included in the equilibrium constant.
- Assuming a weak acid has no ions in solution is wrong because weak acids still ionize partially and reach equilibrium with both reactants and products present.
Practice Questions
- 1 Acetic acid has Ka = 1.8 x 10^-5. Calculate its pKa to two decimal places.
- 2 The conjugate acid of a base has Ka = 6.3 x 10^-10 at 25 °C. Calculate the Kb of the conjugate base.
- 3 Acid X has pKa = 3.2 and acid Y has pKa = 5.8. Which acid is stronger, and what does that imply about the relative strength of their conjugate bases?