Oxoacids of Phosphorus preparation and properties, Group 15 P Block Elements

Oxoacids of Phosphorus preparation and properties, Group 15 P Block Elements

Oxoacids of Phosphorus :

→ Phosphorus forms various oxoacids. In these oxoacids two are very important :

• Phosphorus acid
• Phosphoric acid

→ Other oxo acids are formed by above two acids. The series formed by oxo acids contain a tetrahedral phosphorus atom which is co-ordinated to four atoms. The compositions of the oxo acids are inter-related in terms of loss or gain of H₂O molecule or O atom. The rules related to draw the structure of oxo acids are as follows:

• It must have at least one P = O linkage.
• It must have at least one P – OH linkage.
• It can have either P – H or P – P linkage but not both.
• If a molecule has P – H linkage then it behave as a good reducing agent.

→ P – H bonds are not ionisable to give H⁺ and do not play any role in basicity. Only those H^– atoms which are attached with oxygen in POH form are ionisable and cause the basicity. If a molecule has one-OH bond then its basicity is one or if it has two-OH bond then it basicity is two.

→ The oxo acids in which phosphorus has lower oxidation state (less than +5) contain, P = O, P – OH bonds either with P – P or P – H bonds but not both.

→ Hypophosphorus acid is a good reducing agent as it contains two P-H bonds and reduces for example AgNO₃ into metallic silver.
4AgNO₃ + 2H₂O + H₃PO₂ ? 4Ag + 4HNO₃ + H₃PO₄

→ The acids having +3 oxidation state of phosphorus tends to disproportionate to higher and lower oxidation states. For example, orthophosphorus acid on heating disproportionates to give orthophosphoric acid and phosphine.

4H₃PO₃ — 3H₃PO₄ + PH₃

Structure of Some oxo acids

Some important structure of oxo acids are as follows:

→ The important oxoacids of phosphorus with their formulae, methods of preparation and presence of some characteristic bonds in their structure are given in table 7.9:

Table 7.9: Oxoacids of Phosphorus

Chemistry Notes