Red Phosphorus: Preparation, Properties, Structure & Uses

Red Phosphorus - Preparation, Properties & Uses | NEET | JEE | GATE | CSIR

Red Phosphorus – Preparation, Properties, Structure, Uses and Previous Year Questions

Highly Important for NEET, JEE Main/Advanced, GATE & CSIR-NET

1. Preparation of Red Phosphorus

Method	Description	Key Points (Exam Favorite)
Heating White Phosphorus (Main Industrial Method)	White P₄ is heated to 240–300°C in absence of air (inert atmosphere) for several hours/days.	No catalyst needed in simple heating Iodine acts as catalyst → lowers temperature to ~200°C and speeds up conversion Reaction: White P (molecular) → Red P (polymeric) JEE/NEET frequent question: Red P is obtained by heating white P at 573 K in inert atmosphere
By Sunlight/UV Exposure	White phosphorus kept in sunlight or exposed to UV light slowly converts to red form.	Slow process; often asked as alternative method.

Trick to Remember: White P → Red P is exothermic but has high activation energy, so requires heat or catalyst.

2. Physical Properties of Red Phosphorus

Property	Red Phosphorus	White Phosphorus (Comparison)
Colour	Reddish-brown / Violet (amorphous to crystalline)	Pale yellow
State	Solid, brittle powder	Waxy solid
Density	2.1–2.34 g/cm³	1.82 g/cm³
Melting Point	~590°C (sublimes)	44°C
Solubility	Insoluble in water & CS₂	Soluble in CS₂, insoluble in water
Reactivity	Less reactive, does not glow in dark	Highly reactive, glows in dark (chemiluminescence)
Poisonous Nature	Non-poisonous	Highly poisonous
Ignition Temperature	High (~260°C)	Very low (30°C)

Most asked comparison points in NEET/JEE:
• Red P: No chemiluminescence, non-poisonous, insoluble in CS₂
• White P: Glows in dark, highly poisonous, soluble in CS₂

3. Chemical Properties of Red Phosphorus

Stability: More stable than white P; does not catch fire easily in air.
With Air/Oxygen: Burns only when heated strongly (above 260°C) → P₄O₁₀
With Halogens: Combines directly but less vigorously than white P.
With Alkalis (Important Reaction):
3NaOH + 4P (red) + 3H₂O → PH₃↑ + 3NaH₂PO₂
(Produces phosphine + sodium hypophosphite)
CSIR-NET & JEE Advanced favorite reaction
With Conc. HNO₃: Oxidised to H₃PO₄ (phosphoric acid)
Does NOT show disproportionation like white P with hot conc. NaOH.

4. Structure of Red Phosphorus

Polymeric structure consisting of chains of P₄ tetrahedra linked together.
Amorphous red P → on heating → crystalline forms (e.g., Hittorf’s violet phosphorus).
Each P atom covalently bonded to three others → polymeric network.

Structure of Red Phosphorus

5. Uses of Red Phosphorus (Very Important for Exams)

Use	Reason/Application
Manufacture of safety matches	Present on the striking surface (side of matchbox) – mixture of red P, antimony trisulphide & binder. Ignites only when struck (high ignition temp).
Production of phosphine (PH₃)	In laboratory by reaction with hot conc. alkali.
In smoke screens & incendiary bombs	Burns to produce dense white smoke of P₄O₁₀.
As flame retardant	In plastics and textiles.
Synthesis of organophosphorus compounds	Starting material in organic synthesis.
In semiconductors (doping)	Crystalline red P used in some electronic applications.

One-liner for exams:
Red phosphorus is used on the striking surface of matchbox because it has high ignition temperature and is non-poisonous.

Quick Revision Points (High-Yield for NEET/JEE)

Red P is polymeric, stable, non-poisonous allotrope.
Prepared by heating white P at 573 K (300°C) in inert atmosphere.
Insoluble in CS₂ (key differentiator from white P).
Does not glow in dark.
Ignition temp ≈ 260°C → used in safety matches.
Reacts very slowly with hot conc. NaOH (Unlike White P, which disproportionates easily).

Must Read: Allotropes of Phosphorus MCQs asked in NEET, IIT-JEE, GATE & CSIR-NET, IIT-JAM

Red Phosphorus: Preparation, Properties, Structure & Uses