Why is sulfur trioxide not added directly to water to form sulphuric acid?

Answer 1

In the manufacture of H2SO4, the reaction:

H2O + SO3 → H2SO4 is not used because it is highly exothermic.

Instead, the SO3 is dissolved in conc H2SO4 to form a substance called oleum, H2S2O7. This is then dissolved in water to produce H2SO4

H2S2O7 + H2O → 2H2SO4 This is a much more controllable reaction

Answer 2

The reaction would be:

SO3 (g) + H2O (l) → H2SO4 (l)

This reaction is highly exothermic. Enough heat is generated to vaporize the aqueous solution of Sulfuric acid that would be formed. In order to control this on an industrial scale, a great amount of energy would be need to remove this generated heat. This would be costly, driving up the final price of the H2SO4 produced.

In addition, the vaporized H2SO4 is very difficult to deal with. This is super acid rain and even if contained is highly corrosive.

By dissolving the Sulphur trioxide in Sulphuric acid, oleum is formed:

SO3 (g) + H2SO4 (l) → H2S2O7 (l)

This reaction does generate as much heat, an in a plant which is making H2SO4, the cost of the acid is negligible.

Answer 3

Sulfur trioxide is highly reactive with water, forming sulfuric acid in an extremely exothermic reaction which can lead to thermal hazards like splattering and boiling. In addition, the reaction is difficult to control and can cause a rapid increase in temperature, making it unsafe. Therefore, sulfur trioxide is typically added to a diluted acid solution to generate sulfuric acid more safely.