Q.(i) Write the chemical reaction when PCl5 is strongly heated.

PCl5 dissociates on strong heating; interhalogens are compounds of two different halogens; the Contact process converts SO2 to SO3 catalytically and absorbs it as oleum to manufacture H2SO4.

1. PCl5 on strong heating: PCl5(s) --Δ (strong heat)--> PCl3(l) + Cl2(g) PCl5 dissociates reversibly into phosphorus trichloride and chlorine gas on strong heating.
2. Interhalogen compounds: these are compounds formed by the combination of two different halogen atoms - one halogen (the larger, more electropositive one) combines with one, three, five or seven atoms of a smaller halogen, giving general formulas XX', XX'3, XX'5, XX'7. Examples: ClF (chlorine monofluoride), BrF3 (bromine trifluoride), ICl3 (iodine trichloride), IF7 (iodine heptafluoride).
3. Contact process for manufacture of H2SO4: Step 1 - Burning of sulphur/sulphide ore in air to get SO2: S(s) + O2(g) -> SO2(g) (or roasting iron pyrites: 4FeS2 + 11O2 -> 2Fe2O3 + 8SO2) Step 2 - Purification of SO2: the SO2 gas is purified by removing dust and other impurities (e.g. arsenic compounds). Step 3 - Catalytic oxidation of SO2 to SO3: purified SO2, mixed with excess air/O2, is passed over a V2O5 (vanadium pentoxide) catalyst at about 720 K and 2 bar pressure: 2SO2(g) + O2(g) <=>(V2O5, 720K, 2bar) 2SO3(g) (exothermic, reversible) …