Question

What is an oxyacid?

Answer 1

It is an acidic substance in which one or more hydrogen atoms are bonded to oxygen atoms that are on turn bonded to another element "`X`": `X-O-H`.

Explanation:

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