Production of Sulfuric AcidOxford AQA IGCSE Chemistry: Revision notes

The Contact process is the industrial method for manufacturing sulfuric acid ($\text{H}_2\text{SO}_4$). It takes place in three stages, converting sulfur into sulfur dioxide, then sulfur trioxide, then sulfuric acid.

Sulfur is burned in oxygen to produce sulfur dioxide:

S(s) + O2(g) → SO2(g)

This is a straightforward combustion reaction and goes to completion (it is not reversible).

Sulfur dioxide is reacted with more oxygen to form sulfur trioxide, in a reversible reaction:

2SO2(g) + O2(g) ⇌ 2SO3(g)

This stage uses:

• a vanadium(V) oxide catalyst
• a temperature of approximately 450 °C
• atmospheric pressure

The catalyst speeds up the rate of reaction without being used up. A moderate temperature is used as a compromise: the forward reaction is exothermic, so a lower temperature would give a higher yield, but the rate would be too slow — 450 °C gives an acceptable rate without significantly reducing yield. Atmospheric (normal) pressure is used because the yield is already very high (about 98%) at this pressure, so using higher pressure would add cost for very little extra benefit.

Sulfur trioxide reacts with water to produce sulfuric acid:

H2O(l) + SO3(g) → H2SO4(aq)

This reaction is not reversible and goes to completion, producing concentrated sulfuric acid.