Reactivity with Water & Hydrolysis
Why White P is stored underwater, and the classic NCl₃ vs PCl₃ anomaly.
When discussing the reactivity of Group 15 elements with water, examiners love to deploy a massive conceptual trap. The elements themselves are generally inert to water, but their compounds undergo highly specific, mechanistically driven hydrolysis reactions. Let's explore the inertness of the elements and the violent hydrolysis of their halides.
1. Reaction of the Free Elements with Water
As a general rule, the free elements of Group 15 do not react with cold or hot water under normal conditions.
- Nitrogen ($N_2$): Highly inert due to its strong triple bond ($N \equiv N$). It does not react with water at all.
-
Phosphorus ($P_4$): White phosphorus is extremely reactive in air and catches fire spontaneously. However, it is completely insoluble and unreactive in water.
Exam Fact: This is why highly reactive White Phosphorus is safely stored under water. - Arsenic & Antimony: Inert to cold and hot water.
- Bismuth ($Bi$): Bismuth is the most metallic element in the group. If heated to red-hot temperatures, it can react with steam to form bismuth(III) oxide and liberate hydrogen gas, but this requires extreme conditions.
$2Bi(s) + 3H_2O(g) \xrightarrow{\text{Red Heat}} Bi_2O_3(s) + 3H_2(g)$
2. Hydrolysis of Halides (A High-Yield Topic)
Since the elements don't react with water, competitive exams heavily test the reaction of Group 15 compounds with water—specifically the trihalides ($EX_3$) and pentahalides ($EX_5$).
The Classic Anomaly: $NCl_3$ vs. $PCl_3$
Both Nitrogen and Phosphorus form trichlorides. However, when added to water, they yield completely different sets of products. This is dictated by the availability of vacant d-orbitals.
Nitrogen (Period 2) has no vacant d-orbitals. It cannot act as a Lewis acid to accept the lone pair from water. However, the Chlorine atom does have vacant 3d-orbitals. Therefore, the water molecule attacks the Chlorine atom instead of the Nitrogen atom.
(Ammonia & Hypochlorous acid)
Phosphorus (Period 3) has vacant 3d-orbitals. It acts as a Lewis acid and easily accepts the lone pair of electrons from the oxygen atom in water. The water attacks the central Phosphorus atom.
(Phosphorous acid & Hydrochloric acid)
Figure 1: The presence of vacant d-orbitals determines the site of nucleophilic attack by the water molecule.
Unlike $NCl_3$, $NF_3$ is entirely stable in water. This is because neither Nitrogen nor Fluorine possesses vacant d-orbitals in their valence shells. There is no site for the water molecule to attack!
3. Hydrolysis of Pentahalides & Heavy Halides
Phosphorus pentachloride hydrolyzes violently in two steps. It first forms an intermediate, Phosphorus oxychloride ($POCl_3$), which upon further reaction with water yields Orthophosphoric acid.
$POCl_3 + 3H_2O \rightarrow \mathbf{H_3PO_4} + 3HCl$
The trichlorides of Antimony and Bismuth do not undergo complete hydrolysis. Instead, they undergo partial hydrolysis to form insoluble, white oxo-chloride precipitates causing turbidity in the water.
$BiCl_3 + H_2O \rightleftharpoons \mathbf{BiOCl} \downarrow + 2HCl$
Adding HCl pushes the reaction backward, clearing the turbidity (Le Chatelier's Principle).
4. Hydrolysis of Nitrides & Phosphides
When Group 15 elements react with highly electropositive active metals (like Ca or Mg), they form ionic compounds containing $N^{3-}$ or $P^{3-}$ ions. Because these are salts of very weak acids ($NH_3$, $PH_3$), they undergo rapid and complete hydrolysis in water to liberate the corresponding hydride gas.
Mastery Check: Group 15 Hydrolysis
15 High-Yield Questions to test your JEE/NEET Preparation
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