How is ammonia manufactured by Haber's process ? What happens when ammonia solution is passed through Nessler's reagent ? [4+1]

N₂+3H₂Fe (catalyst), Mo₂O₃ (promoter), 400–500°C, 200 atmIndustrial ammonia synthesis (Le Chatelier’s principle appli2NH₃

Manufacture of Ammonia by Haber’s Process

Ammonia ($\mathrm{NH}{}_3$) is industrially produced by the Haber process, a direct combination of nitrogen and hydrogen gases under high pressure and temperature.

Key Conditions and Catalysts

• Reactants: Nitrogen ($\mathrm{N}{}_2$) from air and hydrogen ($\mathrm{H}{}_2$) from natural gas or water gas.
• Catalyst: Finely divided iron (Fe) with molybdenum oxide (Mo₂O₃) as a promoter to increase efficiency.
• Temperature: 400–500°C (compromise between rate and equilibrium yield; exothermic reaction favors lower temperatures).
• Pressure: 200 atm (high pressure shifts equilibrium toward product formation by Le Chatelier’s principle).
• Equilibrium Reaction: $$\mathrm{N}{}_2(\mathrm{g})+3\mathrm{H}{}_2(\mathrm{g})\rightleftharpoons 2\mathrm{NH}{}_3(\mathrm{g})\mathrm{\Delta H}=-92\text{ kJ mol}{}^{-1}$$ The reaction is exothermic, so low temperature favors ammonia formation, but a catalyst is needed to achieve a reasonable rate.

Process Steps

1. Purification: Impurities like $\mathrm{O}{}_2$ and $\mathrm{CO}$ are removed to prevent catalyst poisoning.
2. Compression: Gases are compressed to 200 atm in a multi-stage compressor.
3. Catalytic Conversion: The gas mixture passes over the Fe catalyst at 400–500°C, producing ammonia.
4. Separation: Ammonia (boiling point: –33°C) is liquefied and separated from unreacted $\mathrm{N}{}_2$ and $\mathrm{H}{}_2$, which are recycled.

Economic Considerations

• Yield: Only 15–20% conversion per pass; unreacted gases are recycled.
• Energy Efficiency: Heat from the exothermic reaction is reused to preheat incoming gases.

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Reaction with Nessler’s Reagent

When ammonia solution is passed through Nessler’s reagent ($\mathrm{K}{}_2[\mathrm{HgI}{}_4]+\mathrm{KOH}$), a brown precipitate or coloration is observed due to the formation of iodomercurate(II) complex: $$2\mathrm{K}{}_2[\mathrm{HgI}{}_4]+\mathrm{NH}{}_3+3\mathrm{KOH}\to [\mathrm{Hg}{}_2\mathrm{N}]\mathrm{I}\cdot \mathrm{H}{}_2\mathrm{O}(\mathrm{brown})+7\mathrm{KI}+2\mathrm{H}{}_2\mathrm{O}$$ This reaction is quantitative and used in analytical chemistry to detect ammonia (even in trace amounts).

NH₃+K₂[HgI₄]Alkaline mediumHgO·Hg(NH₂)I+2KI

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Note: The brown color’s intensity is proportional to ammonia concentration, making Nessler’s reagent useful for quantitative analysis (e.g., in water quality testing).