10.2 Brønsted-Lowry Definition of Acids and Bases

In 1923, the Danish scientist Johannes Brønsted and the English scientist Thomas Lowry independently proposed new definitions for acids and bases. Rather than considering both hydrogen and hydroxide ions, they focused on the hydrogen ion only. A Brønsted-Lowry acid is a compound that supplies a hydrogen ion in a reaction. A Brønsted-Lowry base, conversely, is a compound that accepts a hydrogen ion in a reaction. Thus, the Brønsted-Lowry definitions of an acid and a base focus on the movement of hydrogen ions in a reaction, rather than on the production of hydrogen ions and hydroxide ions in an aqueous solution.  Reactions that involve weak acids and bases are also reversible, indicated by the double arrow, and the chemical that acts as the acid in the reverse reaction is identified as the conjugate acid, while the chemical that acts as the base in the reverse reaction is the conjugate base.

The reaction of ammonia in water demonstrates the Brønsted-Lowry definitions of acid, base, conjugate acid, and conjugate base. Ammonia and water molecules are reactants, while the ammonium ion and the hydroxide ion are products:

NH₃(aq) + H₂O(ℓ) → NH₄⁺(aq) + OH⁻(aq)

What has happened in this reaction is that the original water molecule has donated a hydrogen ion to the original ammonia molecule, which in turn has accepted the hydrogen ion. We can illustrate this as follows:

Because the water molecule donates a hydrogen ion to the ammonia, it is the Brønsted-Lowry acid, while the ammonia molecule—which accepts the hydrogen ion—is the Brønsted-Lowry base. Thus, ammonia acts as a base in both the Arrhenius sense and the Brønsted-Lowry sense.

Brønsted-Lowry View of Reaction of an Acid with Water

Understanding how an Arrhenius acid like hydrochloric acid is still an acid in the Brønsted-Lowry sense requires understanding what really happens when HCl dissolves in water. Recall that the hydrogen atom is a single proton surrounded by a single electron. The hydrogen ion is formed by removing the electron, leaving a bare proton. Are there really bare protons floating around in aqueous solution?  No. What really happens is that the H⁺ ion attaches itself to H₂O to make H₃O⁺, which is called the hydronium ion. For most purposes, H⁺ and H₃O⁺ represent the same species, but writing H₃O⁺ instead of H⁺ recognizes that there are no bare protons floating around in solution. Rather, these protons are actually attached to solvent molecules

With this in mind, how is HCl defined as an acid in the Brønsted-Lowry sense? Consider what happens when HCl dissolves in H₂O:

HCl + H₂O(ℓ) → H₃O⁺(aq) + Cl⁻(aq)

HCl is a strong electrolyte/strong acid, so there is no reverse arrow.  Lewis electron dot diagrams show the movement of a proton:

Now we see that a hydrogen ion is transferred from the HCl molecule to the H₂O molecule to make chloride ions and hydronium ions. As the hydrogen ion donor, HCl acts as a Brønsted-Lowry acid; as a hydrogen ion acceptor, H₂O is a Brønsted-Lowry base. So HCl is an acid not just in the Arrhenius sense but also in the Brønsted-Lowry sense. Moreover, by the Brønsted-Lowry definitions, H₂O is a base in the formation of aqueous HCl. So the Brønsted-Lowry definitions of an acid and a base classify the dissolving of HCl in water as a reaction between an acid and a base—although the Arrhenius definition would not have labeled H₂O a base in this circumstance.