Acids and bases as defined by Brønsted; acid–base pairs

Brønsted’s definition

According to the Danish chemist Brønsted, an acid is a chemical species capable of DONATING an H+ ion (a proton), whilst a base is a chemical species capable of ACCEPTING an H+ ion. This definition, which is widely used in secondary schools, helps us to understand most acid-base reactions.

Example of an acid: ethanoic acid

Ethanoic acid (CH₃COOH, found in vinegar) can donate an H⁺ ion: CH₃COOH → CH₃COO⁻ + H⁺. It is therefore an acid in the Brønsted sense.

Example of a base: ammonia

Ammonia NH₃ can accept an H⁺ ion to form the ammonium ion: NH₃ + H⁺ → NH₄⁺. It is therefore a base in the Brønsted sense.

Acid–base pairs

An acid AH and the base A– resulting from its loss of a proton form an acid–base pair, denoted AH / A–. For example: CH₃COOH / CH₃COO– is an acid–base pair; NH₄⁺ / NH₃ is another (here, the base NH₃ is listed second).

The water pair

Water itself belongs to two pairs, as it can play both roles depending on the circumstances: H₃O⁺ / H₂O (in which case water acts as a base) and H₂O / HO⁻ (in which case water acts as an acid). Water is said to be amphoteric (or an ampholyte).

Common mistake

Do not assume that a species is ‘acidic’ or ‘basic’ in an absolute sense: it is its ability to donate or accept an H+ in a given reaction that defines its role. Water illustrates this idea well, as it can behave as an acid when faced with a strong base, and as a base when faced with a strong acid.