4.8 Introduction to Acid-Base Reactions

Defining acids and bases

By the Arrhenius definition, acids increase [H⁺] and bases increase [OH⁻] in water. The broader Brønsted-Lowry definition describes acids as proton (H⁺) donors and bases as proton acceptors. In an acid-base reaction, a proton transfers from the acid to the base, forming a conjugate base and conjugate acid.

Conjugate acid-base pairs

A conjugate acid-base pair differs by exactly one H⁺. When HA donates a proton to become A⁻, HA is the acid and A⁻ is its conjugate base. The species that accepts the proton (like water becoming H₃O⁺) becomes a conjugate acid. Recognizing these pairs helps identify reactants and products in acid-base equations.

Neutralization reactions

In neutralization, an acid reacts with a base to form water and a salt (an ionic compound). For example: HCl + NaOH → NaCl + H₂O. These reactions can be represented with full molecular, ionic, or net ionic equations, and often involve strong acids/bases that fully dissociate in solution.

Key terms

Brønsted-Lowry acid

A substance that donates a proton (H⁺) in a reaction.

Brønsted-Lowry base

A substance that accepts a proton (H⁺) in a reaction.

Conjugate acid-base pair

Two species that differ by exactly one proton (H⁺), such as HA and A⁻.

Neutralization reaction

A reaction between an acid and a base that produces water and a salt.

Arrhenius definition

Acids increase [H⁺] in solution; bases increase [OH⁻] in solution.

Try an AP-style question

In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, which species acts as the acid?

1. NH₃
2. OH⁻
3. NH₄⁺
4. H₂O