Why does successive acid dissociation constants Ka1 Ka2 etc of a polyprotic acid decrease in magnitude?

Answer 1

Polyprotic acids are acids with more than one ionizable proton. For example: phosphoric acid (H3PO4) has 3 ionizable protons, therefore it has 3 K

a values. The reason successive acid dissociation constants decrease in magnitude is because it is more difficult for an H+ ion to leave a singly charged anion (like H2PO4-) than to leave a neutral molecule (like H3PO4); it's even more difficult to the proton to be "ripped off" from a doubly charged anion (like HPO42-).

All polyprotic molecules follow this pattern:

Ka1>Ka2>>Ka3
The smaller the Ka value is, the easier the proton will be removed.

Think of it as the positively charged proton is very attracted to the negative molecule. The more negative that molecule is, the harder it is to separate the proton. But when the molecule becomes less negative, the proton is easier to be removed because their is less of a negative attraction "clinging" it to the molecule.