The formation of conjugate acids and bases is central to the Brønsted-Lowry definition of acids and bases: the conjugate base is the ion or molecule remaining after the acid has lost its proton, and the conjugate acid is the species created when the base accepts the proton.

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

When a Brønsted-Lowry acid donates its proton, it becomes a base in the reverse reaction. In the example below, the hydrogen sulfate ion (HSO₄⁻ ) donates a proton to water and becomes a sulfate ion (SO ₄ ²⁻ ). The HSO₄ ⁻ and the SO₄²⁻ are linked to one another by the presence or absence of the H ⁺ ion, they are a conjugate acid-base pair.

A conjugate acid is the particle produced when a base accepts a proton. The hydrogen sulfate ion is the conjugate acid of the sulfate ion. A conjugate base is the particle produced when an acid donates a proton. When acting as a base and reacting with water, Ammonia NH₃ can form a conjugate acid when it forms ammonium NH₄⁺.

In the reaction illustrated below, water serves both as acid and base simultaneously. One water molecule serves as an acid and donates a proton. The other water molecule functions as a base by accepting the proton. So, water is capable of being either an acid or a base, a characteristic called amphoterism.

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Key Points

• The formation of conjugate acids and bases is central to the Brønsted-Lowry definition of acids and bases. The conjugate base is the ion or molecule remaining after the acid has lost its proton, and the conjugate acid is the species created when the base accepts the proton.

• Interestingly, water is amphoteric and can act as both an acid and a base. Therefore, it can play all four roles: conjugate acid, conjugate base, acid, and base.

• A Brønsted-Lowry acid-base reaction can be defined as: acid + base $\rightleftharpoons$conjugate base + conjugate acid.

• Ammonia can act as a base by accepting a hydrogen ion to form the conjugate acid ammonium NH₄⁺.

Key Terms

Brønsted-Lowry acid/base reaction: A transfer of a proton from one molecule or ion to another.

Conjugate acid: The particle produced when a base accepts a proton.

Conjugate base: The particle produced when an acid donates a proton.

Conjugate acid-base pair: A pair of substances related by the loss or gain of a single hydrogen ion.

Brønsted-Lowry acid: A molecule or ion that donates a hydrogen ion in a reaction.

Brønsted-Lowry base: A molecule or ion that accepts a hydrogen ion in a reaction.

Amphoterism: The quality of exhibiting the characteristics of an acid and a base and having the capacity to react either as an acid or a base.