Oxoacids of Phosphorous

Phosphorus is known to form a number of oxoacids.

Listed here are some important oxoacids of phosphorus.

The compositions of these oxoacids are interrelated in terms of loss or gain of $H_{2}O$ molecule or O-atom

Now we will be having a look at some important oxoacids:

From the structures it is clear that all acids contain at least 1 P=O bond and 1 P-OH bond.

This shows that in oxoacids, phosphorus is tetrahedrally surrounded by other atoms.

Moving ahead let us have a look at the other bonds possessed by the oxoacids of phosphorus.

The oxoacids of phosphorus may have oxidation states less than +5.

Such oxoacids of phosphorus having a lower oxidation state also contain either P-P or P-H bonds but never both.

The P-P or P-H bonds are present in addition to the previously present P=O and P-OH bonds.

The P-P bond can be observed in $H_4{P}_2{O}_6$:

While the P-H bond can be observed in $H_{3}P{O}_2$:

These acids in +3 oxidation state (eg; in $H_{3}P{O}_2$) tend to disproportionate to higher and lower oxidation states.

For example, phosphorus acids on heating disproportionates to give phosphoric acid and phosphine.

Let us have a look at the reaction :

Further, the acids containing P-H bond have strong reducing properties.

Due to the strong reducing properties these acids themselves get strongly oxidised.

When they get strongly oxidised they either gain an oxygen atom or lose a hydrogen atom.

Thus, hypophosphorous acid is a good reducing agent as it contains two P-H bonds.

As a result hypophosphorous acid $(H_{3}P{O}_2)$ is used to reduce $AgN{O}_3$ to metallic silver.

But these P-H bonds are not ionisable to give $H^{+}$ ions.

The absence of $H^{+}$ ions leads to no role being played in the basicity.

So only those H atoms which are attached with oxygen in P-OH form are ionisable and cause the basicity.

Thus, H3PO3 is dibasic as it has 2 P-OH bonds:

While H3PO4 is tribasic as it has 3 P-OH bonds:

Revision

Phosphorus is known to form a number of oxoacids.

The compositions of these oxoacids are interrelated in terms of loss or gain of $H_{2}O$ molecule or O-atom

All the oxoacids of phosphorus contain at least 1 P=O bond and 1 P-OH bond.

In addition some oxoacids may also contain P-P or P-H bonds but not both.

The acids which contain P-H bond have strong reducing properties.

But these P-H bonds are not ionisable to give $H^{+}$ and therefore do not contribute in the basicity.

The end