Acid-Base Reactions: Using the Brønsted-Lowry System

The Brønsted-Lowry theory states that acid-base reactions are a competition for a proton. For example, take a look at the reaction of ammonia with water:

Ammonia is a base (it accepts the proton), and water is an acid (it donates the proton) in the forward (left to right) reaction.

But in the reverse reaction (right to left), the ammonium ion is an acid and the hydroxide ion is a base. If water is a stronger acid than the ammonium ion, then there is a relatively large concentration of ammonium and hydroxide ions at equilibrium. If, however, the ammonium ion is a stronger acid, much more ammonia than ammonium ion is present at equilibrium.

Brønsted and Lowry said that an acid reacts with a base to form conjugate acid-base pairs. Conjugate acid-base pairs differ by a single H⁺. NH₃ is a base, for example, and NH₄⁺ is its conjugate acid. H₂O is an acid in the reaction between ammonia and water, and OH^– is its conjugate base. In this reaction, the hydroxide ion is a strong base and ammonia is a weak base, so the equilibrium is shifted to the left — there’s not much hydroxide at equilibrium.