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The contact process, for making sulfuric acid, is a process which involves a reversible reaction.

The raw materials needed to make sulfuric acid are:

• sulfur
• air
• water

Stage one – making sulfur dioxide

In the first stage of the contact process, sulfur is burned in air to make sulfur dioxide:

sulfur + oxygen → sulfur dioxide

S(l) + O₂(g) → SO₂(g)

This is not a reversible reaction – (l) means liquid and (g) means gas.

Sulfur dioxide should not be released into the atmosphere as it contributes to acid rain.

Stage two – making sulfur trioxide

In the second stage, sulfur dioxide reacts with more oxygen to make sulfur trioxide:

sulfur dioxide + oxygen ⇌ sulfur trioxide

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

This reaction is reversible. The conditions needed for it are:

• a catalyst of vanadium(V) oxide, V₂O₅
• a temperature of around 450°C (chosen as a compromise temperature, giving a decent yield with a good rate of reaction)
• a pressure of approximately 2 atmospheres (the increased pressure favours the formation of SO₃, but if the pressure is too high the risk of explosion is too great, given that SO₃ is a highly acidic gas)

Stage three – making sulfuric acid

In the final stage, sulfur trioxide reacts with water to make sulfuric acid:

H₂O(l) + SO₃(g) → H₂SO₄(aq)

This is not a reversible reaction, just like the first stage – (aq) means aqueous, or dissolved in water.

The direct addition of sulfur trioxide to water is highly exothermic and leads to the formation of clouds of sulfuric acid. This is highly dangerous. In industry, sulfur trioxide is added to highly concentrated sulfuric acid to form a highly corrosive liquid called oleum to minimise the risk:

SO₃ + H₂SO₄ → H₂S₂O₇

Water is then added to the oleum to form sulfuric acid again:

H₂S₂O₇ + H₂O → 2H₂SO₄