Bronsted-Lowry Acid Definition

Learn What a Bronsted-Lowry Acid Is in Chemistry

Bronsted-Lowry acids give up hydrogen ions in a chemical reaction.
Bronsted-Lowry acids give up hydrogen ions in a chemical reaction. ANDREW MCCLENAGHAN/SCIENCE PHOTO LIBRARY. / Getty Images

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 in allowing acids and bases that don't necessarily contain hydrogen cations and hydroxide anions.

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; it's conjugate acid is the ammonium ion.