Why acidity increases with increasing oxidation state?

Answer 1

This happens for two reasons:

1) When the central atom of an oxyacid is highly oxidized, that implies that there are several highly electronegatives atoms bonded to that central atom. Those electronegative atoms will pull electron density toward themselves, making the bonds in the molecule more polar. Specifically, the O-H bond will become more polar. As the electron density between the O and H lessens, there is less of an attachment of the O to the H, so the dissociation of the H becomes more likely. Therefore, acidity is increased.

2) When the central atom of an oxyacid is highly oxidized, that implies that there are several highly electronegatives atoms bonded to that central atom. When the H has left, and the molecule is left as a conjugate base, the negative charge will generally be stabilized by the presence electronegative atoms, who will gladly take a share in the electron density, thereby smearing out the negative charge that results. A stable base is a weak base, so the fact that the resulting conjugate base does not have a highly condensed, localized charge, implies that it will not have a strong affinity for the H+ now in the solution. Thus most of the acid will dissociate, and will remain that way.