The Brønsted-Lowry Definition of Acids and Bases

Originally, acids and bases were defined by Svante Arrhenius. His original definition stated that acids were compounds that increased the concentration of hydrogen ions (H⁺) in solution, whereas bases were compounds that increased the concentration of hydroxide ions (OH^-) in solutions. Problems arise with this conceptualization because Arrhenius's definition is limited to aqueous solutions, referring to the solvation of aqueous ions, and is therefore not inclusive of acids dissolved in organic solvents. To solve this problem, Johannes Nicolaus Brønsted and Thomas Martin Lowry, in 1923, both independently proposed an alternative definition of acids and bases. In this newer system, Brønsted-Lowry acids were defined as any molecule or ion that is capable of donating a hydrogen cation (proton, H⁺), whereas a Brønsted-Lowry base is a species with the ability to gain, or accept, a hydrogen cation. A wide range of compounds can be classified in the Brønsted-Lowry framework: mineral acids and derivatives such as sulfonates, carboxylic acids, amines, carbon acids, and many more.

Brønsted-Lowry Acid/Base Reaction

Keep in mind that acids and bases must always react in pairs. This is because if a compound is to behave as an acid, donating its proton, then there must necessarily be a base present to accept that proton. The general scheme for a Brønsted-Lowry acid/base reaction can be visualized in the form:

acid + base $\rightleftharpoons$ conjugate base + conjugate acid

Here, a conjugate base is the species that is left over after the Brønsted acid donates its proton. The conjugate acid is the species that is formed when the Brønsted base accepts a proton from the Brønsted acid. Therefore, according to the Brønsted-Lowry definition, an acid-base reaction is one in which a conjugate base and a conjugate acid are formed (note how this is different from the Arrhenius definition of an acid-base reaction, which is limited to the reaction of H⁺ with OH^- to produce water). Lastly, note that the reaction can proceed in either the forward or the backward direction; in each case, the acid donates a proton to the base.

Consider the reaction between acetic acid and water:

$H_3CCOOH(aq)+H_2O(l)\rightleftharpoons H_3CCOO^-(aq)+H_3O^+(aq)$

Here, acetic acid acts as a Brønsted-Lowry acid, donating a proton to water, which acts as the Brønsted-Lowry base. The products include the acetate ion, which is the conjugate base formed in the reaction, as well as hydronium ion, which is the conjugate acid formed.

Note that water is amphoteric; depending on the circumstances, it can act as either an acid or a base, either donating or accepting a proton. For instance, in the presence of ammonia, water will donate a proton and act as a Brønsted-Lowry acid:

$NH_3(aq)+H_2O(l)\rightleftharpoons NH_4^+(aq)+OH^-(aq)$

Here, ammonia is the Brønsted-Lowry base. The conjugate acid formed in the reaction is the ammonium ion, and the conjugate base formed is hydroxide.