Oxoacids of Phosphorus
Structures, Basicity, and the secret of their Reducing Power.
Phosphorus forms a wide variety of oxoacids. In competitive exams, you are rarely asked to just memorize the formulas. Instead, you are tested on your understanding of their structures, specifically the distinction between $P-OH$ bonds and $P-H$ bonds.
1. The Three Structural Rules
Before memorizing any specific acid, you must understand the fundamental rules governing the structure of all phosphorus oxoacids:
- Tetrahedral Geometry: The central Phosphorus atom is always $sp^3$ hybridized and surrounded tetrahedrally by other atoms.
- The Core Requirement: Every single oxoacid contains at least one $P=O$ (phosphoryl) bond and at least one $P-OH$ (hydroxyl) bond.
- The Remainder: Any remaining valency on the phosphorus atom is satisfied either by another $P-OH$ bond or a direct $P-H$ bond. (In complex polymeric acids, it can be satisfied by a $P-O-P$ or $P-P$ bond).
- Basicity ($P-OH$ bonds): Only the Hydrogen atoms attached to highly electronegative Oxygen atoms ($P-OH$) are ionizable. They can be released as $H^+$ ions. Therefore, the basicity of the acid equals the number of $P-OH$ bonds.
- Reducing Power ($P-H$ bonds): The Hydrogen atoms attached directly to the central Phosphorus atom ($P-H$) are tightly held and non-ionizable. However, these $P-H$ bonds are responsible for the reducing properties of the acid.
2. The Three Core Oxoacids
| Name (IUPAC Name) | Formula | Oxidation State of P | Basicity | Reducing Nature |
|---|---|---|---|---|
| Hypophosphorous Acid (Phosphinic acid) |
$H_3PO_2$ | +1 | Monobasic (1 P-OH) |
Strong Reducing Agent (2 P-H bonds) |
| Phosphorous Acid (Phosphonic acid) |
$H_3PO_3$ | +3 | Dibasic (2 P-OH) |
Mild Reducing Agent (1 P-H bond) |
| Orthophosphoric Acid (Phosphoric acid) |
$H_3PO_4$ | +5 | Tribasic (3 P-OH) |
Not Reducing (0 P-H bonds) |
Figure 1: The blue rectangles represent acidic -OH groups. The red circles represent reducing P-H bonds.
3. Thermal Disproportionation
Phosphorus oxoacids in the +3 oxidation state tend to disproportionate to higher and lower oxidation states when heated.
For example, when Phosphorous acid ($H_3PO_3$, O.S. = +3) is heated, it disproportionates to yield Orthophosphoric acid ($H_3PO_4$, O.S. = +5) and highly toxic Phosphine gas ($PH_3$, O.S. = -3).
4. Complex Polymeric Acids
Phosphoric acid can polymerize by eliminating water molecules, creating larger, more complex acids.
Formed by heating two molecules of orthophosphoric acid ($H_3PO_4$) at $250^\circ\text{C}$, losing one molecule of water.
- Structure: Two tetrahedral units linked by a P-O-P bond.
- Basicity: Tetrabasic (It has 4 $P-OH$ bonds).
- Oxidation State: +5
Formed by heating orthophosphoric acid to $316^\circ\text{C}$. It is a polymeric solid. Commonly known as glacial phosphoric acid.
- Structure: Exists as linear chains or cyclic rings (e.g., Cyclotrimetaphosphoric acid, $(HPO_3)_3$, which has a 6-membered ring of alternating P and O atoms).
- Oxidation State: +5
This acid is unique because the two phosphorus atoms are linked directly to each other via a $P-P$ bond (there is no bridging oxygen atom between them).
- Basicity: Tetrabasic (4 $P-OH$ bonds).
- Oxidation State: +4
Mastery Check: Oxoacids of Phosphorus
15 High-Yield Questions to test your JEE/NEET Preparation
🚀 Complete Guide to the p-Block
Master Inorganic Chemistry. Dive deep into the specific compounds, trends, and anomalies of Groups 15, 16, 17, and 18 for JEE Advanced, JEE Main and NEET.
Explore the p-Block Master Hub
No comments:
Post a Comment