What is an oxyacid?

An acid is a substance which is able to donate protons (#H^+#).

In the case of an oxyacid, #XOH#, this ability is due to the high electron withdrawing effect of the #XO# group upon the hydrogen atom, as describet in detail in the following.

The element X is an electronegative nonmetal, as #N, S, Cl# etc. or a high oxidation state metal, as #Mn(VII)# or #Cr(VI)#.

Then, helped by the high electronegativity of the oxygen atom, the couple #XO"-"# causes a withdrawing effect on the electron pair of the #O-H# bond.

The proton (#H^+#), at the end of chain, is partially unshielded and ready to be transferred to bond an electron pair furnished by a basic species.

This process is called ionisation and leaves an oxoanion, #XO^-# as the residue of the oxyacid.

So, the complete acid-base or proton transfer process is:

#XOH + :B^"-" -> XO^"-" + H:B#

Example with nitric acid (#HNO_3 = O_2NOH#, where #X = O_2N#) and hydroxide ion as base:

#HNO_3 + :OH^"-" -> NO_3^"-" + H_2O#

Example with nitric acid and ammonia as base:

#HNO_3 + :NH_3 -> NO_3^"-" + NH_4^+#

In some cases the central element is not highly electronegative, but it gains withdrawing effect by its bonds with extra oxygen atoms, as in carbonic acid (#H_2CO_3 = OC(OH)_2#), phosphoric acid, #H_3PO_4 = OP(OH)_3#, phosphonic acid #H_3PO_3 = OPH(OH)_2#.

In general, the higher is the number of extra oxygen atoms, the stronger is the oxyacid (i.e. it is more easily ionizable).

This can be demonstrated from many evidences, as:

a) nitric acid #HNO_3 = O_2NOH# is stronger than nitrous acid #HNO_2 = ONOH#

b) sulfuric acid #H_2SO_4 = O_2S(OH)_2# is stronger than sulfurous acid #H_2SO_3 = OS(OH)_2#

c) the chlorine oxyacids are stronger in the sequence:

#HClO < HClO_2 < HClO_3 < HClO_4# that is:

#ClOH < OClOH < O_2ClOH < O_3ClOH#

or, by name:

hypochlorous < chlorous < chloric < perchloric acid