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Oxoacids Of Phosphorus -Preparation, Properties

This blog covers oxoacids of phosphorus with their molecular formula, method of preparation, reactions and properties.

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Posted by Mahak Jain, 30/10/2020
Oxoacids Of Phosphorus -Preparation, Properties

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Oxoacids are acids that contain the element oxygen in compound. Phosphorus forms a number of oxoacids like H4P2O5, H3PO4, H3PO3, HPO3, etc.

In oxoacids, other atoms surrounds phosphorus tetrahedrally. All these acids contain at least one P=O bond and one P–OH bond. The oxoacids in which phosphorus has lower oxidation state (less than +5) contain, in addition to P=O and P–OH bonds, either P–P (e.g., in H4P2O6) or P–H (e.g., in H3PO2) bonds but not both. These acids in +3 oxidation state of phosphorus tend to disproportionate to higher and lower oxidation states. 

For example, orthophophorous acid (or phosphorous acid) on heating disproportionates to give orthophosphoric acid (or phosphoric acid) and phosphine.

4H3PO3 → 3H3PO4 + PH3

The acids which contain P–H bond have strong reducing properties. Thus, hypo-phosphorous acid is a good reducing agent as it contains two P–H bonds and reduces, for example, AgNO3 to metallic silver.

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

These P–H bonds are not ionizable to give H+ and do not play any role in basicity. Only those H atoms which attach with oxygen in P–OH form are ionizable and cause the basicity. Thus, H3PO3  and H3PO4 are dibasic and tribasic, respectively as the structure of H3PO3 has two P–OH bonds and H3PO4 has three.

Hypo Phosphorus Acid (H3PO2) or Phosphinic Acid:

  • It is a powerful reducing agent with molecular formula H3PO2.
  • Generally we write H3PO2, but a more descriptive representation is HOP(O)H2, which highlights its monoprotic character. 
Hypo Phosphorus Acid (H3PO2) or Phosphinic Acid
  • Oxidation state of phosphorus- +1
  • One P-OH bond 
  • Two P-H bond
  • One P=O bond
  • Salts derived from this acid are called hypophosphites.

Preparation

  • You can prepare it by the oxidation of phosphine by iodine in the presence of figured amount of water. It is a monobasic acid.
PH3 + 2I2 + 2H2O → H3PO2 + 4 HI
  • Hypophosphite salts of the alkali and alkaline earth metals result from the reaction of white phosphorus with hot aqueous solution of the appropriate hydroxide.
P4 + 4 OH + 4 H2O → 4 H2PO2- + 2 H2
  • The salt is then treated with a strong, non-oxidizing acid to give the free hypo-phosphorous acid.
H2PO2- + H+ → H3PO2

Phosphorus Acid or ortho-phosphorous (H3PO3) or Phosphonic Acid:

  • Phosphorous acid, molecular formula is H3PO3. This acid is diprotic (not triprotic) phosphorous acid. It is an intermediate in the preparation of other phosphorus compounds.
Phosphorous Acid - oxoacids of phosphorous
  • Oxidatative state of phosphorus- +3
  • Two P-OH bond
  • One P-H bond
  • One P=O bond

Preparation

  1. On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam.
PCl3 + 3 H2O → HPO(OH)2 + 3 HCl

Potassium phosphite is also a precursor to phosphorous acid:

K2HPO3 + 2 HCl → 2 KCl + H3PO3

2. It formed by hydrolysis of phosphorous trioxide (P4O6). It a is dibasic acid.

P4O6 + 6H2O → 4H3PO3

Pyrophosphorus Acid (H4P2O5)

Another, one of the most important among oxoacids of phosphorus is pyrophosphorus acid.

Preparation 

PCl3 +H3PO3

Pyrophosphorus acid - oxoacids of phosphorous
  • Oxidatative state of phosphorus- +3
  • Two P-OH bond
  • Two P-H bond
  • Two P=O bond

Hypophosphoric Acid (H4P2O6):

Hypo-phosphoric acid is a mineral acid with the formula H4P2O6. In the solid state it is present as the dihydrate, H4P2O6·2H2O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a P−P bond.

Preparation

  • It is formed by controlled oxidation of red phosphorus with sodium chlorite. Hypophosphoric acid is tetrabasic in nature.
2P + 2NaClO2 + 2H2O → Na2H2P2O6 + 2HCl
Na2H2P2O6 + 2H-resin → H4P2O6 + 2Na-resin
Hypophosphoric Acid - oxoacids of phosphorous
  • Oxidatative state of phosphorus- +4
  • Four P-OH bond
  • Two P=O bond
  • One P-P bond

Orthophosphoric Acid (H3PO4)

  • Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a weak acid. Its molecular formula is H3PO4.
  • The pure compound is a colorless solid.
Orthophosphoric Acid (H3PO4) - oxoacids of phosphorous
  • Oxidatative state of phosphorus- +5
  • Three P-OH bond
  • One P=O bond

Preparation

  1. Phosphoric acid is produced industrially by a wet process. A phosphate-containing mineral such as calcium hydroxyapatite is treated with sulfuric acid.
  2. It is formed by treating P4O10 with bubbled water. It is a tribasic acid.

P4O10 + 6H2O → 4H3PO4

Pyrophosphoric Acid (H4P2O7)

  • Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, [(HO)2P(O)]2O. It is colorless and odorless and is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid crystallizes in two polymorphs, which melt at 54.3 °C and 71.5 °C.

Preparation

  • It is formed by heating orthophosphonic acid at about 250 °C. It is a tetrabasic acid.

2H3PO4 → H4P2O7 + H2O

Pyrophosphoric Acid (H4P2O7) - oxoacid of phosphorous
  • Oxidatative state if phosphorus- +5
  • Four P-OH bond
  • Two P=O bond
  • One P-O-P bond

Meta Phosphoric Acid (HPO3)n

Preparation

  • It is formed by warming orthophosphoric acid to around 850 K. It exists as cyclic trimer, cyclic tetramer or polymer.

H3PO4 → HPO3 + H2O

Cyclotrimetaphosphoric acid - oxoacid of phosphorous
Cyclotrimetaphosphoric acid
Polymetaphosphoric acid - oxoacid of phosphorous
Polymetaphosphoric acid
  • Oxidatative state if phosphorus- +5
  • Three P-OH bond
  • Three P=O bond
  • Three P-O-P bond

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