Sulphur: Allotropes, Compounds, Extraction, Uses & Environmental Impact

TAKEAWAYS:

• Sulphur exists in three allotropes (rhombic, monoclinic, plastic) with different physical properties and uses.
• Hydrogen sulphide (H₂S) and sulphur dioxide (SO₂) are toxic gases with distinct smells and industrial applications.
• The Contact Process produces sulphuric acid (H₂SO₄), the most important industrial chemical.
• Sulphur compounds are essential for life (amino acids, vitamins) but also pollute air and water (acid rain, corrosion).
• NEB exam focus: Extraction methods, chemical reactions of sulphur compounds, and environmental impacts.

1. Occurrence and Extraction of Sulphur

1.1 Occurrence of Sulphur

Sulphur is found in nature in three main forms:

1. Native sulphur (free sulphur) – Found in volcanic regions (e.g., Sicily, Japan, USA) and underground deposits.
2. Combined sulphur – Found in metal sulphides (e.g., pyrite (FeS₂), galena (PbS)) and sulphates (e.g., gypsum (CaSO₄·2H₂O)).
3. In organic compounds – Found in proteins, coal, petroleum, and natural gas.

1.2 Extraction of Sulphur

Sulphur is extracted by two main methods:

(A) Frasch Process (Underground Sulphur)

• Used for deep underground deposits (e.g., Texas, USA).
• Steps:
  1. Superheated water (165°C) is pumped down to melt sulphur.
  2. Compressed air is blown in to force the molten sulphur up through pipes.
  3. Sulphur solidifies and is collected.

flowchart TD
    A["Superheated water (165°C)"] --> B["Melt sulphur"]
    B --> C["Compressed air"]
    C --> D["Force molten sulphur up"]
    D --> E["Sulphur solidifies & collected"]

(B) From Hydrogen Sulphide (H₂S)

• H₂S is obtained from petroleum refining and natural gas.
• Process:
  • H₂S is burned in air to form SO₂: $$2H_{2}S+3O_2\to 2S{O}_2+2H_{2}O$$
  • SO₂ is then converted to sulphur (Claus Process): $$2H_{2}S+S{O}_2\to 3S+2H_{2}O$$

(C) From Pyrite (FeS₂)

• Roasting of pyrite gives SO₂, which is then converted to sulphur: $$4Fe{S}_2+11O_2\to 2F{e}_2{O}_3+8S{O}_2$$ $$2S{O}_2+2H_{2}S\to 3S+2H_{2}O$$

2. Allotropes of Sulphur

Sulphur exists in three allotropic forms with different structures and properties.

2.1 Conversion Between Allotropes

• Rhombic ↔ Monoclinic:
  • Heating rhombic sulphur above 96°C → monoclinic (stable above 96°C).
  • Cooling monoclinic below 96°C → rhombic (stable below 96°C).
• Plastic Sulphur:
  • Formed by rapid cooling of molten sulphur (prevents S₈ ring formation).
  • Elastic and rubber-like due to long Sₙ chains.

3. Chemical Properties of Sulphur

Sulphur reacts with metals, non-metals, and compounds to form various products.

3.1 Reaction with Metals

• Forms metal sulphides (used in pyrotechnics, semiconductors). $$S+Fe\to FeS$$ $$3S+2Al\to A{l}_2{S}_3$$

3.2 Reaction with Non-Metals

• With hydrogen (H₂) → Hydrogen sulphide (H₂S) (rotten egg smell, toxic). $$S+H_2\to H_{2}S$$
• With oxygen (O₂) → Sulphur dioxide (SO₂) (choking smell, pollutant). $$S+O_2\to S{O}_2$$

3.3 Reaction with Compounds

• With steam (H₂O) → H₂S and SO₂. $$3S+2H_{2}O\to 2H_{2}S+S{O}_2$$
• With concentrated HNO₃ → H₂SO₄. $$S+6HN{O}_3\to H_{2}S{O}_4+6N{O}_2+2H_{2}O$$

4. Important Compounds of Sulphur

4.1 Hydrogen Sulphide (H₂S)

• Properties:
  • Colourless gas with rotten egg smell.
  • Highly toxic (paralyses olfactory nerves).
  • Weak acid (turns blue litmus red).
• Preparation: $$FeS+2HCl\to FeC{l}_2+H_{2}S$$
• Uses:
  • Manufacture of sulphur (Claus Process).
  • Leather industry (dehairing).
  • Analytical chemistry (detecting metal ions).

4.2 Sulphur Dioxide (SO₂)

• Properties:
  • Colourless gas with pungent smell.
  • Bleaching agent (reducing agent).
  • Forms acid rain (pollutant).
• Preparation: $$S+O_2\to S{O}_2$$ $$Cu+2H_{2}S{O}_4\to CuS{O}_4+S{O}_2+2H_{2}O$$
• Uses:
  • Preservative in food (E220-E228).
  • Disinfectant (sterilising wine).
  • Manufacture of H₂SO₄ (Contact Process).

4.3 Sulphuric Acid (H₂SO₄)

• Properties:
  • Oily, colourless liquid.
  • Strong dehydrating agent (removes water from substances).
  • Highly corrosive (burns skin).
• Preparation (Contact Process):
  1. Burn sulphur or pyrite → SO₂.
  2. Convert SO₂ to SO₃ (using V₂O₅ catalyst). $$2S{O}_2+O_2\rightleftharpoons 2S{O}_3$$
  3. Absorb SO₃ in H₂SO₄ → Oleum (H₂S₂O₇).
  4. Dilute oleum with water → H₂SO₄.
• Uses:
  • Fertiliser industry (NPK).
  • Petroleum refining (removes impurities).
  • Battery acid (lead-acid batteries).
  • Dye and detergent manufacturing.

flowchart TD
    A["Burn S/Pyrite"] --> B["SO₂"]
    B --> C["SO₂ + O₂ → SO₃ (V₂O₅ catalyst)"]
    C --> D["SO₃ + H₂SO₄ → Oleum"]
    D --> E["Oleum + H₂O → H₂SO₄"]

5. Environmental Effects of Sulphur Compounds

Sulphur compounds pollute air, water, and soil, causing:

1. Acid Rain:
   • SO₂ and NO₂ react with water → H₂SO₄ and HNO₃.
   • Damages crops, buildings, and aquatic life.
2. Respiratory Diseases:
   • SO₂ and H₂S cause asthma, bronchitis.
3. Global Warming:
   • SO₂ forms aerosols that reflect sunlight (cooling effect, but harmful).

5.1 Control of Sulphur Pollution

• Use low-sulphur fuels (diesel, coal).
• Scrubbers in factories (remove SO₂ from exhaust gases).
• Catalytic converters in vehicles.

6. NEB Board-Style Questions & Solutions

Short Answer Questions (SAQ)

Q1. How is sulphur extracted by the Frasch process? Answer:

1. Superheated water (165°C) is pumped down to melt sulphur.
2. Compressed air forces molten sulphur up through pipes.
3. Sulphur solidifies and is collected.

Q2. Why is plastic sulphur elastic? Answer: Plastic sulphur has long chains of sulphur atoms (Sₙ) that can stretch and coil, making it elastic.

Q3. Write the reaction of sulphur with hot concentrated HNO₃. Answer: $$S+6HN{O}_3\to H_{2}S{O}_4+6N{O}_2+2H_{2}O$$

Long Answer Questions (LAQ)

Q4. Describe the Contact Process for the manufacture of sulphuric acid. Answer:

1. Burn sulphur or pyrite to produce SO₂.
2. Convert SO₂ to SO₃ using V₂O₅ catalyst at 450°C.
3. Absorb SO₃ in H₂SO₄ to form oleum (H₂S₂O₇).
4. Dilute oleum with water to get concentrated H₂SO₄ (98%).

Q5. Explain the environmental effects of sulphur dioxide. Answer:

• Forms acid rain (H₂SO₄), damaging soil, crops, and buildings.
• Causes respiratory diseases (asthma, bronchitis).
• Forms aerosols that reflect sunlight, affecting climate.

Exam Tip

✅ NEB Exam Focus Areas:

1. Extraction methods (Frasch Process, Claus Process).
2. Allotropes (rhombic ↔ monoclinic conversion, plastic sulphur).
3. Chemical reactions (sulphur with metals, non-metals, compounds).
4. Contact Process (steps, catalysts, conditions).
5. Environmental impact (acid rain, pollution control).

🔹 Common Mistakes to Avoid:

• Confusing rhombic and monoclinic sulphur (melting point difference).
• Forgetting V₂O₅ catalyst in the Contact Process.
• Writing incorrect balancing of equations (e.g., SO₂ + H₂O → H₂SO₃).

📌 Marks Boosters:

• Draw diagrams (Frasch Process, Contact Process).
• Mention real-world applications (H₂SO₄ in fertilisers, SO₂ in food preservation).
• Explain environmental effects with examples (acid rain damaging Taj Mahal).

Step-by-step industrial production of sulphuric acid (Image: Alexandre Passant & Jodi Schneider, CC BY-SA 3.0, via Wikimedia Commons) Chemical reactions leading to environmental damage (Image: Nino Barbieri, CC BY 2.5, via Wikimedia Commons)