Brønsted acid and Brønsted base

Brønsted acid: A substance capable of donating a proton.

Brønsted base: A substance capable of accepting a proton.

Conjugate acid-base pair: an acid and its conjugate base or a base and its conjugate acid.

Every Brønsted acid has a conjugate base, and every Brønsted base has a conjugate acid. For example, the chloride ion (Cl₂) is the conjugate base formed from the acid HCl, and H₃O⁺ (hydronium ion) is the conjugate acid of the base H₂O.

Similarly, the ionization of acetic acid can be represented as;

The subscripts 1 and 2 designate the two conjugate acid-base pairs. Thus, the acetate ion (CH₃COO₂) is the conjugate base of CH₃COOH. Both the ionization of HCl and the ionization of CH3COOH are examples of Brønsted acid-base reactions. The Brønsted definition also enables us to classify ammonia as a base because of its ability to accept a proton:

In this case, NH⁺₄ is the conjugate acid of the base NH₃, and the hydroxide ion OH^- is the conjugate base of the acid H₂O. Note that the atom in the Brønsted base that accepts a H⁺ ion must have a lone pair.