Q.The question has choice. Answer only one question. Some elements in p-block shows allotropy.

Sulphur exists as rhombic and monoclinic allotropes; sulphuric acid is manufactured industrially by the Contact process via catalytic oxidation of SO2 and absorption as oleum; and interhalogen compounds are covalent compounds formed between two different halogens.

a) Allotropic forms of sulphur

The two well-known crystalline allotropes of sulphur are:

• Rhombic sulphur (α-sulphur) — the stable form below 369 K, yellow, made of S8 puckered-ring molecules.
• Monoclinic sulphur (β-sulphur) — stable above 369 K (obtained by melting rhombic sulphur and cooling slowly), also made of S8 rings but packed differently.

b) (i) Manufacture of sulphuric acid — Contact process

1. Burning of sulphur (or roasting of iron pyrites) in air to obtain sulphur dioxide: S(s) + O2(g) → SO2(g)

2. Purification of the SO2 gas to remove dust and arsenic impurities that would poison the catalyst.

3. Catalytic oxidation: the purified SO2, mixed with excess air, is passed over a V2O5 (vanadium pentoxide) catalyst at about 720 K and 2 atm pressure: 2SO2(g) + O2(g) ⇌(V2O5) 2SO3(g)

4. Absorption: the SO3 is absorbed in concentrated H2SO4 (not directly in water, to avoid forming a difficult-to-condense acid mist/fog) to give oleum (pyrosulphuric acid): SO3 + H2SO4 → H2S2O7 (oleum)

5. Dilution: the oleum is diluted with the required amount of water to obtain sulphuric acid of the desired strength: H2S2O7 + H2O → 2H2SO4

b) (ii) Interhalogen compounds

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