Conjugate Base Definition (Chemistry)

Conjugate Base Definition

The Bronsted-Lowry acid-base theory includes the concepts of conjugate acids and conjugate bases. When an acid dissociates into its ions in water, it loses a hydrogen ion. The species that is formed is the acid's conjugate base. A more general definition is that a conjugate base is the base member, X^-, of a pair of compounds that transform into each other by gaining or losing a proton. The conjugate base is able to gain or absorb a proton in a chemical reaction. The conjugate acid donates the proton or hydrogen in the reaction.

In an acid-base reaction, the chemical reaction is:

Acid + Base ⇌ Conjugate Base + Conjugate Acid

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Conjugate Acid Definition in Chemistry

By Anne Marie Helmenstine, Ph.D.

Conjugate Base Examples

The general chemical reaction between a conjugate acid and a conjugate base is:

HX + H₂O ↔ X⁻ + H₃O⁺

In an acid-base reaction, you can recognize the conjugate base because it is an anion. For hydrochloric acid (HCl), this reaction becomes:

HCl + H₂O ↔ Cl⁻ + H₃O⁺

Here, the chloride anion, Cl⁻, is the conjugate base.

Sulfuric acid, H₂SO₄ forms two conjugate bases as hydrogen ions are successively removed from the acid: HSO₄^- and SO₄^2-.