Introduction
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
1. What are oxoacids of phosphorus?
Oxoacids of phosphorus are chemical compounds that contain phosphorus, oxygen, and hydrogen atoms. A key feature of these acids is that the phosphorus atom is tetrahedrally bonded to other atoms. Almost all of them include at least one double bond to oxygen (P=O) and one hydroxyl group (P–OH) which determines their acidic nature.
2. What are some common examples of phosphorus oxoacids?
Some of the most important oxoacids of phosphorus that students study in the CBSE syllabus include:
- Orthophosphoric acid (H3PO4)
- Pyrophosphoric acid (H4P2O7)
- Orthophosphorous acid (H3PO3)
- Hypophosphorous acid (H3PO2)
- Metaphosphoric acid (HPO3)n
3. How is the structure of oxoacids of phosphorus generally described?
The structure of phosphorus oxoacids is based on a central, tetrahedrally coordinated phosphorus atom. All these acids feature at least one P=O (phosphoryl) group and one P-OH (hydroxyl) group. Depending on the specific acid, you may also find P-H bonds, or P-O-P bridges that link multiple phosphorus atoms together.
4. What determines the basicity of an oxoacid of phosphorus?
The basicity of a phosphorus oxoacid refers to the number of ionisable hydrogen atoms it can donate. This is determined by the number of hydroxyl (P–OH) groups present in its structure. Hydrogen atoms bonded directly to the phosphorus atom (P–H) are not acidic and do not contribute to the basicity.
5. Why is H3PO4 (phosphoric acid) a stronger acid than H3PO3 (phosphorous acid)?
Phosphoric acid (H3PO4) is stronger than phosphorous acid (H3PO3) because it has more electron-withdrawing oxygen atoms. The phosphorus in H3PO4 is in a higher oxidation state (+5), which increases its ability to pull electron density away from the O-H bonds. This makes the bonds more polar and the hydrogen ion (H+) easier to release, resulting in greater acidic strength.
6. How does the oxidation state of phosphorus affect the properties of its oxoacids?
The oxidation state of the central phosphorus atom significantly impacts the acidic strength of the oxoacid. A higher oxidation state (like +5 in H3PO4) makes the phosphorus atom act more electronegatively. This increased pull on electrons weakens the attached O-H bonds, making the acid stronger. Oxoacids with phosphorus in a lower oxidation state (like +1 in H3PO2) are generally weaker.
7. What is the main difference between phosphorous acid (H3PO3) and phosphoric acid (H3PO4)?
The main differences between phosphorous acid and phosphoric acid are their structure and basicity:
- Phosphorous Acid (H3PO3): It is a dibasic acid because it only has two P–OH groups and one P–H bond. The phosphorus atom is in the +3 oxidation state.
- Phosphoric Acid (H3PO4): It is a tribasic acid as it contains three P–OH groups and no P–H bonds. The phosphorus atom is in the +5 oxidation state.
8. Why are hydrogen atoms attached directly to phosphorus, like in H3PO2, not acidic?
Hydrogen atoms bonded directly to phosphorus (P-H bonds) are not acidic because the electronegativity difference between phosphorus and hydrogen is very small. This makes the P-H bond almost non-polar and strong. In contrast, the O-H bond is highly polar due to oxygen's high electronegativity, which weakens the bond and allows the hydrogen to be released as an H+ ion in water.
