Oxoacids of Phosphorus

Oxoacids of Phosphorus – The p-Block Elements – Class 12

Some Important Oxoacids of Phosphorus

 

(1) Phosphinic acid or Hypophosphorous acid, H₃PO₂

Preparation of Phosphinic acid or Hypophosphorous acid

(1) It is prepared by heating phosphorus with alkalies like barium hydroxide when precipitate of barium phosphate is formed. The precipitate is separated and heated with a calculated amount of sulphuric acid.

 

3Ba(OH)₂ + 2P₄ + 6H₂O —–> 2PH₃ +3Ba(H₂PO₂)₂

 

Ba(H₂PO₂)₂ + H₂SO₄→ BaSO₄ + 2H₃PO₂

 

Barium sulphate is separated by filtration and solution is concentrated when hypophosphorous acid separates out.

 

It has one P=0, one P-OH and two P-H bonds. Since it has only one P-OH bond, so it has only one ionizable hydrogen atom. Therefore, it behaves as monobasic acid, ionising as:

 

H₃PO₂ ⇔ H⁺ + H₂PO₂¯

 

The other two hydrogen atoms directly bonded to P are not ionizable. Due to the presence of two P–H bonds, the acid as well as it salt behave as strong reducing agents. 

 

On heating to 313 K it decompose to phosphine

 

3H₃PO₂ ——> PH₃ + 2H₃PO₃

 

However, on heating above 333K, it decomposes to phosphoric acid and phosphine

 

2 H₃PO₂ —–> H₃PO₄ + PH₃

 

(2) Phosphorous acid or Phosphonic acid, H₃PO₃

 

Preparation of Phosphorous acid or Phosphonic acid

 

(1) It is prepared by the hydrolysis of phosphorus trichloride or trioxide.

 

PCl₃ + 3H₂O→ H₃PO₃ + 3HCl

P₄O₆ + 6 H₂O –> 4 H₃PO₃

 

It has one P double bond O, one P-H and two P-OH bonds. Since it has two P-OH bonds, so it has two ionisable H atoms.

 

Therefore, it behaves as dibasic or diprotic.

 

H₃PO₃ ⇔ H⁺ + H₂PO₃¯        K_a1 = 1 x 10-2

 

H₂PO₃¯⇔  H⁺ + H₂PO₃^2-       K_a2 = 2 x 10^-7

 

 

On heating it decomposes into phosphoric acid and phosphine.

 

4 H₃PO₃ ———-> 3 H₃PO₄ + PH₃

It is readily oxidised by air in the presence of a catalyst iodine into phosphoric acid.

2H₃PO₃ + O₂ ———>2H₃PO₄

It is a strong reducing agent.

(3) Orthophosphoric acid, H₃PO₄

Preparation of Orthophosphoric acid

 

(1) It is prepared by adding phosphorus pentaoxide to water and boiling the solution.

P₄O₁₀ + 6H₂O ——–> 4H₃PO₄

(2) It can also be prepared by heating phosphorus with cone. HNO3 or by dissolving PCl5 in water.

PCl₅ + 4H₂O ——-> H₃PO₄ + 5HCl

It has one P=0 and three P-OH bonds. Since it has three P-OH bonds, so it has three ionisable H atoms. 

 

Therefore, it behaves as tribasic or triprotic and ionises in three stages :

 

H₃PO₄ ⇔ H⁺ + H₂PO₄¯        K_a1 = 7.5 x 10^-3

H₂PO₄¯ ⇔ H⁺ + HPO₄^2-        K_a2 = 6.2 x 10^-8

HPO₄^2- ⇔ H⁺ + PO₄^3¯         K_a3 = 1 x 10^-13

Therefore, it forms three series of salts such as NaH₂PO₄ (disodium hydrogen phosphate), Na₃HPO₄ (disodium hydrogen phosphate) and Na₃PO₄ (sodium phosphate).

 

(4) Pyrophosphoric acid, H₄P₂O₇

It contains phosphorus in +5 oxidation state.

Preparation of Pyrophosphoric acid

(1) It is prepared by heating orthophosphoric acid to 523K.

2H₃PO₄ —–> H₄P₂O₇ + H₂O

 

Structure of Pyrophosphoric acid

The acid is formed by the loss of a molecule of water from two molecules of phosphoric acid. 

(a) Since it contains four OH groups, it is tetrabasic. It dissolves in water to form orthophosphoric acid.

2H₄P₂O₇ + H₂O —> 3 H₃PO₄ 
                  Orthophosphoric acid

(b) Upon strong heating, it decomposes to form meta phosphoric acid.

H₄P₂O₇ —->  2HPO₃ + H₂O
Metaphosphoric acid

(5) Metaphosphoric acid, HPO₃

 

It has one P-OH bond and therefore, exists as monobasic.

 

Preparation of Metaphosphoric acid

It is prepared by heating orthophosphoric acid to 525 K.

H₃PO₄ —-> HPO₃+ H₂O

Metaphosphoric acid does not exist as simple monomer, rather it exists as cyclometaphosphoric acid or polymetaphosphoric acid.