Sulfuric Acid

Absorption

\[\ce{SO3(g)} + \ce{H2O(l)} \rightarrow \ce{H2SO4(l)} \qquad \text{ $\Delta$H = -130.4 kJ/mol}\] Since the heat of reaction the above reaction between and producing sulfuric acid is exothermic, it will lead to the formation of acid mist. This mist is difficult to contain.

Instead of the above single step reaction, it is usually carried out in steps: forming oleum by absorption with concentrated sulfuric acid, followed by diluting it with water to get sulfuric acid.