6.5 - The Contact Process

Making sulfuric acid in three stages

Sulfuric acid is manufactured by the Contact process in three stages.

Stage 1 - Making sulfur dioxide

Sulfur is burned in air to make sulfur dioxide.

S(s) + O₂(g) → SO₂(g)

Stage 2 - Making sulfur trioxide

• The sulfur dioxide reacts with more oxygen to make sulfur trioxide.
• This reaction is exothermic and reversible:
  • 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

Carried out over a catalyst of vanadium(V) oxide, V₂O₅, at about 450 °C and atmospheric pressure.

Stage 3 - Making sulfuric acid

The sulfur trioxide reacts with water to give sulfuric acid.

H₂O(l) + SO₃(g) → H₂SO₄(aq)

Contact process conditions

The reaction to make sulfur trioxide is reversible:

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

Temperature - About 450 °C

• The forward reaction is exothermic, so a lower temperature gives a higher yield of sulfur trioxide at equilibrium, but makes the reaction too slow.
• A higher temperature increases the rate, but reduces the yield.
• 450 °C is an optimum temperature that gives an acceptable yield at a reasonable rate.

Pressure - At or just above atmospheric

• An increase in pressure favours the forward reaction because there are 3 moles of gas on the left and only 2 moles on the right.
• However, the yield is already high (around 98%) at atmospheric pressure, so high-pressure equipment is not worth the extra cost.

Catalyst - Vanadium(V) oxide

• Speeds up the reaction without being used up.
• Increases the rate at 450 °C without affecting the position of equilibrium or yield.