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.
Understanding Chemistry: Ka, Kb, and Acid Strength
At equilibrium, acid particles are not all in one form. A weak acid exists as a mixture of intact acid molecules, hydronium ions, and conjugate base ions. The ionization constant describes the balance of that mixture after the forward and reverse reactions occur at equal rates.
Water is left out of the constant expression because liquid water is present in an almost fixed amount. Concentration matters because the constant is based on equilibrium concentrations, not the amounts placed in the flask at the start. Students commonly use an ICE table to organize initial concentrations, concentration changes, and equilibrium concentrations before calculating the constant.
A value for acid strength comes from the structure of the acid. After an acid loses a proton, its conjugate base remains. A stable conjugate base makes proton loss more favorable.
Stability can come from charge spreading across several atoms, called resonance. It can come from an electronegative atom holding negative charge well. It can come from the pull of nearby atoms or groups that attract electrons.
For example, acids with oxygen often form conjugate bases where the negative charge is spread over oxygen atoms. This helps explain why two substances that both contain hydrogen can have very different acid strengths.
Base strength follows the same equilibrium idea. A base accepts a proton from water and produces hydroxide ions. Its base ionization constant tells how far that process proceeds.
The important link is between a substance and its conjugate partner. If an acid gives up protons easily, the conjugate base has little tendency to take a proton back. Therefore a strong acid has a weak conjugate base.
At a fixed temperature, the acid and base constants for a conjugate pair are connected through the ionization of water. This relationship lets students find an unknown acid constant from a known base constant, or the reverse.
These ideas appear in buffer solutions, titrations, biology, and environmental chemistry. A buffer contains a weak acid with its conjugate base, or a weak base with its conjugate acid. It resists sudden changes in pH because each member of the pair reacts with added acid or added base.
During a titration of a weak acid, the half equivalence point is especially useful. At that point, equal amounts of weak acid and conjugate base are present, so the pH equals the pKa. Pay close attention to the direction of the strength scale.
Larger ionization constants mean stronger behavior, while larger pKa or pKb values mean weaker behavior. Keep temperature consistent when using standard relationships, since the familiar values apply at 25 degrees Celsius.
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?