Acids and bases are key players in chemistry, with their strength determining how they interact in solutions. Strong acids and bases fully break apart, while weak ones only partially do, creating a balance between their original form and separated ions.

Understanding acid and base strength is crucial for predicting chemical reactions. By looking at dissociation constants (Ka and Kb), we can compare the strength of different acids and bases, and even figure out how their conjugate pairs will behave.

Acid and Base Strength

Strong vs weak acids and bases

• Strong acids and bases completely ionize in aqueous solutions dissociate into their constituent ions ($\ce{HCl}$ → $\ce{H+}$ + $\ce{Cl-}$)
• Weak acids and bases partially ionize in aqueous solutions exist mostly in their undissociated forms ($\ce{CH3COOH}$ ⇌ $\ce{CH3COO-}$ + $\ce{H+}$)
  • Reach equilibrium between undissociated molecules and dissociated ions
  • Equilibrium favors undissociated form
• Examples of strong acids: $\ce{HCl}$ (hydrochloric acid), $\ce{H2SO4}$ (sulfuric acid), $\ce{HNO3}$ (nitric acid)
• Examples of strong bases: $\ce{NaOH}$ (sodium hydroxide), $\ce{KOH}$ (potassium hydroxide), $\ce{Ca(OH)2}$ (calcium hydroxide)
• Examples of weak acids: $\ce{CH3COOH}$ (acetic acid), $\ce{HF}$ (hydrofluoric acid), $\ce{H2CO3}$ (carbonic acid)
• Examples of weak bases: $\ce{NH3}$ (ammonia), $\ce{CH3NH2}$ (methylamine), $\ce{C5H5N}$ (pyridine)

Acid and base dissociation constants

• Acid dissociation constant ($K_a$) equilibrium constant for the dissociation of a weak acid in water
  • General equation for acid dissociation: $\ce{HA + H2O <=> H3O+ + A-}$
  • $K_a$ expression: $K_a = \frac{[\ce{H3O+}][\ce{A-}]}{[\ce{HA}]}$
  • Larger $K_a$ values indicate stronger acids more dissociation at equilibrium
• Base dissociation constant ($K_b$) equilibrium constant for the dissociation of a weak base in water
  • General equation for base dissociation: $\ce{B + H2O <=> BH+ + OH-}$
  • $K_b$ expression: $K_b = \frac{[\ce{BH+}][\ce{OH-}]}{[\ce{B}]}$
  • Larger $K_b$ values indicate stronger bases more dissociation at equilibrium

Relationship of Ka and Kb to strength

• Acid strength directly proportional to $K_a$ value
  • Stronger acids have larger $K_a$ values ($\ce{HCl}$: $K_a$ ≈ $10^6$, $\ce{CH3COOH}$: $K_a$ ≈ $1.8 \times 10^{-5}$)
  • Larger $K_a$ more dissociation higher concentration of $\ce{H+}$ ions
• Base strength directly proportional to $K_b$ value
  • Stronger bases have larger $K_b$ values ($\ce{NaOH}$: $K_b$ ≈ $10^6$, $\ce{NH3}$: $K_b$ ≈ $1.8 \times 10^{-5}$)
  • Larger $K_b$ more dissociation higher concentration of $\ce{OH-}$ ions
• Inverse relationship between strength of conjugate acid-base pairs
  • Stronger acid weaker conjugate base ($\ce{HCl}$ strong acid, $\ce{Cl-}$ weak base)
  • Stronger base weaker conjugate acid ($\ce{NH3}$ weak base, $\ce{NH4+}$ weak acid)

Predicting strengths with Ka and Kb

1. Compare $K_a$ or $K_b$ values of acids or bases
2. Acid with larger $K_a$ is stronger ($\ce{HNO3}$: $K_a$ ≈ $10^1$, $\ce{HF}$: $K_a$ ≈ $7.2 \times 10^{-4}$, $\ce{HNO3}$ is stronger)
3. Base with larger $K_b$ is stronger ($\ce{CH3NH2}$: $K_b$ ≈ $4.4 \times 10^{-4}$, $\ce{C5H5N}$: $K_b$ ≈ $1.7 \times 10^{-9}$, $\ce{CH3NH2}$ is stronger)
4. For conjugate acid-base pairs, compare $K_a$ of acid to $K_b$ of base
5. Stronger acid has weaker conjugate base ($\ce{HCl}$: $K_a$ ≈ $10^6$, $\ce{Cl-}$: $K_b$ ≈ $10^{-8}$)
6. Stronger base has weaker conjugate acid ($\ce{CH3COO-}$: $K_b$ ≈ $5.6 \times 10^{-10}$, $\ce{CH3COOH}$: $K_a$ ≈ $1.8 \times 10^{-5}$)