Writing acid-base reactions follows all the general rules of writing balanced equations. In this case, you know that one reactant (your acid) will be losing a proton during the reaction (and becoming one unit less positive / more negative along the way), and that the proton will be added to your basic reactant (and also gaining the proton’s positive charge).
Write separate equations representing the reaction of $HSO_3^-$
(a) as an acid with OH−
(b) as a base with HI
Solution
(a) $HSO_3^-\;(aq)+OH^-\;(aq)⇌SO_3^{2-}\;(aq)+H_2O\;(l)$
(b) $HSO_3^-\;(aq)+HI\;(aq)⇌H_2SO_3\;(aq)+I^-\;(aq)$
Check Your Learning
Write separate equations representing the reaction of $H_2PO_4^-$
(a) as a base with HBr
(b) as an acid with OH−
Answer:
(a) $H_2PO_4^-\;(aq)+HBr\;(aq)⇌H_3PO_4\;(aq)+Br^-\;(aq)$
(b) $H_2PO_4^-\;(aq)+OH^-\;(aq)⇌HPO_4^{2-}\;(aq)+H_2O\;(l)$