From the sour taste of vinegar in salad dressings to the caustic liquids used as drain cleaners, acids and bases have many applications.

According to the Brønsted-Lowry theory, an acid is a proton donor, and a base is a proton acceptor; thus, an acid–base reaction is a proton-transfer reaction.  

Examine the reaction between the Brønsted acid — acetic acid — and the Brønsted base — ammonia. The nitrogen atom in ammonia, owing to its lone pair, serves as the proton receptor site.

Proton transfer from the acid to the base is shown using curved arrows, which illustrate the movement of electrons.

The arrow from the base denotes the shift of nonbonding electrons to the proton of the acid, forming a new covalent bond.

The arrow on the acid shows that the O–H bond breaks to release a proton and that its electrons shift entirely to the oxygen atom.

The acetate ion that remains from the deprotonation of acetic acid is the conjugate base. The ammonium ion, resulting from the protonation of ammonia, is the conjugate acid. 

Notice that acetic acid has two potential proton receptor sites — the carbonyl oxygen and the hydroxyl oxygen — and that it functions as a Brønsted base in its reaction with sulfuric acid. 

Proton transfer from the acid to the carbonyl oxygen of the base gives cation A, whereas transfer to the hydroxyl oxygen of the base results in cation B.

The positive charge in cation A can delocalize over three atoms, resulting in three resonance structures of cation A. In cation B, however, the delocalization occurs  over only two atoms, limiting the number of contributing resonance structures to two.

The greater charge delocalization in cation A makes the carbonyl oxygen of acetic acid the preferred protonation site in an acid–base reaction.