Polyprotic Acids and Amphiprotic Species

POLYPROTIC ACIDS

Most of the acids we work with can only donate one proton (H⁺ ion). However, some acids have more than one hydrogen ion that can be donated when they react (or dissociate in water).

For example, sulfuric acid:

H₂SO₄ + 2H₂O → SO₄^2- + 2H₃O⁺

However, this is not the whole picture.

First, sulfuric acid loses just one hydrogen ion to become the hydrogen sulfate ion. This ion will become important shortly... it can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base!!

H₂SO₄ + H₂O ⇋ HSO₄^- + H₃O⁺

Secondly, the hydrogen sulfate ion loses a hydrogen ion to complete the dissociation of the acid:

HSO₄^- + H₂O ⇋ SO₄^2- + H₃O⁺

AMPHIPROTIC SPECIES

Amphiprotic means "can act as both an acid or as a base". Water is the amphiprotic species you have met before:

H₂O + H₂O → OH^- + H₃O⁺

The red water molecule is acting as a Brønsted-Lowry acid - it is losing a proton to become the hydroxide ion.

The blue water molecule is acting as a Brønsted-Lowry base - it is gaining a proton to become the hydronium ion.

Hydrogen sulfate can also do this because:

• it is negatively charged, so can attract a hydrogen ion (proton). Opposites attract.
• it also has a hydrogen ion, which it can lose.

Acting as an acid:

HSO₄^- + H₂O ⇋ SO₄^2- + H₃O⁺

^{Acting as a base:}

HSO₄^- + H₂O ⇋ H₂SO₄ + OH^-