Conjugate Base Definition (Chemistry)

Bronsted Lowry Acids and Bases

The conjugate base of hydrochloric acid is the chloride anion.
The conjugate base of hydrochloric acid is the chloride anion. Josh Westrich / Getty Images

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

Key Takeaways: Conjugate Base

  • Conjugate acids and bases are part of the Bronsted-Lowry theory of acids and bases.
  • According to this theory, the species that donates a hydrogen cation or proton in a reaction is a conjugate acid, while the remaining portion or the one that accepts a proton or hydrogen is the conjugate base.
  • The conjugate base is an anion.

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 SO42-.

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Your Citation
Helmenstine, Anne Marie, Ph.D. "Conjugate Base Definition (Chemistry)." ThoughtCo, Nov. 28, 2022, Helmenstine, Anne Marie, Ph.D. (2022, November 28). Conjugate Base Definition (Chemistry). Retrieved from Helmenstine, Anne Marie, Ph.D. "Conjugate Base Definition (Chemistry)." ThoughtCo. (accessed June 8, 2023).