Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 8

Big Idea #6: Equilibrium, Acids and Bases, Titrations, and Solubility

POLYPROTIC ACIDS AND AMPHOTERIC SUBSTANCES

Some acids, such as H₂SO₄ and H₃PO₄, can give up more than one hydrogen ion in solution. These are called polyprotic acids.

Polyprotic acids are always more willing to give up their first protons than later protons. For example, H₃PO₄ gives up an H⁺ ion (proton) more easily than does , so H₃PO₄ is a stronger acid. In the same way, is a stronger acid than .

Substances that can act as either acids or bases are called amphoteric substances.

For instance

·  can act as an acid, giving up a proton to become , or it can act as a base, accepting a proton to become H₃PO₄

·  can act as an acid, giving up a proton to become , or it can act as a base, accepting a proton to become H₂SO₄

· H₂O can act as an acid, giving up a proton to become OH⁻, or it can act as a base, accepting a proton to become H₃O⁺