Nitrous acid, HNO2, is weaker than
nitric acid, HNO3; and phosphorous
is weaker than phosphoric acid, H3PO4.
Sulfurous acid, H2SO3,
is not as strong as H2SO4.
Chlorine can form an entire series of oxyacids with decreasing oxygen
content and decreasing acid strength:
or H-O-ClO3 Perchloric
- HClO3 or H-O-ClO2
- HClO2 or H-O-ClO Chlorous
- HClO or H-O-Cl Hypochlorous Acid
In each of these acids, shown at the left, the dissociating
proton is bonded to an oxygen atom; thus the second structure given
above for each acid is a more accurate model of the molecule.
The greater the number of oxygen atoms bonded to the Cl, the greater
is the attraction by the rest of the molecule for the electrons
of the W-O bond, and the more likely the acid molecule is to dissociate
into H+ and an oxyanion.
Perchloric acid is the strongest common inorganic acid known, but
bypochlorous acid is weak.
The third-row nonmetals also form acids other than oxyacids, in
which the protons are attached directly to the central atom, as
Hydrochloric acid, HCl, is a strong acid and, with HNO3
makes up the trio of acids that you are most likely to encounter
in the laboratory.
Sulfur is less electronegative than chlorine, therefore hydrogen
sulfide, H2S, is a much weaker acid.
Even so, it is stronger than its second-row analogue, H2O.
This is so because the sulfur atom is large enough that the protons
cannot get as close to the center of charge on the sulfur atom,
and the attraction between S and H is weakened.