Question

Rank the relative acid strengths of the following compounds: H2O, H2S, and H2Se

___ > ___ > ___

Answer 1

Acidity of a compound can be analyzed by looking at the stability of the CONJUGATE base. Recall that the loss of H+ from these compounds results in an anion.

A STRONGER/LESS STABLE ACID will yield a WEAKER/MORE STABLE CONJUGATE BASE!

As a general rule for these "series" of molecules, stability of the conjugate base increases with the size of the central electronegative atom because the larger atom can better spread out the anion's negative charge. Remember that separating or spreading like-charge is energetically preferrable.

The central atom size increases as: O < S < Se

Thus, the order of stability for the conjugate bases:
-OH < -SH < -SeH

OR, in terms of conjugate basicity:
-SeH < -SH < -OH

Finally, acid-base conjugate pairs are related as I first mentioned. So, in terms of the original acids:

H2Se > H2S > H2O in terms of acid strength.
(i.e. the weakest conjugate base came from the strongest acid)