Chapter 14 · Acids and Bases
14.4Molecular Definitions of Acids and Bases
10 min · two checks
Predict
In Brønsted–Lowry terms, what is a base?
The idea
- State Arrhenius and Brønsted–Lowry definitions.
- Identify conjugate pairs.
Arrhenius: acids produce H⁺ in water, bases produce OH⁻ in water. It fits HCl and NaOH and struggles to call ammonia a base without an extra sentence. Brønsted–Lowry: an acid donates a proton, H⁺, and a base accepts it. HCl + H₂O → H₃O⁺ + Cl⁻ makes water the base. NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ makes water the acid. Water can play either role.
Every Brønsted acid–base reaction has two conjugate pairs. The acid becomes its conjugate base by losing H⁺. The base becomes its conjugate acid by gaining H⁺. In the ammonia reaction, NH₄⁺ is the conjugate acid of NH₃, and OH⁻ is the conjugate base of H₂O. A conjugate pair differs by exactly one H⁺. If your pair differs by two, you grabbed the wrong partner.
Keep these
- Arrhenius focuses on H⁺ or OH⁻ in water.
- Brønsted–Lowry: donate or accept a proton.
- A conjugate pair differs by one H⁺.