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I've read that in the preparation of $\ce{H2SO4}$ through the Contact process, on the absortion tower, the gas $\ce{SO3}$ is getting in contact with $\ce{H2SO4} (l)$ , because adding $\ce{SO3}$ to $\ce{H2O}$ is going to cause an exothermic reaction, so:

$\ce{SO3 + H2SO4 -> H2S2O7}$

After this, it is controllable to add water to the oleum in order to produce liquid sulfuric acid.

$\ce{H2S2O7 + H2O → 2 H2SO4}$

But I've seen in other websites that:

$\ce{SO3 + H2O -> H2SO4}$

So I don't know which is the real reaction of the absortion tower, because there are other pages that state that the liquid entering is 98% sulfuric acid, and 2% water, this water reacts with the sulfur trioxide (this little water avoids the heat problem), producing sulfuric acid.

I would like to know which premise is correct.

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    $\begingroup$ All is correct. Reactions of sulphur trioxide and sulphuric acid or residual water are obviously different. The latter can be also considered as the net reaction of the whole absorption/dilution process. $\endgroup$
    – Poutnik
    Commented Oct 7, 2021 at 16:50
  • $\begingroup$ If you carry out the algebraic sum of your two first equations, you will obtain the third equation. So the third equation does not really happens. But it means that if you take $\ce{1 SO3}$ and $\ce{1 H2O}$, and that these substances are introduced in some unexplained ... machinery, you will always get at the end $\ce{1 H2SO4}$ $\endgroup$
    – Maurice
    Commented Oct 7, 2021 at 21:02
  • $\begingroup$ There are many questions related to "Contact process" in chem.SE. I am sure you can gain some insight from them. Here, have a look: chemistry.stackexchange.com/search?q=Contact+process $\endgroup$
    – Nilay Ghosh
    Commented Oct 8, 2021 at 2:34
  • $\begingroup$ The issue for the contact process is that, while the final reaction describes the overall result (water plus sulfur trioxide -> sulfuric acid) the constraints of a practical industrial process mean that the earlier reactions are a better overall way to achieve the product. It is all about practical chemical engineering not the chemistry. $\endgroup$
    – matt_black
    Commented Oct 8, 2021 at 11:33

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