7.7 Polyprotic Acids

polyprotic acids

can donate more than one proton per molecule

examples of polyprotic acids

H₂SO₄ (sulfuric) and H₃PO₄ (phosphoric acid)

how do polyprotic acids dissociate

one proton at a time, stepwise manner

How does carbonic acid (H₂CO₃) dissociate?

H₂CO₃ + H₂O ⇋ H₃O⁺ + HCO₃^-

HCO₃^- + H₂O ⇋ H₃O⁺ + CO₃^2-

triprotic acids

lose 3 H⁺

ex: Phosphoric acid

for a weak polyprotic acid, how does K_a change

K_a1 > K_a2 > K_a3

for a weak polyprotic acid, why is  K_a1 > K_a2 > K_a3

greater negative charge on acid (acid loses a proton so becomes more negative) makes it more difficult to remove H⁺

for a weak tripotic acid, which dissociation step makes the largest contribution to [H⁺]

first step, use it to determine pH

Sulfuric Acid

• Strong acid in the first dissociation step, weak acid in second step

  • Only the first step makes an important contribution to [H⁺]

However, in dilute H₂SO₄ solutions, second dissociation step contributed significantly to [