MCAT Content / Acid Base Equilibria / Bronsted Lowry Definition Of Acid Base

Brønsted–Lowry Definition of Acid, Base

Topic: Acid Base Equilibria

A compound that donates a proton to another compound is called a Brønsted-Lowry acid, and a compound that accepts a proton is called a Brønsted-Lowry base.

The Arrhenius definition of acids and bases is somewhat limited. A Brønsted-Lowry acid is a molecule or ion that donates a hydrogen ion in a reaction. A Brønsted-Lowry base is a molecule or ion that accepts a hydrogen ion in a reaction. A hydrogen ion is commonly referred to as a proton, and so acids and bases are proton donors and proton acceptors respectively according to the Brønsted-Lowry definition. All substances that are categorized as acids and bases under the Arrhenius definition are also defined as such under the Brønsted-Lowry definition. The new definition, however, includes some substances that are left out according to the Arrhenius definition.

A Brønsted-Lowry acid is a molecule or ion that donates a hydrogen ion in a reaction. A Brønsted-Lowry base is a molecule or ion that accepts a hydrogen ion in a reaction. A hydrogen ion is commonly referred to as a proton, and so acids and bases are proton donors and proton acceptors respectively according to the Brønsted-Lowry definition.

A conjugate acid-base pair is a pair of substances related by the loss or gain of a single hydrogen ion. A conjugate acid is the particle produced when a base accepts a proton. A conjugate base is the particle produced when an acid donates a proton. In this image, we can see sulphuric acid acting as an acid by donating a proton and the forming of a hydrogen sulfate ion which can act as its conjugate base.

A typical Brønsted-Lowry acid-base reaction contains two conjugate acid-base pairs as shown below.

One conjugate acid-base pair is HNO / NO − , while the other pair is HPO 2− / PO 3− .


Key Points

• Brønsted-Lowry acids and bases is a way of defining acidic and basic compounds that extends the Arrhenius theory.

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

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

• A conjugate acid-base pair is a pair of substances related by the loss or gain of a single hydrogen ion. A conjugate acid is the particle produced when a base accepts a proton. A conjugate base is the particle produced when an acid donates a proton.

• A typical Brønsted-Lowry acid-base reaction contains two conjugate acid-base pairs.


Key Terms

Arrhenius definition: Arrhenius acids form hydrogen ions in aqueous solution with Arrhenius bases forming hydroxide ions.

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.

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

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

Conjugate base is the particle produced when an acid donates a proton.



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