Question

why is so3 not directly dissolved in water during preparation of sulphuric acid

Answer 1

SO3 dissolution in water is extremely exothermic. This forces the sulfuric acid to form a mist of tiny droplets. These are highly corrosive which can even attack and corrode the lead pipelines and linings which cover the tower in contact process. Also, this mist does not condense so easily. So, sulfur trioxide is dissolved in concentrated sulfuric acid to produce oleum/ pyrosulfuric acid (H2S2O7). equialent to sulfuric acid content of about 110% )which can be diluted with a required amount of water (in a much less exothermic pathway) to yield con (98%) sulfuric acid.

Answer 2

Because when mixed with water, sulfur trioxide causes the formation of thick fog comprising of sulfuric acid that is hard to condense.

Explanation:

• Sulfur trioxide is never dissolved directly in the water but rather through the contact process.
• The key reason is that $SO{3}$ is highly exothermic in nature.
• Thus, when sulfur trioxide is mixed with water, it leads to the formation of aerosol(a dense fog/mist) of sulfuric acid which is highly corrosive and hard to liquidize/condense.
• Hence, it is first dissolved in concentrated sulfuric acid.

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