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Oxyacids of Phosphorus

Chapter 5:- Oxyacids of Boron, Phosphorus, and Sulphur
B.sc 1st year Book
(Page 2)

Oxyacids of Phosphorus

Phosphorus forms a much larger number of acids with the general formulae H3POn (ortho acids, n = 2, 3, 4, 5, and 6), HPOn-1 (meta-acids n = 3 and 4) H4P2On (diacids, n = 4, 5, 6, 7 and 8) and several polyphosphoric acids with three or more phosphorus atoms per molecules. The polyphosphoric acids have the general formula Hn+2PnO3n+1.

The methods of preparation, properties, and structures of some oxyacids of phosphorus are discussed below:

1. Hypophosphorous Acid or Phosphinic acid: H3PO2:

This monobasic acid contains one protonated oxygen atom and one non-protonated oxygen atom.

Preparation:

(ii) By heating white phosphorus with baryta Ba(OH)2:Oxyacids of Phosphorus

(ii) By hydrolysis of white phosphorus:

Oxyacids of Phosphorus

(iii) Commercial Method: In industry, the free acid is obtained by treating the calcium salt, Ca(H2PoO2)2 with H2SO4.

Calcium hypophosphite

When the aqueous solution is evaporated the acid crystallizes as colorless leaflets.

(iv) It may also be obtained when PH3 is oxidized with I2.

Properties:

  1. It is a colorless crystalline solid.
  2. Its bp is 26.5 °C
  • It is monobasic acid. It ionizes as:

ionaizatins

This acid and its salts are strong reducing agents and reduce CuSO4 into Cu (OH) and Hg (II) chloride to Hg (1) chloride.

choloride

  1. This acid itself reduced to PH3 when treated with Zn and dil HCI.

hcl

  1. Decomposition:

DECOMPOSITION

  • It is disproportionate when heated to 130-140°C giving phosphine and phosphonic acid.

phosphonic acid.

  • In a highly alkaline solution H3PO2 also reduces the proton of water to H2. This occurs more rapidly as the pH increases

pH increases

Structure:  Since on ionization it gives only one H+ ion this suggests the presence of only one -OH group in its molecule. X-ray and Raman spectra show that it has a tetrahedral shape. The P-H distance = 1.5A: O-P-O bond angle = 120° and H-P-H bond angle =92.

 

Tetrahedral shape of H3PO2Figure 7.04: Tetrahedral shape of H3PO2

Uses: Sodium salt is particularly used for the non-electrical nickel plating of metals.

  1. Phosphorus Acid or Phosphonic acid: H3PO3:

Preparation: By hydrolysis of phosphorus trichloride and (P203)2.

Preparation

In the industrial synthesis, PCI3 is sprayed into steam at 190 °C. The heat is used to distill off the HCl and excess water vapour.

Properties:

  1. It is a colorless crystalline solid, bp 70.1°C
  2. It is extremely soluble in water.
  • it is strong dibasic acid. Although it contains three hydrogen atoms but only two are ionizable. Hence it ionizes as:

are ionizable

  1. It gives two series of salts, such as NaH2PO3, Na2HPO3, known as primary and secondary phosphites respectively.
  2. H3PO3 and its salts are strong reducing agents.
  3. it is readily oxidizable to phosphoric acid and phosphates respectively.

Thus, H3PO3 reduces the salt of Cu, Ag, Au, etc. to their respective metals.

  • On heating, phosphorus acid decomposes into phosphine and phosphoric acid.

vii. On heating phosphorus

Structure:

Tetrahedral shape of H3PO3

Figure 7.05: Tetrahedral shape of H3PO3

The dibasic nature of H3PO3, suggests the presence of two-OH groups. The third H-atom is not ionized. It has the following structure.

Uses:

  1. It is used as a reducing agent.
  2. It is also used in the production of basic lead phosphonate PVC stabilizers and various organic derivatives.
  3. Hypodiphosphoric Acid: H4P2O6:

Preparation:

By controlled oxidation of red phosphorus with sodium chlorite solution at room temperature.

temperature.

Properties:

It is a solid and its melting point is 70°C.

It is stable towards alkalis and does not decompose even when heated with 80% NaOH at 200°C. While in the acidic medium it is readily hydrolyzed giving a mixture of phosphorus and phosphoric acids.

acids.

Structure:

It is a tetrabasic acid and lionizes in stages as:

stages

Thus, Hypodiphosphoric acid has the following structure:

Structure of H4P2O6

Fig. 7.06 Structure of H4P2O6

  1. Orthophosphoric Acid or Phosphoric acid: H3PO4:

Preparation:

  1. By the hydrolysis of P205:

P205

  1. Laboratory Method

ii. Laboratory

  • By the action of conc. H2SO4 with rock phosphate or bone ash:

or bone ash

Properties:

  1. The conc. H3PO4 (85%) is a clear syrupy liquid.
  2. It combines with water giving the semi-hydrate of the formula 2H3PO4.H2O which melts at about 29 °C.
  • It decomposes on heating giving pyrophosphoric acid at 250°C.

pyrophosphoric acid

  1. It gives metaphosphoric acid at the higher temperature.

metaphosphoric

  1. It is a tribasic acid and ionizes in three stages as represented below.

Structure:

The structure of this acid is represented below:

Structure of H3PO4

Fig. 7.07. Structure of H3PO4

Causes of syrupy nature:

The crystalline H3PO4 has a hydrogen-bonded layer structure in which tetrahedral PO43-, groups are bonded by hydrogen bonds. The hydrogen bonding is responsible for the syrupy nature of H3PO4, as shown in figure 7.07

  1. Pyrophosphoric Acid or Di phosphoric acid: H4P2O7:

This acid is also called Di phosphonic acid.

Preparation:

  1. By heating orthophosphoric acid at 250 °C:

heating orthophosphoric

  1. By heating orthophosphoric acid with POCI3:

POCI3

Properties:

  1. It is a colorless crystalline solid.
  2. Its boiling point is 60 °C.
  • It is reconverted into H3PO4, on boiling with water.

water.

  1. It is a tetrabasic acid.
  2. The dissociation constant for the four stages of dissociation is

dissociation

Structure:

Structure of H4P2O7.

Fig. 7.08: Structure of H4P2O7.

6. Metaphosphoric Acid, HPO3:

Preparation:

  1. The reaction of P2O5 with water:

Reaction of P2O5

  1. By heating orthophosphoric acid or pyrophosphoric acid strongly

or pyrophosphoric

Properties:

  1. it is a transparent glassy solid and therefore known as Glacial phosphoric acid
  2. It is soluble in water
  • It melts at 38.4°C
  1. Its aqueous solution on boiling changes into H3PO4.

aqueous solution

  1. It is monobasic acid, forming salts known as metaphosphates e.g., NaPO3.

Structure:

The structure of the monomer is based on its formation from H3PO4, by the elimination of water molecules as shown in figure 7.09. However, it does not exist as a simple monomer but as a polymer (see Figure 7.10 and 7.11). The simplest metaphosphoric acids are (HPO3)3trimer-timetaphosphoric acid and (H3PO4)4tetramer-tetrametaphosphoric acid which has cyclic structures.

Structure of HPO3

Fig. 7.09 – Structure of HPO3

 

Straight chain structure of (HPO3) n

Fig. 7.10: Straight chain structure of (HPO3) n

 

Cyclic structures of metaphosphoric acid

Fig. 7.11: Cyclic structures of metaphosphoric acid

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