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8.3.2The Periodic Table

Production of Sulfuric Acid — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 4 parts Lesson 12 of 23 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Stages of the Contact Process

Sulfuric acid is made by the Contact process in three main stages.

1
Stage 1: sulfur is burned in oxygen to form sulfur dioxide: S(s) + O₂(g) → SO₂(g).
2
Stage 2: sulfur dioxide reacts with oxygen over a vanadium(V) oxide catalyst to form sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).
3
Stage 3: sulfur trioxide is absorbed into concentrated sulfuric acid, and the product is diluted with water to give sulfuric acid.

Part 2 of 4Conditions in Stage 2

The conditions in Stage 2 are chosen carefully as a compromise.

Temperature
  • Temperature About 450 °C is used because the forward reaction is exothermic, so a higher temperature lowers the yield.
Compromise
  • Compromise A lower temperature would give more sulfur trioxide, but the rate would be too slow to be economic.
Pressure
  • Pressure About 2 atm (200 kPa) is used, because the equilibrium mixture already contains a very high proportion of sulfur trioxide.
Catalyst
  • Catalyst Vanadium(V) oxide speeds up the reaction without being used up and without changing the position of equilibrium.

Part 3 of 4Why Pressure Is Kept Low

Higher pressure would favour the product but is not worth the cost.

Why Pressure Is Kept Low
Gas molecules
  • Gas molecules The forward reaction produces fewer gas molecules, so a higher pressure would favour sulfur trioxide.
Small gain
  • Small gain The yield is already very high, so the extra gain from higher pressure is small.
Cost
  • Cost The extra energy and equipment needed for high pressure are not worth spending for such a small gain.
Safety
  • Safety High-pressure plant is more hazardous, so atmospheric-level pressure is preferred.

Part 4 of 4Exam Checklist & Tips

8.3.2 – Key Points for the Exam

  • Contact process The industrial manufacture of sulfuric acid from sulfur, oxygen and water.
  • Vanadium(V) oxide The catalyst used to convert sulfur dioxide into sulfur trioxide.
  • Sulfur trioxide The gas SO₃ formed in Stage 2 of the Contact process.
  • Concentrated sulfuric acid The acid that absorbs sulfur trioxide in Stage 3.

Common Mistakes

  • Writing the Stage 2 equation without the reversible sign; this stage is an equilibrium.
  • Saying a higher temperature increases the yield of sulfur trioxide;
    • the reaction is exothermic, so a higher temperature lowers it.
  • Stating that the catalyst increases the yield of sulfur trioxide; it only speeds up the reaction.
  • Forgetting that the oxygen used in Stage 2 is obtained from the air.

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