Phosphorus: Allotropes, Compounds, and Uses

TAKEAWAYS:

• Phosphorus exists in three allotropes (white, red, black) with distinct properties and uses.
• Phosphorus oxides (P₄O₆, P₄O₁₀) react with water to form phosphoric and phosphorous acids, crucial in fertilizers and food additives.
• Phosphate rock (Ca₅(PO₄)₃(OH)) is the primary source of phosphorus, processed via the electric furnace method.
• Phosphorus compounds (e.g., PCl₅, Ca₃(PO₄)₂) are used in detergents, matches, and pesticides.
• Environmental impact: Excess phosphorus causes eutrophication (water pollution).
• NEB exam focus: Allotrope properties, reactions of P₄O₁₀, and industrial extraction.

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### **1. Introduction to Phosphorus**
Phosphorus is a **non-metal** (Group 15, Period 3) that **never occurs free in nature**—it is always found in **compounds**, mainly in **phosphate rocks** (e.g., Ca₅(PO₄)₃(OH)). It is **highly reactive** and forms **three allotropes** (different forms of the same element). Phosphorus is **essential for life** (found in DNA, ATP, and bones) and has **industrial uses** in fertilizers, matches, and detergents.

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### **2. Allotropes of Phosphorus**
Allotropes are **different structural forms of the same element**. Phosphorus has **three main allotropes**:

```figure
{"type":"cylinder","r":"1.5 cm","h":"2 cm","caption":"Tetrahedral structure of white phosphorus (P₄)"}

A. White Phosphorus (P₄)

• Structure: Tetrahedral (4 P atoms bonded in a pyramid).
• Appearance: Waxy, translucent solid (glows in the dark due to oxidation).
• Reactivity: Highly reactive—ignites spontaneously in air (used in smoke screens and incendiary bombs).
• Toxicity: Poisonous (causes "phossy jaw" in workers).
• Storage: Kept under water to prevent oxidation.

B. Red Phosphorus

• Structure: Polymeric chains (long chains of P atoms).
• Appearance: Reddish-brown powder.
• Reactivity: Less reactive than white phosphorus (does not ignite in air).
• Uses:
  • Matchbox strikers (mixed with glass powder).
  • Flare pistols (in military applications).
  • Semiconductors (in solar cells).

C. Black Phosphorus

• Structure: Layered sheets (like graphite).
• Appearance: Black, metallic luster.
• Reactivity: Least reactive.
• Uses:
  • Semiconductor industry (better than silicon in some cases).
  • Lubricants (due to its layered structure).

3. Extraction of Phosphorus

Phosphorus is extracted from phosphate rock (Ca₅(PO₄)₃(OH)) using the electric furnace method:

Step-by-Step Process

1. Mix phosphate rock with sand (SiO₂) and coke (C).
2. Heat in an electric furnace (1400–1500°C).
3. Reactions:
   • Ca₅(PO₄)₃(OH) + 3 SiO₂ → 3 Ca₃(PO₄)₂ + CaSiO₃ (slag)
   • Ca₃(PO₄)₂ + 5 C + 3 SiO₂ → 3 CaSiO₃ + 5 CO + 2 P (gaseous)
4. Gaseous P₄ is collected and cooled to form white phosphorus.

flowchart TD
    A["Phosphate Rock<br/>Ca₅(PO₄)₃(OH)"] -->|"+ SiO₂, C"| B["Electric Furnace<br/>1400°C"]
    B --> C["Ca₃(PO₄)₂ + CaSiO₃<br/>(slag)"]
    B --> D["5 CO + 2 P₄<br/>(gas)"]
    D --> E["Cool P₄<br/>→ White Phosphorus"]

Why Sand (SiO₂) is Added?

• Removes calcium as calcium silicate (CaSiO₃), preventing reformation of phosphate.

4. Phosphorus Oxides

Phosphorus burns in air to form two oxides:

A. Phosphorus(III) Oxide (P₄O₆)

• Formation: P₄ + 3 O₂ → P₄O₆
• Reaction with water: P₄O₆ + 6 H₂O → 4 H₃PO₃ (phosphorous acid)
• Uses: Rubber industry, food preservatives.

B. Phosphorus(V) Oxide (P₄O₁₀)

• Formation: P₄ + 5 O₂ → P₄O₁₀
• Reaction with water: P₄O₁₀ + 6 H₂O → 4 H₃PO₄ (phosphoric acid)
• Uses:
  • Fertilizers (superphosphate).
  • Food industry (acidulant in sodas).
  • Detergents (as sodium phosphate).

5. Phosphoric and Phosphorous Acids

Properties of Phosphoric Acid (H₃PO₄)

• Triprotic acid (can donate 3 H⁺ ions).
• Weak acid (does not dissociate completely).
• Uses:
  • Phosphate fertilizers (e.g., superphosphate).
  • Cola drinks (acidulant).
  • Cleaning agents.

6. Phosphorus Halides

Phosphorus reacts with halogens (F₂, Cl₂, Br₂) to form phosphorus halides:

Reaction of PCl₅ with Water

PCl₅ + 4 H₂O → H₃PO₄ + 5 HCl

• Used to generate HCl gas in labs.

7. Phosphorus in Fertilizers

Phosphorus is a key nutrient for plants (along with N, K). The main phosphorus fertilizers are:

1. Superphosphate (Ca(H₂PO₄)₂)

   • Prepared by treating phosphate rock with sulfuric acid (H₂SO₄).
   • Reaction: Ca₃(PO₄)₂ + 2 H₂SO₄ → Ca(H₂PO₄)₂ + 2 CaSO₄
   • Advantage: Highly soluble, readily absorbed by plants.
   • Disadvantage: Can acidify soil over time.

2. Basic Slag (Ca₃(PO₄)₂)

   • Prepared by heating phosphate rock with silica and coke.
   • Advantage: Less acidic than superphosphate.
   • Disadvantage: Less soluble, slower release.

8. Environmental Impact of Phosphorus

• Eutrophication: Excess phosphorus in water bodies causes algal blooms, leading to oxygen depletion and fish kills.
• Soil pollution: Overuse of phosphorus fertilizers can reduce soil fertility over time.
• Biological role: Essential for DNA, ATP, and cell membranes.

Excess phosphorus causing algal bloom in a lake. (Image: Goran_tek-en, CC BY-SA 4.0, via Wikimedia Commons)

9. Industrial Applications of Phosphorus

Exam Tip: How to Score Full Marks in NEB Exams

1. Allotropes Comparison:

   • Always compare structure, reactivity, and uses of white, red, and black phosphorus.
   • Example: "White phosphorus is highly reactive and stored under water, while red phosphorus is used in matchboxes."

2. Extraction Process:

   • Memorize the electric furnace method steps and role of sand (SiO₂).
   • Example: "Sand removes calcium as calcium silicate, preventing reformation of phosphate."

3. Reactions of Phosphorus Oxides:

   • Know how P₄O₆ → H₃PO₃ and P₄O₁₀ → H₃PO₄.
   • Example: "P₄O₁₀ reacts with water to form phosphoric acid, used in fertilizers."

4. Fertilizer Preparation:

   • Differentiate between superphosphate (H₂SO₄) and basic slag (heat + coke).
   • Example: "Superphosphate is prepared by treating phosphate rock with sulfuric acid."

5. Environmental Impact:

   • Always mention eutrophication when discussing phosphorus pollution.

NEB Board-Style Questions (Practice)

Short Answer (3 marks each)

1. Describe the structure and uses of white and red phosphorus.
2. How is phosphoric acid prepared from phosphorus? Give one use.
3. What is eutrophication? How does phosphorus contribute to it?

Long Answer (5 marks)

4. Explain the electric furnace method for extracting phosphorus from phosphate rock. Why is sand added in this process?
5. Compare P₄O₆ and P₄O₁₀ in terms of:
   • Formation
   • Reaction with water
   • Uses

Numerical (2 marks)

6. Calculate the mass of P₄O₁₀ formed when 124 g of phosphorus (P₄) burns completely in excess oxygen. (Atomic mass: P = 31, O = 16)

Solutions to NEB-Style Questions

1. Structure and Uses of White & Red Phosphorus

• White Phosphorus:
  • Structure: Tetrahedral (P₄ molecules).
  • Uses: Smoke screens, incendiary bombs (highly reactive).
• Red Phosphorus:
  • Structure: Polymeric chains.
  • Uses: Matchbox strikers, flares (less reactive).

4. Electric Furnace Method

• Steps:
  1. Mix phosphate rock (Ca₅(PO₄)₃(OH)) + sand (SiO₂) + coke (C).
  2. Heat in electric furnace (1400–1500°C).
  3. P₄ gas is collected and cooled to form white phosphorus.
• Role of Sand: Removes calcium as CaSiO₃, preventing reformation of phosphate.

6. Mass of P₄O₁₀ Calculation

• Given: 124 g P₄ → Molar mass of P₄ = 4 × 31 = 124 g/mol.
• Moles of P₄ = 124 g / 124 g/mol = 1 mol.
• Reaction: P₄ + 5 O₂ → P₄O₁₀.
• Moles of P₄O₁₀ = 1 mol (1:1 ratio).
• Molar mass of P₄O₁₀ = 4 × 31 + 10 × 16 = 284 g/mol.
• Mass of P₄O₁₀ = 1 mol × 284 g/mol = 284 g.

Final Notes for NEB Exam

✅ Memorize allotropes (structure, reactivity, uses). ✅ Know extraction steps (electric furnace, role of sand). ✅ Reactions of P₄O₆ and P₄O₁₀ with water. ✅ Fertilizer preparation (superphosphate vs. basic slag). ✅ Environmental impact (eutrophication).

Good luck! 🚀