Oxoacids of Phosphorus


Oxoacids are the acids that contain the element oxygen. Phosphorus is known to form many oxoacids. For example, H3PO4, H3PO3, and more. In the phosphorus oxoacids, it is surrounded tetrahedrally by other atoms. In general, all these acids are known to form at least one P-OH bond and P=O bond.

The bonds P–P or P–H are also found in addition to P=O and P-OH bonds in oxoacids of Phosphorus, where the oxidation state of Phosphorus is less than ‘+5’. Generally, these acids are seen to be disproportionate to lower and higher oxidation states. For suppose, when the phosphorus acid is heated, it results in phosphine and phosphoric acid.

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4H3PO3 → 3H3PO4 + PH3

The P-H bonds available in the oxoacids cannot go through ionization to produce H+ ions, while the hydrogen (H) atoms attached with oxygen in P-OH form are ionisable. Therefore, we can say only the hydrogen (H) atoms attached to oxygen cause the basicity. Resultantly, phosphorus acid, H3PO3, is dibasic oxoacid of phosphorus because of the presence of two P-OH bonds, and the phosphoric acid, H3PO4, is tribasic because of the presence of three P-OH bonds. The oxoacids of Phosphorus, consisting of the P-H bonds, have strong reducing properties. For suppose, the hypophosphorous acid containing two P-H bonds acts as a good reducing agent.

4 AgNO3 + 2H2O + H3PO2 → 4Ag + 4HNO3 + H3PO4

Some Popular Oxoacids of Phosphorus

Phosphorus Acid, H3PO3

Phosphorous acid is a diprotic acid. Which means it ionizes two protons. Better, it is described with the structural formula, HPO(OH)2. The phosphorous acid is made by hydrolysis of phosphorus trichloride with either steam or acid.

PCl3 + 3 H2O ---> HPO(OH)2 + 3 HCl

Phosphoric Acid, H3PO4

Phosphoric acid is a triprotic acid. Which means it ionizes three protons. It is solid at room temperature and pressure, and it is a non-toxic acid, when pure. Phosphoric acid is made by adding the sulfuric acid to tricalcium phosphate rock.

Ca5(PO4)3X + 5 H2SO4 + 10 H2O ---> 3H3PO4 + 5 CaSO4.2H2O + HX

Here, X can be the F, Cl, Br, and OH.

MetaPhosphoric (HPO3)n

This acid forms by warming the orthophosphoric acid at nearly 850 K. Metaphosphoric acid exists as a cyclic trimer, cyclic tetramer, or polymer, but not as a monomer.

H3PO4 → HPO3 + H2O

Hypophosphoric Acid (H4P2O6)

Hypophosphoric acid forms by conducting controlled oxidation of red Phosphorus with sodium chlorite. Once the disodium salt of the acid is formed, it moves via a cation exchanger that eventually creates hypophosphoric acid. The acid has a tetrabasic nature.

2P + 2NaClO2 + 2H2O ———> Na2H2P2O6 + 2HCl

Na2H2P2O6 + 2H —–(resin)—–> H4P2O6 + 2Na (resin)

Pyrophosphoric Acid (H4P2O7)

When orthophosphoric acid is heated nearly at 250°C, it forms a tetrabasic acid.

2H3PO4 ———> H4P2O7 + H2O

Orthophosphoric Acid (H3PO4)

Orthophosphoric acid is formed when P4O10 is treated with the bubbled water. This acid is tribasic.

P4O10 + 6H2O ———-> 4H3PO4

Acidic strength order of the Oxoacids of Phosphorus: H₃PO₂, H₃PO₃, and H₃PO₄

The strength of oxoacids of phosphorus acidity order is, H3PO4 > H3PO3 > H3PO2.

In the first case, all hydrogen atoms are attached to an oxygen atom as an OH group from which hydrogen can be easily released as protons. Hence, the acidity of oxyacids of phosphorus will be the highest.

In the second case, two hydrogens are attached to the oxygen atoms, and the third hydrogen is attached to the P atom, which cannot come out as a proton.

Finally, in the third case, only one hydrogen atom is attached to oxygen, and the other two hydrogen atoms are attached directly to P, which cannot come out as protons. Also, only one hydrogen can come out as a proton. Thereby, the acidity of a protonic acid is decided by the ease by which the species releases the H+ ion. Thus, the resultant order is that which is given above.

Basicity of Oxoacids of Phosphorus

Basicity of oxoacids of phosphorus is defined as the number of ionizable H+ ions or protons in that respective acid.

The structure of Oxoacids of Phosphorus (H3PO3) is given below.

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Although the structure of Oxoacids of Phosphorus has three H atoms, only the ones directly attached to the Oxygen (O) atom will be easily ionized. But, there are only two such H atoms present.

Therefore, the basicity of oxoacids of phosphorus is 3.

Uses of Oxoacids of Phosphorus

Few of the uses of oxoacids of phosphorus include,

  • Phosphorus is used in the preparation of HI and HBr as a substitute of sulphuric acid.

  • It is added as a souring agent in the preparation of soft drinks.

  • It is used in the preparation of phosphate salts of sodium, ammonium, and potassium.

  • It is used in the manufacturing of phosphatic fertilizers.

Why is H3PO3 Dibasic?

The above diagram describes the structure of Oxoacids of Phosphorus, H3PO3.

If we observe the structure, two -OH groups are bonded to the phosphorus atom, and one hydrogen atom is bonded directly to the phosphorus atom.

Breaking the O-H bond is easy. Therefore, to make this compound as diprotic, the two -OH groups are responsible. Whereas another Hydrogen is directly bonded to the phosphorus atom, it is difficult to break the bond due to its highly stable nature.

Because of this, H3PO3 is a diprotic acid, but not the triprotic.

FAQ (Frequently Asked Questions)

1. Is H3PO2 an acid?

If we observe the structure of Oxoacids of Phosphorus, H3PO2, P is bonded to an oxygen atom with a double bond, two hydrogens with every single bond and to a hydroxyl group finally.

Acids are the compounds that can give free H+ ions.

In the compound listed above, there are three hydrogens.

The basicity here is 2, but the hydrogen that is attached to the electronegative atom, to the oxygen atom, which is donated as a hydrogen ion. This might result in confusion.

Because, here P-H bonds are stronger than the O-H bond, and hence the weaker bond is easily broken.

As this compound is capable of giving an H+ ion, it's an acid obviously.

2. Which is more acidic, H3PO2 or H3PO4?

H3PO4 is a tribasic acid. Whereas, H3PO2 is a mono-basic acid. The ionization of one mole of H3PO4 results in three moles of H+ cation. In the case of  H3PO2, H+ ion formed one mole from one mole. H3PO4 is, therefore, a stronger acid than H3PO2 .

The oxidation number of 'P' in H3PO4  is +5, while the 'P' atom in H3PO2 is +1. Phosphorus in H3PO4 is, therefore, more electronegative than H3PO2 phosphorus. The O-H bond in H3PO4 is also more ionic than the O-H bond in H3PO2

As a result, H3PO4 is a stronger acid than H3PO2.