76. The number of phosphorous-hydrogen bonds in H3PO2 is
Number of Phosphorus-Hydrogen Bonds in H₃PO₂ (Hypophosphorous Acid)
Correct Answer
✅ Correct Answer: 2
Understanding the Structure of H₃PO₂
H₃PO₂ is known as hypophosphorous acid. Although its molecular formula contains three hydrogen atoms, not all of them are attached to oxygen atoms. The actual structure reveals that phosphorus is bonded to one oxygen atom through a double bond, one hydroxyl group through a single bond, and two hydrogen atoms directly through phosphorus-hydrogen bonds.
The structural formula can be represented as:
HOP(H)₂=O
or equivalently,
HP(O)(OH)H
In this structure:
- One phosphorus-oxygen double bond (P=O)
- One phosphorus-hydroxyl bond (P–OH)
- Two phosphorus-hydrogen (P–H) bonds
Therefore, the number of P–H bonds present in H₃PO₂ is 2.
Why Only One Hydrogen is Acidic
An important feature of oxyacids of phosphorus is that only hydrogen atoms attached to oxygen can be released as protons (H⁺). Hydrogen atoms directly bonded to phosphorus are non-ionizable and therefore do not contribute to the acidity of the compound.
Since H₃PO₂ contains only one hydroxyl (–OH) group, it is a monobasic acid, despite having three hydrogen atoms in its molecular formula.
Relationship Between P–H Bonds and Reducing Nature
The presence of phosphorus-hydrogen bonds gives hypophosphorous acid its strong reducing properties. Compounds containing P–H bonds can easily donate hydrogen during redox reactions, making them effective reducing agents. The greater the number of P–H bonds, the stronger the reducing character of the oxyacid.
Because H₃PO₂ contains two P–H bonds, it is a much stronger reducing agent than phosphorous acid (H₃PO₃), which contains only one P–H bond, and phosphoric acid (H₃PO₄), which contains no P–H bonds.
Comparison of Common Oxyacids of Phosphorus
| Oxyacid | P–H Bonds | Basicity | Reducing Nature |
|---|---|---|---|
| H₃PO₂ (Hypophosphorous acid) | 2 | Monobasic | Strong reducing agent |
| H₃PO₃ (Phosphorous acid) | 1 | Dibasic | Moderate reducing agent |
| H₃PO₄ (Phosphoric acid) | 0 | Tribasic | Not a reducing agent |
Concept Behind the Question
This question evaluates the understanding of the actual structures of phosphorus oxyacids rather than their molecular formulas. Students often assume that every hydrogen atom in an oxyacid is bonded to oxygen, leading to incorrect answers. In reality, the chemical properties of phosphorus oxyacids depend on whether hydrogen is attached to oxygen or directly to phosphorus. The number of hydroxyl groups determines basicity, while the number of phosphorus-hydrogen bonds determines reducing power.
Final Answer
Hypophosphorous acid (H₃PO₂) contains two hydrogen atoms directly bonded to phosphorus. Therefore, the number of phosphorus-hydrogen bonds is:
✅ Correct Answer: 2
