# Question 27a2d

Jun 29, 2015

Yes, the bicarbonate ion is amphoteric.

#### Explanation:

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, ${H}_{2} C {O}_{3}$, or give off a proton to become the carbonate ion, $C {O}_{3}^{2 -}$.

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

${\underbrace{H C {O}_{3 \left(a q\right)}^{-}}}_{\textcolor{b l u e}{\text{acts as base}}} + {H}_{2} {O}_{\left(l\right)} r i g h t \le f t h a r p \infty n s {H}_{2} C {O}_{3 \left(a q\right)} + O {H}_{\left(a q\right)}^{-}$

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

${\underbrace{H C {O}_{3 \left(a q\right)}^{-}}}_{\textcolor{g r e e n}{\text{acts as acid}}} + {H}_{2} {O}_{\left(l\right)} r i g h t \le f t h a r p \infty n s C {O}_{3 \left(a q\right)}^{2 -} + {H}_{3} {O}_{\left(a q\right)}^{+}$

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

${\underbrace{H C {O}_{3 \left(a q\right)}^{-}}}_{\textcolor{b l u e}{\text{acts as base}}} + {H}_{3} {O}_{\left(a q\right)}^{+} r i g h t \le f t h a r p \infty n s {H}_{2} C {O}_{3 \left(a q\right)} + {H}_{2} {O}_{\left(l\right)}$

${\underbrace{H C {O}_{3 \left(a q\right)}^{-}}}_{\textcolor{g r e e n}{\text{acts as acid}}} + O {H}_{\textrm{\left(a q\right]}}^{-} r i g h t \le f t h a r p \infty n s C {O}_{3 \left(a q\right)}^{2 -} + {H}_{2} {O}_{\left(l\right)}$

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

${H}_{2} C {O}_{3 \left(a q\right)} r i g h t \le f t h a r p \infty n s C {O}_{2 \left(a q\right)} + {H}_{2} {O}_{\left(l\right)}$