Question #27a2d

1 Answer
Jun 29, 2015

Yes, the bicarbonate ion is amphoteric.


An amphoteric substance can act both as an acid in one reaction, and as a base in another reaction.

In your case, the bicarbonate ion, #HCO_3""^(-)#, can either accept a proton to become carbonic acid, #H_2CO_3#, or give off a proton to become the carbonate ion, #CO_3^(2-)#.

When placed in aqueous solution, the bicarbonate ion can pick up a proton from water and form carbonic acid and hydroxide ions, thus acting as a base

#underbrace(HCO_(3(aq))^(-))_(color(blue)("acts as base")) + H_2O_((l)) rightleftharpoons H_2CO_(3(aq)) + OH_((aq))^(-)#

or donate a proton to water to form carbonate ions and hydronium ions, thus acting as an acid.

#underbrace(HCO_(3(aq))^(-))_(color(green)("acts as acid")) + H_2O_((l)) rightleftharpoons CO_(3(aq))^(2-) + H_3O_((aq))^(+)#

Likewise, it can react with hydronium ions to form carbonic acid and with hydroxide ions to form bicarbonate ions.

#underbrace(HCO_(3(aq))^(-))_(color(blue)("acts as base")) + H_3O_((aq))^(+) rightleftharpoons H_2CO_(3(aq)) + H_2O_((l))#

#underbrace(HCO_(3(aq))^(-))_(color(green)("acts as acid")) + OH_text((aq])^(-) rightleftharpoons CO_(3(aq))^(2-) + H_2O_((l))#

SIDE NOTE As you can see in the diagram, carbonic acid is actually carbon dioxide and water.

#H_2CO_(3(aq)) rightleftharpoons CO_(2(aq)) + H_2O_((l))#