9.3 - 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.

sulfur + oxygen → sulfur dioxide

S(s) + O₂(g) → SO₂(g)

Stage 2 - Making sulfur trioxide

• The sulfur dioxide is mixed with more oxygen and passed over a catalyst, oxidising it to sulfur trioxide.
• This reaction is exothermic and reversible:
  • sulfur dioxide + oxygen ⇌ sulfur trioxide
  • 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

Conditions used:

• Temperature - About 450 °C
• Pressure - At or just above atmospheric
• Catalyst - Vanadium(V) oxide, V₂O₅ (vanadium pentoxide)

Stage 3 - Making sulfuric acid

The overall reaction turns sulfur trioxide and water into sulfuric acid:

sulfur trioxide + water → sulfuric acid

SO₃(g) + H₂O(l) → H₂SO₄(aq)

Why it isn't done directly

• Adding sulfur trioxide straight to water releases so much heat that the acid boils, forming a dangerous acidic mist.
• Instead, sulfur trioxide is dissolved in concentrated sulfuric acid to produce oleum:
  • sulfur trioxide + concentrated sulfuric acid → oleum
  • SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l)

The oleum is then safely diluted with water to make sulfuric acid:

oleum + water → sulfuric acid

H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)

Uses of sulfuric acid

Sulfuric acid is manufactured in huge quantities as an industrial raw material.

Main uses

• Fertilisers - Making phosphate fertilisers and ammonium sulfate (the largest single use).
• Other uses - Making detergents, paints, dyes, plastics, and synthetic fibres.

Concentrated sulfuric acid as a dehydrating agent

Concentrated sulfuric acid is a dehydrating agent: it removes water (or the elements of water) from other substances.

Hydrated and anhydrous salts

• Hydrated salts contain water of crystallisation within their crystal lattice.
• Anhydrous salts have had this water removed.

Hydrated copper(II) sulfate

Concentrated sulfuric acid removes water of crystallisation from blue hydrated copper(II) sulfate, leaving white anhydrous copper(II) sulfate powder:

CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(l)

The mixture gets very hot, because the acid gives out a lot of heat as it takes up the water.

Sugar (sucrose)

Concentrated sulfuric acid removes hydrogen and oxygen (as water) from sucrose, leaving behind carbon:

C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(l)

The mixture turns black and expands into a steaming black column of carbon as the heat of reaction boils the released water into steam.