State which of the following species are amphiprotic and write chemical equations illustrating the amphiprotic character of these species:

\({\rm{\;(a)\;}}{{\rm{H}}_2}{\rm{O}}\)

\({\rm{\;(b)\;}}{{\rm{H}}_2}{\rm{PO}}_4^ - \)

\({\rm{\;(c)\;}}{{\rm{S}}^{2 - }}\)

\({\rm{\;(d)\;C}}{{\rm{O}}_3}2 - \)

\({\rm{\;(e)\;HSO}}_4^ - \)

Amphiprotic species sometimes react as a Bronsted-Lowry's acid (proton donor) or as a base (proton acceptor).

a) Water is amphiprotic and we can show this with equation of water ionization:

\({{\bf{H}}_2}{\bf{O}}(l) + {H_2}{\rm{O}}(l) \to {{\bf{H}}_3}{{\bf{O}}^ + }(aq) + O{H^ - }(aq)\)

Molecule of water that is bolded acts as a base, it takes proton from the other water. Other molecule of water is an acid. So with this equation we see that water can act as an acid and as a base.

b) \({{\rm{H}}_2}{\rm{PO}}_4^ - \)is also amphiproticand it can be seen in its reactions with water. Here acts like an acid:

\({{\rm{H}}_2}{\rm{PO}}_4^ - (aq) + {{\rm{H}}_2}{\rm{O}}(l) \to {\rm{HPO}}_4^{2 - }(aq) + {{\rm{H}}_3}{{\rm{O}}^ + }(aq)\)

In following equation acts as a base:

\({{\rm{H}}_2}{\rm{PO}}_4^ - (aq) + {{\rm{H}}_2}{\rm{O}}({\rm{l}}) \to {{\rm{H}}_3}{\rm{P}}{{\rm{O}}_4}(aq) + {\rm{O}}{{\rm{H}}^ - }(aq)\)

c) \({S^{2 - }}\)is not amphiprotic. It can act like a base - proton acceptor, but it cannot be an acid because it does not have any proton to donate.

d) \({\rm{CO}}_3^{2 - }\)is also not amphiprotic. It can act like a base - proton acceptor, but it cannot be an acid because it does not have any proton to donate.

e) \({\rm{HSO}}_4^ - \) is amphiprotic. In reaction with water can be an acid-proton donor:

\({\rm{HSO}}_4^ - (aq) + {{\rm{H}}_2}{\rm{O}}(l) \to {\rm{SO}}_4^{2 - }(aq) + {{\rm{H}}_3}{{\rm{O}}^ + }(aq)\)

Also, in reaction with water it can be a base - proton acceptor:

\({\rm{HSO}}_4^ - (aq) + {{\rm{H}}_2}{\rm{O}}({\rm{l}}) \to {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}(aq) + {\rm{O}}{{\rm{H}}^ - }(aq)\)