Brønsted–Lowry Theory
Simple Explanation
The Brønsted–Lowry theory defines an acid as a proton (H⁺) donor and a base as a proton (H⁺) acceptor — a broader definition than Arrhenius theory that works even without water, and even for substances with no OH⁻ in their formula at all.
Why Do We Need It?
This theory explains why ammonia (NH₃), which contains no hydroxide ion at all, can still act as a base — it accepts a proton from water to form NH₄⁺ and OH⁻, satisfying the Brønsted–Lowry definition even though it fails a strict reading of Arrhenius theory.
Why Does This Work?
By defining acids and bases purely in terms of proton transfer, this theory works for any solvent (or no solvent at all) — every acid-base reaction, viewed this way, is simply a proton moving from a donor (the acid) to an acceptor (the base), producing a conjugate base (what's left of the acid) and a conjugate acid (what the base becomes after accepting the proton).
Real-Life Example
Ammonia acting as a base in water
Ammonia gas dissolved in water produces a basic (alkaline) solution, even though NH₃ contains no OH⁻.
NH₃ accepts a proton from a water molecule: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ — ammonia is the Brønsted–Lowry base (proton acceptor), and water acts as the acid (proton donor) in this reaction.
Practice
In the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, which species is the Brønsted–Lowry acid?
MediumCommon mistake
Assuming a substance must be classified as ONLY an acid or ONLY a base — many substances (like water) are amphoteric, meaning they can act as either a Brønsted–Lowry acid or base depending on what they are reacting with.
Quick Review
- Brønsted–Lowry acid: a proton (H⁺) donor.
- Brønsted–Lowry base: a proton (H⁺) acceptor.
- Broader than Arrhenius theory — explains bases like NH₃ with no OH⁻.