Brønsted-Lowry acid-base theory

In chemistry, the Bronsted-Lowry acid-base theory or Bronsted-Lowry concept is an acid-base theory describing the reaction mechanism between acids and bases, and was independently proposed by Johannes Nicolaus Bronsted and Thomas Martin Lowry in 1923. In this system, a Bronsted-Lowry acid is defined as any species which has the tendency to lose, or "donate" a hydrogen ion (proton), and a Brønsted-Lowry base as a substance with a tendency to gain or "accept" a hydrogen ion (proton). Thus, other substances such as water molecules can have behaviours not defined in the common definition of an acid or base.

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In the reaction between acetic acid and water, acetic acid acts as an acid by donating a proton to water, which acts as a base. The equation given is:

Water can also act as an acid, for instance when it reacts with ammonia. The equation given for this reaction is:

in which H₂O donates a proton to NH₃. Thus water is amphoteric: able to act as either an acid or a base. These reactions can act in reverse as well as in the forward direction. The reverse of the reaction between acetic acid and water is:

In the reverse reactions H₃O⁺ acts as an acid to donate a proton to CH₃COO^-, which is acting as a base. In the forward reaction, CH₃COOH acts as an acid to donate a proton, and water as a base to receive it. H₂O and H₃O⁺ are therefore considered one acid-base conjugate pair, and CH₃COOH and CH₃COO^- as another conjugate pair.

See also

• Acid-base reaction theories
• Lewis acid
• Lewis base