base
Summary
A base is a species that accepts a proton (Brønsted-Lowry), raises [OH⁻] in water (Arrhenius), or donates an electron pair (Lewis). Most questions use the Brønsted-Lowry view: a stronger base has a larger Kb and a weaker conjugate acid.
Detail
Meaning: A base has three accepted definitions, each broader than the last. Arrhenius: raises [OH⁻] in water (NaOH). Brønsted-Lowry: a proton acceptor (NH₃ + H⁺ → NH₄⁺). Lewis: an electron-pair donor, which covers reactions with no proton transfer. A strong base ionizes essentially completely in water; a weak base ionizes partly and has a small Kb. The stronger the base, the weaker its conjugate acid.
Equation: B + H₂O ⇌ BH⁺ + OH⁻; Kb = [BH⁺][OH⁻]/[B]; Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25 °C. B is the base, BH⁺ its conjugate acid, brackets mean molar concentration (mol/L), and Kb, Ka and Kw have no units.
On the MCAT: Given a Kb or a concentration, find pOH and then pH (pH + pOH = 14 at 25 °C). On a titration curve of a weak base with a strong acid, pOH = pKb at the half-equivalence point, where [B] = [BH⁺]. In organic questions, amines are more basic than amides because resonance delocalizes the amide lone pair.
Common confusion: A base does not need to contain OH⁻. NH₃ is a base because it accepts H⁺. Strong is also not the same as concentrated: strength is the extent of ionization, while concentration is the amount dissolved.
Sources
- Chemistry: The Central Science
- Chemistry: A Molecular Approach
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.