The Contact Process
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
What is the Contact Process? The Contact Process is an industrial process used to manufacture sulfuric acid (H₂SO₄), one of the most important chemicals in the world. Why is it important? Sulfuric acid is used to make fertilizers, detergents, paints, dyes, plastics, and many other chemicals.
More sulfuric acid is produced than any other chemical! The Three Main Stages The Contact Process involves three key stages: Stage 1: Making sulfur dioxide S(s) + O₂(g) → SO₂(g) OR: 4FeS₂(s) + 11O₂(g) → 2Fe₂O₃(s) + 8SO₂(g) (Burn sulfur or roast iron pyrite in air) Stage 2: Converting SO₂ to SO₃ (KEY REVERSIBLE STEP) 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = -197 kJ/mol This is REVERSIBLE and EXOTHERMIC Stage 3: Making sulfuric acid SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l) (oleum) Then: H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l) (SO₃ is NOT added directly to water as this is too violent) Why Not Add SO₃ Directly to Water?
Problem: SO₃ + H₂O → H₂SO₄ is extremely exothermic and violent. It would produce a dangerous mist of sulfuric acid. Solution: Dissolve SO₃ in concentrated H₂SO₄ first to make oleum (H₂S₂O₇), then carefully dilute with water.
Stage 2: The Reversible Reaction (Most Important) 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = -197 kJ/mol This is the KEY step that determines efficiency Key features: Reversible: Shown by ⇌ symbol Exothermic: ΔH is negative (forward releases heat) Fewer molecules on right: 3 molecules left (2+1) → 2 molecules right Conditions Used in the Contact Process Condition Value Used Why This Value?
Temperature 450°C (moderate) Compromise between rate and yield Pressure 1-2 atmospheres (low) Yield already ~98% at low pressure, not worth extra cost Catalyst Vanadium(V) oxide (V₂O₅) Speeds up reaction without being used up Effect of TEMPERATURE - The Compromise Forward reaction is EXOTHERMIC (ΔH = -197 kJ/mol) Temperature Effect on Yield Effect on Rate Low temperature (e.g., 300°C) VERY HIGH yield (~99%) - favors exothermic forward reaction VERY SLOW rate - uneconomical High temperature (e.g., 700°C) LOW yield - favors endothermic backward reaction FAST rate COMPROMISE: 450°C High yield (~98%) Reasonable rate with catalyst Le Chatelier's Principle: Lower temperature favors exothermic direction (forward), giving higher yield.
But 450°C is chosen because lower temperatures make the reaction too slow even with a catalyst. Effect of PRESSURE - Low Pressure is Fine Molecule count: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) Left side: 2 + 1 = 3 molecules Right side: 2 molecules Pressure Effect on Yield Cost/Safety Why Not Used?
Low (1-2 atm) ~98% yield (already very high!) Cheap, safe equipment This IS what's used! High (200 atm) Slightly higher yield (~99%) Very expensive, dangerous Only 1% improvement - not worth the cost Key difference from Haber Process: Unlike ammonia production (only 15-20% yield), the Contact Process already achieves ~98% yield at LOW pressure.
Increasing pressure is not economically worthwhile for such a small improvement. Le Chatelier's Principle: Increasing pressure would shift equilibrium to the right (fewer molecules), but the yield is already so high that the extra cost isn't justified.
Effect of CATALYST - Vanadium(V) Oxide Vanadium(V) oxide (V₂O₅) is used as the catalyst. Aspect Without Catalyst With V₂O₅ Catalyst Rate of reaction Too slow to be economical Much faster - economically viable Yield at equilibrium ~98% at 450°C ~98% at 450°C (same!) Temperature needed Would need very high temp for good rate 450°C gives good rate with catalyst Important: The catalyst allows a moderate temperature (450°C) to be used while still maintaining a fast rate.
This gives both high yield AND fast production! Contact Process Flow Diagram Step-by-step process: Produce SO₂: Burn sulfur (or roast iron pyrite) in air to make SO₂ Purify gases: Remove impurities that would poison the catalyst Mix gases: SO₂ mixed with excess air (provides O₂) Pass over catalyst: Mixture passed over vanadium(V) oxide at 450°C and 1-2 atm Equilibrium reached: About 98% converts to SO₃ Cool gases: SO₃ cooled Absorb in H₂SO₄: SO₃ dissolved in concentrated sulfuric acid to make oleum (H₂S₂O₇) Add water carefully: Oleum diluted with water to produce concentrated H₂SO₄ Why Use Excess Air?
Reason: Using excess oxygen (from air) shifts the equilibrium position to the right by Le Chatelier's Principle, increasing the yield of SO₃. Also ensures all the SO₂ reacts (oxygen is cheap - it's free from air!) Uses of Sulfuric Acid Sulfuric acid is one of the most widely used chemicals in industry: Fertilizers: Making ammonium sulfate, phosphate fertilizers (largest use) Detergents: Manufacturing cleaning products Paints and pigments: Production of titanium dioxide (white paint) Plastics and fibers: Making nylon, polyester Batteries: Car batteries use dilute sulfuric acid Metal processing: Cleaning metals before plating Petroleum refining: Removing impurities from crude oil Comparing Contact Process and Haber Process Aspect Haber Process Contact Process Product Ammonia (NH₃) Sulfuric …
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