Question

In the contact process, concentrated sulphuric acid instead of water is used to absorb the sulphur trioxide gas produced.One of the reasons suggested online is that the reaction between sulphur trioxide and water is highly exothermic.

But I have these concerns:

Both the reaction between sulphur trioxide with concentrated sulphuric acid and that between oleum and water are exothermic
If the reaction is too exothermic, a coolant can be applied in the industrial process
So, I propose two possible reasons.

The two previously mentioned are less exothermic
More concentrated sulphuric acid can be produced by the reaction between concentrated sulphuric acid and sulphur trioxide, hence the process can become more economical
What should be the best explanation for why water is not used to absorb sulphur trioxide?

Answer 1

Concentrated sulfuric acid is used to absorb sulfur trioxide in the contact process to form oleum, which is safer and more controlled than the exothermic reaction with water, while also allowing for economical production of various sulfuric acid concentrations.

Step-by-step explanation:

In the contact process for the manufacture of sulfuric acid, concentrated sulfuric acid is used to absorb sulfur trioxide (SO3) rather than water due to several reasons.

One major reason is that the direct reaction of sulfur trioxide with water is highly exothermic and can lead to a violent reaction, making it difficult to control in an industrial setting. When sulfur trioxide is absorbed into concentrated sulfuric acid, it forms oleum, which can then be safely diluted with water to produce concentrated sulfuric acid.

This approach is less hazardous and allows for the production of sulfuric acid at various desired concentrations more efficiently and economically, leveraging the dehydrating properties of sulfuric acid without the excessive release of heat that would occur with the direct hydration by water.

Another reason is that the formation of oleum prevents the release of large amounts of heat at once, as would occur if sulfur trioxide were directly combined with water. It's also preferable because sulfuric acid can be kept in a liquid state in a range of concentrations, allowing for better handling and storage than with a fully diluted solution.

Therefore, the use of concentrated sulfuric acid not only mitigates safety risks but also improves the economy and control of sulfuric acid production.