Undergraduate · 20 min

The Haber–Bosch process: ammonia from air

Compress $\ce{N2}$ and $\ce{H2}$ over an iron catalyst, condense the ammonia and recycle the unreacted gas.

Goal

Explain how pressure, temperature, catalyst and recycling work together to fix nitrogen at industrial scale.

Apparatus and reagents

Virtual synthesis loop: $\ce{N2}$ / $\ce{H2}$ feeds, compressor, promoted iron catalyst bed, condenser, recycle.

Procedure

Name the five units as the gas flows: feed, compressor, converter, condenser, recycle.
Apply Le Chatelier to $\ce{N2 + 3H2 <=> 2NH3}$ ( $\Delta H < 0$ ): why high pressure and why moderate — not minimal — temperature?
Follow the recycle: with ≈15 % conversion per pass, what happens to the unreacted $\ce{N2}$ / $\ce{H2}$ ?
Discuss why condensing $\ce{NH3}$ out of the loop both harvests the product and shifts the equilibrium forward.

What to observe

The equilibrium $\ce{N2 + 3H2 <=> 2NH3}$ releases heat and shrinks the gas volume — so high P and moderate T are chosen.
Because conversion per pass is limited, the membrane/recycle stage sends unreacted gas back — nothing is wasted.

Explanation

Ammonia synthesis is an exothermic, mole-reducing equilibrium: 4 gas moles become 2. High pressure (~200 bar) favours the product, but lowering temperature too far slows the rate, so ~450 °C over a promoted iron catalyst is the industrial compromise. Removing NH₃ by condensation and recycling the feed pushes overall conversion far above the single-pass value — a masterpiece of reaction engineering that feeds the world via fertilisers.

Chemists behind it

Fritz Haber Carl Bosch