By the Bronsted-Lowry definition:
- A Bronsted acid donates protons.
- A Bronsted base accepts protons.
#"HPO"_4^(2-) + "H"_2"O" rightleftharpoons "H"_2"PO"_4^(-) + "OH"^(-)#
#color(blue)(stackrel("Bronsted Base")overbrace("HPO"_4^(2-)) + stackrel("Bronsted Acid")overbrace("H"_2"O") rightleftharpoons stackrel("Conjugate Acid")overbrace("H"_2"PO"_4^(-)) + stackrel("Conjugate Base")overbrace("OH"^(-)))#
- The Bronsted base and the conjugate acid are one of the two acid-base pairs.
- The Bronsted acid and the conjugate base are the other acid-base pair.
If you compare this back to the Lewis definition:
- A Lewis acid accepts electron pairs.
- A Lewis base donates electron pairs.
then an interesting connection between the two is that:
- In acid-base reactions, electrons are often accepted by a Lewis acid in exchange for a donated proton that is accepted by a Bronsted base.
- In acid-base reactions, electrons are often donated by a Lewis base in exchange for an accepted proton donated from a Bronsted acid.
- A Lewis base that becomes protonated is also a Bronsted base.
- A Lewis acid that becomes deprotonated is also a Bronsted acid.