The Contact Process

The Contact Process is the industrial method to make sulfuric acid, an acid used to make fertilizers, detergents, and car batteries. It turns sulfur into sulfuric acid in safe, controlled steps.

Key steps and equations

• Make sulfur dioxide (SO₂): burn sulfur or roast sulfide ores.
• Convert sulfur dioxide to sulfur trioxide (SO₃) in a reversible reaction.
• Absorb SO₃ into concentrated sulfuric acid to make oleum, then add water to get more sulfuric acid.

Equations:

$$\mathrm{S} + \mathrm{O}_2 \rightarrow \mathrm{SO}_2$$

$$2\,\mathrm{SO}_2(g) + \mathrm{O}_2(g) \rightleftharpoons 2\,\mathrm{SO}_3(g)$$

$$\mathrm{SO}_3(g) + \mathrm{H}_2\mathrm{SO}_4(l) \rightarrow \mathrm{H}_2\mathrm{S}_2\mathrm{O}_7(l)$$

$$\mathrm{H}_2\mathrm{S}_2\mathrm{O}_7(l) + \mathrm{H}_2\mathrm{O}(l) \rightarrow 2\,\mathrm{H}_2\mathrm{SO}_4(aq)$$

Sources of reactants

• Sulfur dioxide: from burning sulfur or roasting sulfide ores (e.g., zinc sulfide).
• Oxygen: from air.

Conditions and equilibrium (why these values?)

The key reversible step is exothermic in the forward direction (it releases heat): $2\,\mathrm{SO}_2 + \mathrm{O}_2 \rightleftharpoons 2\,\mathrm{SO}_3$.

• Temperature ~ 450 °C: Lower temperature gives higher SO₃ yield (favours exothermic forward reaction), but is too slow. Around 450 °C balances a good rate with a good yield.
• Pressure ~ 200 kPa (≈ 2 atm): Going from 3 gas molecules to 2 favours higher pressure. A small pressure increase slightly improves yield and is cheaper and safer than very high pressures.
• Catalyst: vanadium(V) oxide, V₂O₅: Speeds up both forward and reverse reactions equally, reaching equilibrium faster. It does not change the equilibrium position.

Analogy: Imagine people moving between two rooms (reactants and products). Temperature is like how energetic they are; pressure is how crowded the rooms are; a catalyst is a shortcut corridor that makes moving faster but does not change how many end up in each room at equilibrium.

Why not add SO₃ directly to water?

SO₃ reacts violently with water, making a hot acidic mist that is hard to collect. Absorbing SO₃ in concentrated sulfuric acid first (forming oleum) is safer and controllable; oleum is then diluted with water.

Industrial notes

• Gases are cleaned and dried to protect the V₂O₅ catalyst.
• Unreacted SO₂ and O₂ are recycled to increase overall yield.

Common misconceptions

• A catalyst increases rate; it does not increase equilibrium yield.
• Higher temperature is not always better; for exothermic equilibria it lowers yield.
• Very high pressure is not used here because the yield gain is small and equipment costs and safety risks rise.