$\text{Iron pyrites} \xrightarrow{\text{A}} \text{Colourless acidic gas} \xrightarrow{\text{B}} \text{Sulphur trioxide} \xrightarrow{\text{C}} \text{Oleum} \xrightarrow{\text{D}} \text{Sulphuric acid}$

1. Give a balanced equation for the conversion 'A'.
2. The gaseous mixture of the product of conversion 'A' and air contains dust particles as an impurity. Name another impurity in the same mixture.
3. Is the conversion 'B' an exothermic or an endothermic reaction. Would lowering the temperature favour or retard the forward reaction.
4. If the product of conversion 'B' is an acid anhydride of H₂SO₄, the anhydride of conversion 'A' is …………… .
5. State why water is added for the conversion 'D' and not for the conversion 'C'

1. 4FeS₂ + 11O₂ ⟶ 2Fe₂O₃ + 8SO₂
2. Arsenious oxide (As₄O₆)
3. The catalytic oxidation of SO₂ to SO₃ is an exothermic reaction. According to Le Chatelier's principle, higher yield of the product is obtained by lowering the temperature.
4. acid anhydride of H₂SO₃ (Sulphurous acid)
5. Sulphur trioxide is not directly absorbed in water to give sulphuric acid as the reaction is highly exothermic resulting in production of a dense fog of sulphuric acid particles which do not condense easily.
   Hence, sulphur trioxide vapours are dissolved in conc. sulphuric acid to give oleum which on dilution with the requisite amount of soft water in the dilution tank gives sulphuric acid of the desired concentration.