# Question #bc43f

Jan 15, 2015

When an acid reacts with water, it forms a hydronium ion plus the conjugate base of the original acid. Likewise, when a base reacts with water, it forms hydroxide ion plus the conjugate acid of the original base.

Using generic symbol HA for a weak acid, we can write the following balanced proton transfer reaction with water:

$H A + {H}_{2} O \leftrightarrow {A}^{-} + {H}_{3} {O}^{+}$

Because ${A}^{-}$ differs from $H A$ only by the proton that was lost to water, we say that ${A}^{_}$ is the conjugate base of the acid $H A$. Likewise, HA is the conjugate acid of the base ${A}^{-}$. In other words, $H A$ and ${A}^{-}$ form a conjugate acid-base pair.

In the same way, we can write a balanced proton transfer reaction of a base, $B$, with water:

$B + {H}_{2} O \leftrightarrow B {H}^{+} + O {H}^{-}$

For the same reasons as above, we say that $B {H}^{+}$ and $B$ form a conjugate acid-base pair.