Oxoacids of Phosphorus: List, Structure and Uses

Imagine a world without matches, fertilizers, and detergents. All these rather mundane products have a common chemical underpinning associated with them: the oxoacids of Phosphorus . These compounds form the backbone of industries and biological systems alike. From the matchsticks that light our stoves to the fertilizers that enrich our soil, phosphorus oxoacids are all-pervasive in our lives. For example, the head of a match is red phosphorus, while in fertilizers, phosphoric acid is used to provide crucial nutrition for the plants.

Oxoacids of Phosphorus

The oxoacids of phosphorus can be considered as those acids composed of phosphorous, oxygen, and hydrogen. Basically, they are formed by the reaction of phosphorus pentoxide with water. Some common oxoacids of phosphorus include phosphoric acid, H₃PO₄, phosphorous acid, H₃PO₃, and hypophosphorous acid, H₃PO₂. Each of the aforementioned acids has variable structures and properties that enable them in different chemical reactions and applications. As such, phosphoric acid is a triprotic acid that donates three protons to H⁺ ions. In contrast, hypophosphorous acid is monoprotic, meaning that it donates only a single proton. Further study of these acids has the advantage that a basic structure and properties become important in many areas.

Phosphorus forms several oxoacids. The important oxoacids of phosphorus with their formulas, methods of preparation, and the presence of some characteristic bonds in their structures are given in the Table below.

The compositions of the oxoacids are interrelated in terms of loss or gain of H₂O molecule or O-atom. The structures of some important oxoacids are given in later pictures. In oxoacids, phosphorus is tetrahedrally surrounded by other atoms. All these acids contain at least one P=O bond and one P–OH bond. The oxoacids in which phosphorus has a 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 the +3 oxidation state of phosphorus tend to be 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.

$4{\mathrm{H}}_3{\mathrm{PO}}_3\to 3{\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{PH}}_3$

Types of Phosphorus Oxoacids

The oxyacids of phosphorus exist in some rather different types, with the following characteristics:

1. Phosphoric acid, H₃PO₄: This is the best-known phosphorus oxoacid. It is used in fertilizers, soft drinks, and detergents. The phosphorus is central, bonded to four oxygen atoms; one of the bonds is a double bond.:.
2. Phosphorous Acid (H₃PO₃): This acid is truly reducing in nature. In this acid, the structure is built up by one central atom of phosphorus joined to three oxygen atoms and one hydrogen atom, which makes the atom of phosphorus less oxidized as compared with phosphoric acid.

3. Hypophosphorous Acid (H₃PO₂): It is mainly used for electroless plating. It contains two oxygen atoms and two hydrogen atoms bonded to one phosphorus atom. This serves as a very good reducing agent.

4. Polyphosphoric Acids: Condensation of this phosphoric acid molecule obtained by the formed acid, gives rise to different compounds. For instance, pyrophosphoric acid is H₄P₂O₇, and tripolyphosphoric acid is H₅P₃O₁₀. This class of acids has several applications in various industries on a large scale as dehydrating agents.

Importance and uses

Phosphorus oxoacids find a great mass of applications; that span from the industry down to the learning institutions.

1. Agriculture: Phosphoric acid is a major component in fertilizers since it provides plants with the required food for their growth. Without phosphoric acid, crop yields would shred to tatters, hence impacting on food supply.

2. Food and Beverages: Food-grade phosphoric acid finds a huge market as an acidity regulator in various soft drinks, jams, and other foodstuff. It gives the right level of acidity to such food staff and is responsible for a certain taste during consumption.

3. Water treatment: Polyphosphates, resulting from polyphosphoric acids, find application in the processes of water treatment with the purpose of inhibiting scale formation and pipe corrosion.

4. Pharmaceuticals: Hypophosphorous acid finds an application in the synthesis of several pharmaceuticals.

5. Academic Research: These acids, in chemistry and biochemical research, are studied for their specific properties and reactions. Knowledge of the acids' behavior regarding such phenomena is useful in developing new materials and chemical processes.

It would hence not be too challenging to note how crucial oxoacids of phosphorus are to daily life and technological advancement—this roots from their significant spectrum of potential applications, from increasing productivity in agriculture to assuring quality in foods and beverages, these acids play crucial roles.

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Some Solved Examples

Example 1

Question: White phosphorus reacts with concentrated NaOH solution in an inert atmosphere of CO₂ to give phosphine and compound (X). (X) on acidification with HCl gives compound (Y). What is the basicity of compound (Y)?

1)4

2)3

3)2

4)1

Solution:
When white phosphorus reacts with concentrated NaOH in an inert atmosphere of CO₂, the reaction produces phosphine (PH₃) and sodium hypophosphite (Na₂H₂PO₂) as compound (X).

The reaction is as follows:
$[ \text{P}_4 + 3\text{NaOH} + 3\text{H}_2\text{O} \rightarrow \text{PH}_3 + 3\text{NaH}_2\text{PO}_2 ]$ $[{\text{P}}_4+3\text{NaOH}+3{\text{H}}_2\text{O}\to {\text{PH}}_3+3{\text{NaH}}_2{\text{PO}}_2]$

Upon acidification with HCl, sodium hypophosphite (Na₂H₂PO₂) converts to hypophosphorous acid (H₃PO₂) as a compound (Y).

The reaction is as follows:
$[ \text{Na}_2\text{H}_2\text{PO}_2 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_3\text{PO}_2 ]$ Hypophosphorous acid (H₃PO₂) has only one ionizable hydrogen atom, thus its basicity is 1.

Therefore, the basicity of compound (Y) is 1.

Example 2
Question: Match List I (Name of Oxo acid) with List II (Oxidation state of 'P'):

List I:
- (a) Hypophosphorous acid
- (b) Orthophosphoric acid
- (c) Hypophosphoric acid
- (d) Orthophosphorous acid

List II:
- (i) +5
- (ii) +4
- (iii) +3
- (iv) +2
- (v) +1

Choose the correct answer:
1. (a) - (v), (b) - (iv), (c) - (ii), (d) - (iii)
2. (a) - (v), (b) - (i), (c) - (ii), (d) - (iii)
3. (a) - (iv), (b) - (v), (c) - (ii), (d) - (iii)
4. (a) - (iv), (b) - (i), (c) - (ii), (d) - (iii)

Solution:
- Hypophosphorous acid (H₃PO₂) has an oxidation state of +1 for phosphorus.
- Orthophosphoric acid (H₃PO₄) has an oxidation state of +5 for phosphorus.
- Hypophosphoric acid (H₄P₂O₆) has an oxidation state of +4 for phosphorus.
- Orthophosphorous acid (H₃PO₃) has an oxidation state of +3 for phosphorus.

Thus, the correct matching is:
(a) - (v), (b) - (i), (c) - (ii), (d) - (iii)

Therefore, the correct answer is option (2).

Example 3
Question: The number of ionizable hydrogens present in the product obtained from the reaction of phosphorus trichloride (PCl₃) and phosphonic acid (H₃PO₃) is:

1. 3
2. 0
3. 1
4. 2

Solution:
The reaction of phosphorus trichloride (PCl₃) and phosphonic acid (H₃PO₃) yields pyrophosphorous acid (H₄P₂O₆).

The reaction is as follows:
$[ \text{PCl}_3 + 2\text{H}_3\text{PO}_3 \rightarrow \text{H}_4\text{P}_2\text{O}_6 + 3\text{HCl}]$ $[{\text{PCl}}_3+2{\text{H}}_3{\text{PO}}_3\to {\text{H}}_4{\text{P}}_2{\text{O}}_6+3\text{HCl}]$

The structure of pyrophosphorous acid (H₄P₂O₆) reveals that it has two ionizable hydrogen atoms.

Therefore, the number of ionizable hydrogens in the product is 2.

Thus, the correct option is (4).

Summary

The oxoacids of phosphorus are very versatile and essential compounds whose applications reach the fields of agriculture, industry, and the academic world. These involve phosphoric, phosphorous, and hypophosphorous acids and their congeners of polyphosphoric acid. Each one of these features special structures and physical properties that predestine them for certain uses. Their importance in the manufacture of fertilizers, control of food acidity, water treatment, and the fabrication of pharmaceuticals underlines their status as integral factors in modern life. Knowing these compounds, therefore not only enriches our knowledge of chemistry but also opens up avenues for innovation and development.