What are the two acid-base pairs in water?
The two acid-base pairs are H₃O⁺-H₂O and H₂O-OH⁻.
A Brønsted-Lowry acid is a proton donor. A Brønsted-Lowry base is a proton acceptor.
When a Brønsted-Lowry acid donates a proton to something else, the particle that is missing the proton is the conjugate base. Together the acid and the base make a conjugate acid-base pair.
When a Brønsted-Lowry base accepts a proton from something else, the particle that now has the proton is the conjugate acid. Together the base and the acid make a conjugate acid-base pair.
Water undergoes autoionization according to the equation:
The first H₂O has donated a proton (H⁺) to the second H₂O. The first H₂O is a Brønsted-Lowry acid, and the second H₂O is a Brønsted-Lowry base.
The first H₂O lost a proton to become OH⁻. So OH⁻ is the conjugate base of H₂O. A conjugate base always has one less H atom and one more negative charge than the original acid.
The second H₂O gained a proton to form H₃O⁺. So H₃O⁺ is the conjugate acid of H₂O. A conjugate acid always has one more H atom and one more positive charge than the original acid.
Thus, H₂O and OH⁻ are a conjugate acid-base pair, and H₃O⁺ and H₂O are a conjugate acid-base pair. A Brønsted-Lowry acid-base reaction always contains two conjugate acid-base pairs.