What are the types structures and basicity of oxoacids of phosphorus
Oxoacids are the acids that contain the element oxygen. Phosphorus is known to form many oxoacids. For example, H3PO4, H3PO3, and more. In the phosphorus oxoacids, it is surrounded tetrahedrally by other atoms. In general, all these acids are known to form at least one P-OH bond and P=O bond.
The bonds P–P or P–H are also found in addition to P=O and P-OH bonds in oxoacids of Phosphorus, where the oxidation state of Phosphorus is less than ‘+5’. Generally, these acids are seen to be disproportionate to lower and higher oxidation states. For suppose, when the phosphorus acid is heated, it results in phosphine and phosphoric acid.
\[ 4H_{3}PO_{3} \rightarrow 3H_{3}PO_{4} + PH_{3} \]
The P-H bonds available in the oxoacids cannot go through ionization to produce H+ ions, while the hydrogen (H) atoms attached with oxygen in P-OH form are ionisable. Therefore, we can say only the hydrogen (H) atoms attached to oxygen cause the basicity. Resultantly, phosphorus acid, H3PO3, is dibasic oxoacid of phosphorus because of the presence of two P-OH bonds, and the phosphoric acid, H3PO4, is tribasic because of the presence of three P-OH bonds. The oxoacids of Phosphorus, consisting of the P-H bonds, have strong reducing properties. For suppose, the hypophosphorous acid containing two P-H bonds acts as a good reducing agent.
\[ 4 AgNO_{3} + 2H_{2}O + H_{3}PO_{2} \rightarrow 4Ag + 4HNO_{3} + H_{3}PO_{4} \]
Some Popular Oxoacids of Phosphorus
Phosphorus Acid, H3PO3
Phosphorous acid is a diprotic acid. Which means it ionizes two protons. Better, it is described with the structural formula, HPO(OH)2. The phosphorous acid is made by hydrolysis of phosphorus trichloride with either steam or acid.
\[ PCl_{3} + 3 H_{2}O \rightarrow HPO(OH)_{2} + 3 HCl \]
Phosphoric Acid, H3PO4
Phosphoric acid is a triprotic acid. Which means it ionizes three protons. It is solid at room temperature and pressure, and it is a non-toxic acid, when pure. Phosphoric acid is made by adding the sulfuric acid to tricalcium phosphate rock.
\[Ca_{5}(PO_{4})_{3}X + 5 H_{2}SO_{4} + 10 H_{2}O \rightarrow 3H_{3}PO_{4} + 5 CaSO_{4}.2H_{2}O + HX \]
Here, X can be the F, Cl, Br, and OH.
MetaPhosphoric (HPO3)n
This acid forms by warming the orthophosphoric acid at nearly 850 K. Metaphosphoric acid exists as a cyclic trimer, cyclic tetramer, or polymer, but not as a monomer.
\[ H_{3}PO_{4} \rightarrow HPO_{3} + H_{2}O \]
Hypophosphoric Acid (H4P2O6)
Hypophosphoric acid forms by conducting controlled oxidation of red Phosphorus with sodium chlorite. Once the disodium salt of the acid is formed, it moves via a cation exchanger that eventually creates hypophosphoric acid. The acid has a tetrabasic nature.
\[2P + 2NaClO_{2} + 2H_{2}O \rightarrow Na_{2}H_{2}P_{2}O_{6} + 2HCl\]
\[Na_{2}H_{2}P_{2}O_{6} + 2H --(resin)\rightarrow H_{4}P_{2}O_{6} + 2Na (resin)\]
Pyrophosphoric Acid (H4P2O7)
When orthophosphoric acid is heated nearly at 250°C, it forms a tetrabasic acid.
\[ 2H_{3}PO_{4} \rightarrow H_{4}P_{2}O_{7} + H_{2}O \]
Orthophosphoric Acid (H3PO4)
Orthophosphoric acid is formed when P4O10 is treated with the bubbled water. This acid is tribasic.
\[ P_{4}O_{10} + 6H_{2}O \rightarrow 4H_{3}PO_{4} \]
Acidic strength order of the Oxoacids of Phosphorus: H3PO2, H3PO3, and H3PO4
The strength of oxoacids of phosphorus acidity order is, H3PO4 > H3PO3 > H3PO2.
In the first case, all hydrogen atoms are attached to an oxygen atom as an OH group from which hydrogen can be easily released as protons. Hence, the acidity of oxyacids of phosphorus will be the highest.
In the second case, two hydrogens are attached to the oxygen atoms, and the third hydrogen is attached to the P atom, which cannot come out as a proton.
Finally, in the third case, only one hydrogen atom is attached to oxygen, and the other two hydrogen atoms are attached directly to P, which cannot come out as protons. Also, only one hydrogen can come out as a proton. Thereby, the acidity of a protonic acid is decided by the ease by which the species releases the H+ ion. Thus, the resultant order is that which is given above.
Basicity of Oxoacids of Phosphorus
Basicity of oxoacids of phosphorus is defined as the number of ionizable H+ ions or protons in that respective acid.
The structure of Oxoacids of Phosphorus (H3PO3) is given below.
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Although the structure of Oxoacids of Phosphorus has three H atoms, only the ones directly attached to the Oxygen (O) atom will be easily ionized. But, there are only two such H atoms present.
Therefore, the basicity of oxoacids of phosphorus is 3.
Uses of Oxoacids of Phosphorus
Few of the uses of oxoacids of phosphorus include,
Phosphorus is used in the preparation of HI and HBr as a substitute of sulphuric acid.
It is added as a souring agent in the preparation of soft drinks.
It is used in the preparation of phosphate salts of sodium, ammonium, and potassium.
It is used in the manufacturing of phosphatic fertilizers.
Why is H3PO3 Dibasic?
The above diagram describes the structure of Oxoacids of Phosphorus, H3PO3.
If we observe the structure, two -OH groups are bonded to the phosphorus atom, and one hydrogen atom is bonded directly to the phosphorus atom.
Breaking the O-H bond is easy. Therefore, to make this compound as diprotic, the two -OH groups are responsible. Whereas another Hydrogen is directly bonded to the phosphorus atom, it is difficult to break the bond due to its highly stable nature.
Because of this, H3PO3 is a diprotic acid, but not the triprotic.
Conclusion
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FAQs on Oxoacids of Phosphorus Structure Properties and Reactions
1. What are oxoacids of phosphorus?
Oxoacids of phosphorus are acids that contain hydrogen, oxygen, and phosphorus in their molecules. These acids are formed when phosphorus oxides react with water. They generally contain one or more P=O and P–OH bonds and may also have P–H bonds in some cases. Common examples include H3PO4 (phosphoric acid), H3PO3 (phosphorous acid), and H3PO2 (hypophosphorous acid).
2. What is the general formula of oxoacids of phosphorus?
The general formula of oxoacids of phosphorus can be written as HnP Om, where hydrogen and oxygen are bonded to phosphorus in different arrangements. These acids typically contain one P=O bond and several P–OH groups. For example:
- H3PO4 (orthophosphoric acid)
- H4P2O7 (pyrophosphoric acid)
- (HPO3)n (metaphosphoric acid)
3. How are oxoacids of phosphorus prepared?
Oxoacids of phosphorus are prepared by the reaction of phosphorus oxides with water. For example:
- P4O10(s) + 6H2O(l) → 4H3PO4(aq)
- PCl3(l) + 3H2O(l) → H3PO3(aq) + 3HCl(aq)
4. Why is H3PO3 dibasic but H3PO4 tribasic?
H3PO3 is dibasic because it contains only two ionizable P–OH hydrogen atoms, while H3PO4 is tribasic because it contains three ionizable P–OH hydrogens. In H3PO3, one hydrogen is directly bonded to phosphorus (P–H) and is not acidic. In H3PO4, all three hydrogens are attached to oxygen as –OH groups, so all three can be released as H+ ions.
5. What is the basicity of oxoacids of phosphorus?
The basicity of oxoacids of phosphorus equals the number of ionizable –OH groups present in the molecule. Only hydrogen atoms attached to oxygen (P–OH) are acidic. For example:
- H3PO4 → Basicity = 3
- H3PO3 → Basicity = 2
- H3PO2 → Basicity = 1
6. What is the difference between orthophosphoric acid and pyrophosphoric acid?
Orthophosphoric acid is H3PO4, while pyrophosphoric acid is H4P2O7 formed by condensation of two phosphoric acid molecules. The reaction is:
- 2H3PO4(l) → H4P2O7(l) + H2O(l)
7. Which oxoacids of phosphorus show reducing properties?
Oxoacids of phosphorus containing at least one P–H bond show reducing properties. Examples include:
- H3PO3 (phosphorous acid)
- H3PO2 (hypophosphorous acid)
8. What is the oxidation state of phosphorus in common oxoacids?
The oxidation state of phosphorus varies in different oxoacids and can be calculated using standard oxidation number rules. Examples:
- In H3PO4, P = +5
- In H3PO3, P = +3
- In H3PO2, P = +1
9. What are the types of oxoacids of phosphorus?
The main types of oxoacids of phosphorus are orthophosphoric, pyrophosphoric, metaphosphoric, phosphorous, and hypophosphorous acids. These include:
- Orthophosphoric acid: H3PO4
- Pyrophosphoric acid: H4P2O7
- Metaphosphoric acid: (HPO3)n
- Phosphorous acid: H3PO3
- Hypophosphorous acid: H3PO2
10. How do you calculate the basicity of an oxoacid of phosphorus?
The basicity of an oxoacid of phosphorus is calculated by counting the number of –OH groups attached to phosphorus. Steps:
- Draw or analyze the structural formula.
- Identify hydrogen atoms bonded to oxygen (P–OH).
- Ignore hydrogen atoms directly bonded to phosphorus (P–H).
