Acid Strength
pKa values, conjugate base stability, and the factors that make a proton easy or hard to remove. Electronegativity, resonance, induction, and atom size all play a role. Essential for understanding buffer systems and organic reaction mechanisms.
Hydrofluoric acid
HF
Hydrochloric acid
HCl
Despite fluorine being the most electronegative element, HF is a weak acid (pKa 3.17) because the very strong H-F bond resists dissociation in water. HCl dissociates completely and is classified as a strong acid.
HCl is a much stronger acid than HF in water. Although fluorine is more electronegative, the H-F bond is significantly stronger (568 vs 431 kJ/mol), making it much harder to break. Bond strength dominates over electronegativity for this comparison.
Ethanol
C₂H₅OH
Acetic acid
CH₃COOH
Ethanol is a very weak acid because its conjugate base, ethoxide, has the negative charge localized on a single oxygen atom with no resonance stabilization. This makes deprotonation much less favorable.
Acetic acid (pKa 4.76) is far more acidic than ethanol (pKa ~16). The acetate conjugate base is stabilized by resonance delocalization of the negative charge across two equivalent oxygen atoms.
Cyclohexanol
C₆H₁₁OH
Phenol
C₆H₅OH
Cyclohexanol lacks an aromatic ring, so its conjugate base cannot delocalize the negative charge. The resulting cyclohexanoxide ion is much less stable than phenoxide, making cyclohexanol a weaker acid.
Phenol is about a million times more acidic than cyclohexanol. The phenoxide conjugate base is stabilized by delocalization of the negative charge into the aromatic ring through resonance.
Phosphoric acid
H₃PO₄
Sulfuric acid
H₂SO₄
Phosphoric acid is a moderate-strength acid (pKa₁ = 2.15) but not a strong acid. It does not fully dissociate in water, unlike sulfuric acid which completely donates its first proton.
Sulfuric acid is a much stronger acid than phosphoric acid. Sulfur is more electronegative than phosphorus and has a higher oxidation state in H₂SO₄, which stabilizes the conjugate base more effectively through charge delocalization.