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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:

  1. Both the reaction between sulphur trioxide with concentrated sulphuric acid and that between oleum and water are exothermic
  2. If the reaction is too exothermic, a coolant can be applied in the industrial process

So, I propose two possible reasons.

  1. The two previously mentioned are less exothermic
  2. 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?

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The explanation is kinetics. $\ce{SO3}$ reacts with water producing fumes of sulfuric acid. On the contrary it is dissolved into sulfuric acid without fumes, producing oleum. This is why sulfuric acid is used in practice instead of water for absorbing $\ce{SO3}$ produced in the contact process.

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    $\begingroup$ AFAIK, SO3 reacts with water rather violently and forming thick fumes, while it reacts with conc. H2SO4 relatively peacefully, forming oleum, a mixture of mainly H2SO4 and H2S2O7. // Two step production of conc. H2SO4 is similar to preferred two step dilution of H2SO4: Mixing remaining diluted H2SO4 with water and after cooling, mixing conc. H2SO4 with the prior mixture. The exothermic dilution is then divided in 2 steps, easier to manage. $\endgroup$
    – Poutnik
    Sep 7, 2023 at 16:27

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