acid
Summary
An acid is a species that donates a proton (Brønsted-Lowry), raises H⁺ in water (Arrhenius), or accepts an electron pair (Lewis). The MCAT most often uses Brønsted-Lowry: a stronger acid has a larger Ka, a lower pKa and a more stable conjugate base.
Detail
Meaning: Three definitions are accepted. Arrhenius: an acid raises the H⁺ (H₃O⁺) concentration in water. Brønsted-Lowry: an acid is a proton donor, and what remains is its conjugate base. Lewis: an acid is an electron-pair acceptor, the broadest definition, covering BF₃, AlCl₃ and metal cations that have no H to give. Strength reflects how far the acid dissociates, which depends on how stable the conjugate base is. Stabilizers include a charge on a larger or more electronegative atom, resonance, and nearby electron-withdrawing groups.
Equation: HA + H₂O ⇌ A⁻ + H₃O⁺, with Ka = [H₃O⁺][A⁻]/[HA] and pKa = −log Ka. HA is the acid, A⁻ its conjugate base, brackets are concentrations in mol/L, and Ka is the acid dissociation constant (unitless in its thermodynamic form).
On the MCAT: Expect rankings of acid strength from structure, titration curves where pH = pKa at the half-equivalence point of a weak acid, and identification of Lewis acids. A larger Ka means a lower pKa and a stronger acid.
Common confusion: Strong does not mean concentrated. A strong acid dissociates essentially completely in water, while a weak acid dissociates only partly; its Ka is fixed, though the fraction dissociated rises as the solution is diluted.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.