Phosphorus (P)

Chemical Properties Overview

Phosphorus (P) is a non-metallic element in Group 15 (p-block) of the periodic table. It exists in several allotropic forms, with White Phosphorus and Red Phosphorus being the most common and studied.

Reactivity Series Position: P is a relatively reactive non-metal. White phosphorus is exceptionally reactive, while red phosphorus is much less so.
Electronegativity: 2.19 (Pauling scale). It is less electronegative than nitrogen but more electronegative than silicon.
Oxidation States: Common oxidation states include -3 (in phosphides and phosphine), +3 (in PCl₃, H₃PO₃), and +5 (in PCl₅, H₃PO₄). It can also exhibit +1 (in hypophosphites) and +4 (in some oxides).
General Reactivity: Due to its larger size and lower electronegativity compared to nitrogen, phosphorus can expand its octet, forming compounds with a valency of 5 (e.g., PCl₅). White phosphorus is highly poisonous, translucent, and wax-like, existing as discrete P₄ tetrahedral molecules. Red phosphorus is polymeric and more stable.

Action of Air and Oxygen

Phosphorus reacts readily with oxygen to form oxides, primarily phosphorus trioxide (P₄O₆) and phosphorus pentoxide (P₄O₁₀).

With excess oxygen (combustion):
- White phosphorus spontaneously ignites in air at room temperature (ignition temperature ≈ 303 K or 30 °C) to form dense white fumes of phosphorus pentoxide.
- Red phosphorus requires heating (ignition temperature ≈ 533 K or 260 °C) to burn in air, also forming phosphorus pentoxide.
```
P₄(s) + 5O₂(g) → P₄O₁₀(s)
```
(Note: P₄O₁₀ is often referred to as phosphorus pentoxide, based on its empirical formula P₂O₅.)
With limited oxygen:
- Phosphorus reacts with a limited supply of oxygen to form phosphorus trioxide.
```
P₄(s) + 3O₂(g) → P₄O₆(s)
```

Action of Water and Steam

Elemental phosphorus does not react with cold water or steam under normal conditions.

White phosphorus is insoluble in water and is typically stored under water to prevent its spontaneous combustion in air.
Red phosphorus is also insoluble in water and unreactive towards it.

(Note: While phosphorus itself doesn’t react with water, its oxides react vigorously with water to form corresponding acids, e.g., P₄O₁₀ forming H₃PO₄.)

Action of Acids and Bases

Action with Acids

Phosphorus does not react with non-oxidizing acids like dilute HCl or H₂SO₄. However, it is oxidized by strong oxidizing acids.

With Concentrated Nitric Acid (HNO₃):
- Phosphorus is oxidized to phosphoric acid (H₃PO₄), while nitric acid is reduced, typically to nitrogen monoxide (NO).
```
3P(s) + 5HNO₃(conc) + 2H₂O(l) → 3H₃PO₄(aq) + 5NO(g)
```
With Concentrated Sulphuric Acid (H₂SO₄):
- Phosphorus is oxidized to phosphoric acid (H₃PO₄), and concentrated sulphuric acid is reduced to sulphur dioxide (SO₂).
```
2P(s) + 5H₂SO₄(conc) → 2H₃PO₄(aq) + 5SO₂(g) + 2H₂O(l)
```

Action with Bases (Alkalis)

With Hot Concentrated Alkali Solution:
- White phosphorus disproportionates (undergoes both oxidation and reduction) when heated with a concentrated solution of a strong alkali (e.g., NaOH). This reaction produces phosphine (PH₃) and a hypophosphite salt. Red phosphorus does not undergo this reaction.
```
P₄(s) + 3NaOH(aq) + 3H₂O(l) → PH₃(g) + 3NaH₂PO₂(aq)
```
(Note: This is a common laboratory preparation method for phosphine.)

Key Laboratory Test/Identification Reactions

Elemental phosphorus is not typically identified in qualitative analysis. Instead, its most stable and common anion, the phosphate ion (PO₄³⁻), is identified.

1. Ammonium Molybdate Test (for Phosphate ions, PO₄³⁻)

This is the most definitive test for phosphate ions.

Procedure: To a solution containing phosphate ions, add ammonium molybdate solution and concentrated nitric acid, then warm gently.
Observation: Formation of a canary-yellow precipitate of ammonium phosphomolybdate.
Reaction (ionic representation):
```
PO₄³⁻(aq) + 12MoO₄²⁻(aq) + 24H⁺(aq) + 3NH₄⁺(aq) → (NH₄)₃[PMo₁₂O₄₀](s) + 12H₂O(l)
```
(Note: The full molecular equation is complex and often simplified in high school. The characteristic yellow precipitate is the key observation.)

2. Magnesia Mixture Test (for Phosphate ions, PO₄³⁻)

This test is performed in the presence of ammonium chloride (NH₄Cl) and ammonium hydroxide (NH₄OH) to maintain an alkaline medium and prevent the precipitation of magnesium hydroxide.

Procedure: Add magnesia mixture (a solution containing MgCl₂, NH₄Cl, and NH₄OH) to the phosphate solution.
Observation: Formation of a white crystalline precipitate of magnesium ammonium phosphate.

Reaction:

PO₄³⁻(aq) + Mg²⁺(aq) + NH₄⁺(aq) → MgNH₄PO₄(s)

Phosphorus (P) - Chemical Properties and Reactions