The Contact Process
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
The Contact Process
The Contact Process manufactures sulfuric acid (H₂SO₄). Its key reversible step converts sulfur dioxide to sulfur trioxide:
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
- Source of SO₂: burning sulfur, or roasting sulfide ores, in air
- Source of O₂: air
- SO₃ is then dissolved in concentrated H₂SO₄ (not water directly, which would cause a dangerous, violent reaction) to form oleum, which is carefully diluted with water to give H₂SO₄
Typical Conditions and Why They Are Used
| Condition | Value used |
|---|---|
| Temperature | 450°C |
| Pressure | 200 kPa / 2 atm (low) |
| Catalyst | vanadium(V) oxide, V₂O₅ |
Temperature: a Compromise
The forward reaction (2SO₂ + O₂ ⇌ 2SO₃) is exothermic. A lower temperature would give an even higher yield, but the rate would be too slow. 450°C is a compromise that still gives a high yield (~98%) with an acceptable rate.
Pressure: Low Pressure is Sufficient
3 mol of gas (2SO₂ + O₂) → 2 mol of gas (2SO₃), so higher pressure would shift equilibrium further towards SO₃. But the yield is already about 98% at just 1-2 atm, so the extra 1-2% gain from very high pressure does not justify the cost and safety risk of high-pressure plant. Unlike the Haber process, low pressure is used here because the yield is already high enough without it.
Catalyst: Vanadium(V) Oxide
V₂O₅ speeds up the rate, allowing the moderate temperature of 450°C to still give an economically useful rate. Like all catalysts, it has no effect on the equilibrium yield.
Comparing the Haber and Contact Processes
| Aspect | Haber process | Contact process |
|---|---|---|
| Product | ammonia (NH₃) | sulfuric acid (H₂SO₄) |
| Key equilibrium | N₂ + 3H₂ ⇌ 2NH₃ | 2SO₂ + O₂ ⇌ 2SO₃ |
| Temperature | 450°C (compromise) | 450°C (compromise) |
| Pressure | 200 atm (high - yield per pass is low, so pressure is pushed up) | 1-2 atm (low - yield is already ~98%, not worth the cost of going higher) |
| Catalyst | iron | vanadium(V) oxide |
Common Mistakes
- ❌ Assuming high pressure must always be used to maximise yield
- ✓ the Contact Process deliberately uses LOW pressure, because the yield is already ~98% and high-pressure equipment isn't worth the extra cost for a 1-2% gain
- ❌ Adding SO₃ directly to water to make sulfuric acid
- ✓ SO₃ + water is violently exothermic and produces a dangerous acid mist - SO₃ must first be dissolved in concentrated H₂SO₄, then the oleum is diluted carefully
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