acid dissociation
Summary
Acid dissociation is the reversible loss of a proton from an acid, HA, to water or another base, giving the conjugate base A⁻ and H₃O⁺. Its extent is measured by Ka or pKa. In organic chemistry, a more stable conjugate base means a larger Ka, a lower pKa and a stronger acid.
Detail
Meaning: In water an acid donates a proton: HA + H₂O ⇌ A⁻ + H₃O⁺. A strong acid dissociates nearly completely. A weak acid dissociates only partly, so the equilibrium lies toward HA. In organic chemistry, acidity is mostly a question of how stable the conjugate base is. Stabilizing factors include a more electronegative atom carrying the charge, resonance delocalization, inductive withdrawal by nearby electronegative groups, higher s-character (sp carbon is more acidic than sp³), and larger atom size down a group. Approximate pKa values: carboxylic acids 4 to 5, phenols about 10, alcohols about 16, terminal alkynes about 25.
Equation: Ka = [H₃O⁺][A⁻]/[HA], and pKa = −log Ka. Concentrations are in mol/L, and pKa has no unit. The Henderson–Hasselbalch form is pH = pKa + log([A⁻]/[HA]).
On the MCAT: Passages give structures or pKa values and ask you to rank acidity, pick the most acidic proton, or choose a base strong enough to deprotonate a given acid. The base's conjugate acid should have a higher pKa than the acid. You may also be asked about the protonation state at a given pH. At pH = pKa the acid is half dissociated.
Common confusion: A lower pKa means a stronger acid, not a weaker one. A stronger acid also has a weaker conjugate base. Across a period, electronegativity dominates, so HF is a stronger acid than H₂O. Down a group, size dominates, so HI is a stronger acid than HF.
Sources
- Organic Chemistry
- Organic Chemistry: Structure and Function
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.