Q.Write the important reactions involved in the manufacture of sulphuric acid by 'contact process'. OR Write the preparation of the compounds XeF2 and XeF4. Give their structures.

The contact process manufactures sulphuric acid in three main steps: burning sulphur to SO2, catalytically oxidising SO2 to SO3, and converting SO3 to H2SO4 via oleum.

Manufacture of sulphuric acid by the contact process:

Step 1 — Burning of sulphur/ore to produce SO2:

Sulphur, or a sulphide ore (such as iron pyrites, FeS2), is burnt in a furnace with dry air to produce sulphur dioxide gas:

$S(s) + O_2(g) \rightarrow SO_2(g)$

or for iron pyrites:

$4FeS_2 + 11O_2 \rightarrow 2Fe_2O_3 + 8SO_2$

The SO2 gas obtained is purified (dust removed) and dried.

Step 2 — Catalytic oxidation of SO2 to SO3:

The purified and dried SO2 gas, mixed with excess air (O2), is passed over a catalyst, vanadium(V) oxide, $V_2O_5$, at about 720 K and a pressure of about 2 atm. This is a reversible, exothermic reaction:

$2SO_2(g) + O_2(g) \underset{V_2O_5,\ 720\,K,\ 2\,atm}{\rightleftharpoons} 2SO_3(g) \quad \Delta H < 0$

The conditions (moderate temperature, pressure, and excess air) are chosen according to Le Chatelier's principle to maximise the yield of SO3, achieving about 96–98% conversion.

Step 3 — Absorption of SO3 and formation of H2SO4: …