The final product obtained from the complete hydrolysis of phosphorus pentachloride (PCl5) is phosphoric acid (H3PO4), and the number of non-ionisable hydrogen atoms in H3PO4 is zero.
Understanding PCl5 Hydrolysis
Phosphorus pentachloride (PCl5) is a highly reactive compound that undergoes hydrolysis in the presence of water. This process can be viewed in stages, ultimately leading to the formation of phosphoric acid.
- Partial Hydrolysis: When PCl5 reacts with a limited amount of water, it typically forms phosphoryl chloride (POCl3) and hydrogen chloride (HCl).
PCl5 (s) + H2O (l) → POCl3 (l) + 2HCl (g) - Complete Hydrolysis: With an excess of water, the phosphoryl chloride (POCl3) further hydrolyzes to produce phosphoric acid (H3PO4) and additional hydrogen chloride. This is an irreversible reaction, meaning the process goes to completion.
POCl3 (l) + 3H2O (l) → H3PO4 (aq) + 3HCl (aq)
Combining these steps, the overall complete hydrolysis reaction of phosphorus pentachloride yields phosphoric acid as the primary phosphorus-containing product:
PCl5 (s) + 4H2O (l) → H3PO4 (aq) + 5HCl (aq)
Therefore, the final product referred to in the question, resulting from the complete hydrolysis of PCl5, is phosphoric acid (H3PO4).
Structure and Acidity of Phosphoric Acid (H3PO4)
The molecular structure of phosphoric acid is key to understanding the nature of its hydrogen atoms and their ability to ionise.
- Molecular Structure: In H3PO4, a central phosphorus atom is double-bonded to one oxygen atom (P=O) and single-bonded to three hydroxyl (-OH) groups. This means that all three hydrogen atoms in phosphoric acid are directly attached to oxygen atoms. The structure can be represented as
P(=O)(OH)3. - Ionisability of Hydrogen Atoms: The acidity of hydrogen atoms (their tendency to be released as H+ ions in solution) in oxyacids largely depends on their bonding environment:
- Ionisable Hydrogen Atoms: Hydrogen atoms that are bonded to highly electronegative oxygen atoms (O-H bonds) are typically ionisable. The oxygen atom pulls electron density away from the hydrogen, making the O-H bond polar and facilitating the release of H+ in an aqueous solution. Phosphoric acid possesses three such O-H bonds.
- Non-ionisable Hydrogen Atoms: Hydrogen atoms directly bonded to the central atom (e.g., P-H bonds found in some phosphorus acids like hypophosphorous acid (H3PO2) or phosphorous acid (H3PO3)) are generally considered non-ionisable. These P-H bonds are less polar, and the hydrogen atoms are not readily released as protons.
In the specific case of H3PO4, all three hydrogen atoms are part of -OH groups (P-O-H). Consequently, all three hydrogen atoms are acidic and ionisable in aqueous solution. This characteristic makes phosphoric acid a triprotic acid, meaning it is capable of donating three protons in a stepwise manner.
Conclusion on Non-Ionisable Hydrogen Atoms
Given that all hydrogen atoms in phosphoric acid (H3PO4) are bonded to oxygen atoms and are therefore ionisable, there are no hydrogen atoms directly bonded to the phosphorus atom (no P-H bonds). As a result, the number of non-ionisable hydrogen atoms present in phosphoric acid, the final product obtained from the hydrolysis of PCl5, is 0. This finding is consistent with the understanding that H3PO4 is a triprotic acid.
Summary Table: Properties of Phosphoric Acid (H3PO4)
| Property | Description |
|---|---|
| Chemical Formula | H3PO4 |
| Common Name | Phosphoric Acid |
| Structure | P(=O)(OH)3 |
| Total Hydrogen Atoms | 3 |
| Bonding of Hydrogen Atoms | All H atoms bonded to Oxygen (O-H) |
| Ionisability of H Atoms | All 3 H atoms are ionisable (acidic) |
| Non-ionisable H Atoms | 0 |
| Acidity | Triprotic acid |
