# What is an oxyacid?

Dec 29, 2015

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, $X O H$, this ability is due to the high electron withdrawing effect of the $X O$ group upon the hydrogen atom, as describet in detail in the following.

The element X is an electronegative nonmetal, as $N , S , C l$ etc. or a high oxidation state metal, as $M n \left(V I I\right)$ or $C r \left(V I\right)$.

Then, helped by the high electronegativity of the oxygen atom, the couple $X O \text{-}$ 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, $X {O}^{-}$ as the residue of the oxyacid.

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

$X O H + : {B}^{\text{-" -> XO^"-}} + H : B$

Example with nitric acid ($H N {O}_{3} = {O}_{2} N O H$, where $X = {O}_{2} N$) and hydroxide ion as base:

$H N {O}_{3} + : O {H}^{\text{-" -> NO_3^"-}} + {H}_{2} O$

Example with nitric acid and ammonia as base:

$H N {O}_{3} + : N {H}_{3} \to N {O}_{3}^{\text{-}} + N {H}_{4}^{+}$

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 (${H}_{2} C {O}_{3} = O C {\left(O H\right)}_{2}$), phosphoric acid, ${H}_{3} P {O}_{4} = O P {\left(O H\right)}_{3}$, phosphonic acid ${H}_{3} P {O}_{3} = O P H {\left(O H\right)}_{2}$.

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 $H N {O}_{3} = {O}_{2} N O H$ is stronger than nitrous acid $H N {O}_{2} = O N O H$

b) sulfuric acid ${H}_{2} S {O}_{4} = {O}_{2} S {\left(O H\right)}_{2}$ is stronger than sulfurous acid ${H}_{2} S {O}_{3} = O S {\left(O H\right)}_{2}$

c) the chlorine oxyacids are stronger in the sequence:

$H C l O < H C l {O}_{2} < H C l {O}_{3} < H C l {O}_{4}$ that is:

$C l O H < O C l O H < {O}_{2} C l O H < {O}_{3} C l O H$

or, by name:

hypochlorous < chlorous < chloric < perchloric acid