Brønsted-Lowry acid-base theory
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In chemistry, the Brønsted-Lowry acid-base theory or Brønsted-Lowry concept is an acid-base theory describing the reaction mechanism between acids and bases, and was independently proposed by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. In this system, a Brønsted-Lowry acid is defined as any species that 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, under this concept, an acid is synonymous with proton donor, or hydrogen donor, while base means a proton acceptor.
A wide range of compounds fall within this category of acids, from simple mineral acids to ordinary acids, carboxylic acids, amines, sulfonic acids to less obvious ones like carbon acids such as ethyl acetoacetate or Meldrum's acid. The pKa is a measure of the strength of a proton donor.
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.
