How we make fertilizer from air
Haber–Bosch ammonia synthesis · Synthesis · Equilibrium · Exothermic
Roughly half the nitrogen atoms in your body passed through an ammonia factory first.
Reactants approach
Drag to rotate
ΔH = -92 kJ · exothermic, releases heat
ΔH per mol N2.
Meet the molecules
- Reactantgas
N2
nitrogen
- Reactantgas
H2
hydrogen
- Productgas
NH3
ammonia
What you notice
Plants need nitrogen to make proteins and DNA, and although air is 78% nitrogen, the N≡N triple bond is so strong that most organisms cannot use it. Before 1913, farmers relied on manure, guano and mined saltpetre. Fritz Haber and Carl Bosch found a way to combine nitrogen and hydrogen into ammonia on an industrial scale. Today about 180 million tonnes of ammonia are made each year, mostly for fertilizer, and roughly half the nitrogen in your body passed through a Haber–Bosch plant. The process uses about 1–2% of the world's energy.
Try it
Read the label on a bag of garden fertilizer: the first number in the N-P-K code is the percentage of nitrogen, most of it made from Haber–Bosch ammonia.
Safety: Ammonia gas is toxic and corrosive to the eyes and lungs, so farm ammonia tanks have strict handling rules.
Step by step
- 1
Gases meet iron
N2 and H2 are passed over an iron catalyst, promoted with potassium and aluminium oxides, at about 400–450 °C and 150–250 atm.
- 2
Splitting the triple bond
N2 adsorbs on the iron and its triple bond breaks; this is the slowest step. H2 also splits into H atoms on the surface.
- 3
Building NH3
Hydrogen atoms add one at a time to each surface nitrogen, through NH and NH2, until NH3 forms and leaves the surface.
- 4
A compromise with equilibrium
The forward reaction is exothermic and turns 4 mol of gas into 2, so Le Chatelier's principle favours low temperature and high pressure. Low temperature is too slow, so plants accept about 15% conversion per pass and recycle the unreacted gas.