Q.Compounds of nitrogen, phosphorus and sulphur such as Ammonia, phosphoric acid and sulphuric acid are used in fertilizer industry.

Ammonia is made industrially by the catalytic, high-pressure Haber process; phosphoric acid can be obtained by heat-induced disproportionation of orthophosphorous acid; and sulphuric acid is manufactured by the Contact process via catalytic oxidation of SO2 to SO3 and absorption as oleum.

a) Haber process for ammonia

N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92.4 kJ mol⁻¹ (exothermic, reversible)

A 1:3 mixture of nitrogen and hydrogen gases is compressed to about 200 atm pressure and passed over a finely divided iron catalyst promoted with a small amount of molybdenum (or Al2O3/K2O as promoters), maintained at about 450°C. Since the forward reaction is exothermic and proceeds with decrease in moles, high pressure and moderately low temperature favour the yield (as per Le Chatelier's principle), while the catalyst is used to increase the rate at a temperature where the equilibrium yield is still reasonable. The ammonia formed is liquefied and removed, and the unreacted N2/H2 gases are recycled back into the reactor.

b) H3PO4 from H3PO3

Orthophosphorous acid (H3PO3) is a diprotic acid that disproportionates on heating (in absence of air) to give orthophosphoric acid and phosphine:

4H3PO3 →(heat) 3H3PO4 + PH3

c) Contact process for sulphuric acid

Step 1 — Burning of sulphur (or roasting of sulphide ore) to obtain sulphur dioxide:

S(s) + O2(g) → SO2(g)

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