Bronsted-Lowry Acid Definition

Learn What a Bronsted-Lowry Acid Is in Chemistry

Bronsted-Lowry acids give up hydrogen ions in a chemical reaction

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In 1923, chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently described acids and bases based on whether they donate or accept hydrogen ions (H+). The groups of acids and bases defined in this manner came to be known as either Bronsted, Lowry-Bronsted, or Bronsted-Lowry acids and bases.

A Bronsted-Lowry acid is defined as a substance that gives up or donates hydrogen ions during a chemical reaction. In contrast, a Bronsted-Lowry base accepts hydrogen ions. Another way of looking at it is that a Bronsted-Lowry acid donates protons, while the base accepts protons. Species that can either donate or accept protons, depending on the situation, are considered to be amphoteric.

The Bronsted-Lowry theory differs from the Arrhenius theory is allowing acids and bases that don't necessarily contain hydrogen cations and hydroxide anions.

Key Takeaways: Bronsted-Lowry Acid

  • The Bronsted-Lowry theory of acids and bases was proposed independently in 1923 by Johannes Nicolaus Brønsted and Thomas Martin Lowry.
  • A Bronsted-Lowry acid is a chemical species that donates one or more hydrogen ions in a reaction. In contrast, a Bronsted-Lowry base accepts hydrogen ions. When it donates its proton, the acid becomes its conjugate base.
  • A more general look at the theory is an acid as a proton donor and a base as a proton acceptor.

Conjugate Acids and Bases in Bronsted-Lowry Theory

Every Bronsted-Lowry acid donates its proton to a species which is its conjugate base. Every Bronsted-Lowry base similarly accepts a proton from its conjugate acid.

For example, in the reaction:

HCl (aq) + NH3 (aq)→ NH4+ (aq) + Cl- (aq)

Hydrochloric acid (HCl) donates a proton to ammonia (NH3) to form the ammonium cation (NH4+) and the chloride anion (Cl-). Hydrochloric acid is a Bronsted-Lowry acid; the chloride ion is its conjugate base. Ammonia is a Bronsted-Lowry base; its conjugate acid is the ammonium ion.

Sources

  • Brönsted, J. N. (1923). "Einige Bemerkungen über den Begriff der Säuren und Basen" [Some observations about the concept of acids and bases]. Recueil des Travaux Chimiques des Pays-Bas. 42 (8): 718–728. doi:10.1002/recl.19230420815
  • Lowry, T. M. (1923). "The uniqueness of hydrogen". Journal of the Society of Chemical Industry. 42 (3): 43–47. doi:10.1002/jctb.5000420302