Concept understanding — Ammonia and Nitric Acid Preparation
Ammonia and Nitric Acid: From Air to Fertiliser to Explosives
You already know that plants need nitrogen to grow, but the air around us — 78% nitrogen gas — is useless to them in that form. The nitrogen molecule is incredibly stable; two nitrogen atoms triple-bonded together, N≡N, simply won't react with anything mild. To make nitrogen useful, we must first break that bond and force it to combine with other elements. That is the entire point of ammonia and nitric acid production: we take inert atmospheric nitrogen and turn it into reactive chemical forms that can feed the world or, in the case of nitric acid, make explosives and fertilisers.
Ammonia by the Haber Process
The Haber process is the industrial method for fixing nitrogen — turning N2 from the air into ammonia, NH3. The reaction is deceptively simple:
N2(g)+3H2(g)⇌2NH3(g)ΔH=−92.4kJ/mol
The hydrogen comes from natural gas (methane) via steam reforming, and the nitrogen is obtained by fractional distillation of liquid air. The challenge is that this reaction is reversible and exothermic — it releases heat. Le Chatelier's principle tells us that low temperature would favour ammonia formation (since the forward reaction is exothermic), but low temperature also makes the reaction painfully slow because breaking that N≡N triple bond requires a huge activation energy.
The solution is a compromise: a temperature around 450–500°C, a very high pressure of 150–200 atm, and an iron catalyst (with promoters like K2O and Al2O3) to speed things up. Under these conditions, about 15–20% of the reactants convert to ammonia per pass. The unreacted gases are recycled, so overall conversion is nearly 100%.
Important
Key conditions for Haber process: 450–500°C, 150–200 atm, iron catalyst. High pressure favours product (fewer gas molecules on right), moderate temperature balances rate and yield.
The ammonia is removed by cooling the gas mixture — ammonia liquefies at about -33°C at atmospheric pressure, while nitrogen and hydrogen remain gases — and the unreacted gases are pumped back into the reactor.
Nitric Acid by the Ostwald Process
Now take that ammonia and oxidise it. That is the Ostwald process: burning ammonia in air over a platinum-rhodium catalyst to produce nitric oxide, which then goes through a series of spontaneous reactions to become nitric acid.
The first step is the catalytic oxidation of ammonia:
4NH3(g)+5O2(g)Pt/Rh,900∘C4NO(g)+6H2O(g)
This is done at about 900°C and just above atmospheric pressure. The hot gases are then cooled rapidly to prevent the NO from decomposing back. The nitric oxide then reacts with more oxygen from the air to form nitrogen dioxide:
2NO(g)+O2(g)→2NO2(g)
This happens spontaneously as the gases cool. Finally, the nitrogen dioxide is absorbed in water to give nitric acid:
3NO2(g)+H2O(l)→2HNO3(aq)+NO(g)
Notice that NO is regenerated in this last step — it gets recycled back to be oxidised again. The overall reaction from ammonia to nitric acid is: …
This question covers two things: the industrial principle behind Haber's ammonia synthesis, and the distinct colour change ammonia produces when added to copper sulphate solution. …
Haber's process makes NH3 from N2 + 3H2 (⇌, exothermic) at ~200 atm, ~450–500 °C over Fe/Mo catalyst. NH3 turns CuSO4 solution from blue precipitate of Cu(OH)2 to a deep-blue [Cu(NH3)4]2+ solution.
Principle of manufacture (Haber's process):
Ammonia is manufactured by combining nitrogen (from air) and hydrogen (from water gas/natural gas) directly:
N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92.4 kJ
The reaction is reversible, exothermic and proceeds with a decrease in the number of gaseous moles (4 volumes → 2 volumes). Applying Le Chatelier's principle, the forward reaction (more NH3) is favoured by:
High pressure: about 200 atm (fewer moles on the product side).
Low temperature is thermodynamically favourable, but too low a temperature makes the rate very slow; hence a moderate/optimum temperature of about 450–500 °C is used.
Catalyst: finely divided iron with molybdenum (or K2O/Al2O3) as a promoter speeds up the attainment of equilibrium.
Reaction of ammonia with copper sulphate solution:
Ammonia is basic and reacts with CuSO4 solution. Initially a pale blue precipitate of copper hydroxide is formed:
CuSO4 + 2NH4OH → Cu(OH)2(down arrow, pale blue) + (NH4)2SO4
Q.Which of the following catalysts is used in the manufacture of ammonia by Haber's process?
(a) Al2O3
(b) Fe + Mo
(c) CuO
(d) Pt
›Reveal solutionSolution
Ammonia is manufactured by the Haber process using finely divided iron as catalyst and molybdenum as a promoter.
The Haber process synthesises ammonia from its elements:
N2(g) + 3H2(g) <=> 2NH3(g) (exothermic)
Optimum conditions are about 200 atm pressure and 700 K temperature. The reaction is very slow without a catalyst, so finely divided iron is used to speed it up. A small amount of molybdenum is added as a promoter to increase the activity and life of the iron catalyst.