Chemistry » Acid-Base Equilibria » Brønsted-Lowry Acids and Bases

Amphiprotic Species

Amphiprotic Species

Like water, many molecules and ions may either gain or lose a proton under the appropriate conditions. Such species are said to be amphiprotic. Another term used to describe such species is amphoteric, which is a more general term for a species that may act either as an acid or a base by any definition (not just the Brønsted-Lowry one). Consider for example the bicarbonate ion, which may either donate or accept a proton as shown here:

$$\begin{array}{}{\text{HCO}}_{3}{}^{\text{−}}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\phantom{\rule{0.2em}{0ex}}⇌\phantom{\rule{0.2em}{0ex}}{\text{CO}}_{3}{}^{\text{2−}}\left(aq\right)+{\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)\\ {\text{HCO}}_{3}{}^{\text{−}}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\phantom{\rule{0.2em}{0ex}}⇌\phantom{\rule{0.2em}{0ex}}{\text{H}}_{2}{\text{CO}}_{3}\left(aq\right)+{\text{OH}}^{\text{−}}\left(aq\right)\end{array}$$

Example

Representing the Acid-Base Behavior of an Amphoteric Substance

Write separate equations representing the reaction of $${\text{HSO}}_{3}{}^{\text{−}}$$

(a) as an acid with OH

(b) as a base with HI

Solution

(a) $${\text{HSO}}_{3}{}^{\text{−}}\left(aq\right)+{\text{OH}}^{\text{−}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⇌\phantom{\rule{0.2em}{0ex}}{\text{SO}}_{3}{}^{\text{2−}}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)$$

(b) $${\text{HSO}}_{3}{}^{\text{−}}\left(aq\right)+\text{HI}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⇌\phantom{\rule{0.2em}{0ex}}{\text{H}}_{2}{\text{SO}}_{3}\left(aq\right)+{\text{I}}^{\text{−}}\left(aq\right)$$