Final answer:
Polyprotic acids have different Ka values for each proton because they ionize stepwise, with each step having a different strength of ionization and corresponding pKa.
Step-by-step explanation:
Polyprotic acids, such as H₂SO₄ (sulfuric acid), H₃PO₄ (phosphoric acid), and H₂CO₃ (carbonic acid), have different Ka values because they contain more than one ionizable proton. Each proton ionizes in a stepwise fashion, with each step having its own acid ionization constant, denoted as Ka.
For polyprotic acids, acid strength decreases with the sequential loss of protons, and, correspondingly, the pKa value increases. This reflects that it's easier to remove a proton from a neutral molecule than from a negatively charged ion. Often, the difference in the successive ionization constants is on the order of 10⁵ to 10⁶, signifying that each successive ionization is less extensive.