The Synthesis of Ammonia from its Elements
Forcing air's inert nitrogen into ammonia made fertiliser from the sky — feeding billions, and arming a war.
The air is four-fifths nitrogen, yet crops can starve for it. Haber found a way to grab that nitrogen out of the sky.
The big idea
Every plant — and so every animal and person — needs nitrogen to build proteins and DNA. The strange thing is that we are swimming in it: nitrogen makes up about 78% of the air. But that air-nitrogen, N₂, is locked up in a molecule so stable that almost nothing alive can pry it apart and use it.
Fritz Haber found a way to force it open. Under great pressure and heat, with the help of a metal that speeds the reaction along, nitrogen from the air joins with hydrogen to make ammonia — the raw material of fertiliser. For the first time, the nitrogen plants need could be manufactured from thin air, instead of dug out of the ground.
How it came about
By 1900 the world had a quiet crisis brewing. Crops were fertilised mainly with nitrate mined in Chile and seabird droppings, and the chemist William Crookes had warned that these were running out — that the growing population might outstrip its food. Whoever could pull nitrogen from the air would feed the world.
Around 1909 Haber, in Karlsruhe, built a small high-pressure apparatus that did exactly that, producing a steady trickle of ammonia. The chemical company BASF bought the idea, and there the engineer Carl Bosch performed a second miracle: he scaled a delicate bench reaction up into giant steel vessels that could survive the crushing pressures, and a colleague found a cheap iron catalyst to replace Haber's rare metals. The first factory opened in 1913. But the story has a dark second half: Haber threw himself into Germany's war effort, becoming the father of poison-gas warfare, and the same reaction that made fertiliser also made the nitrates for explosives. He won the Nobel Prize in 1918 to international outrage, and — himself Jewish — was later driven out of Germany by the Nazis.
Why it mattered
It is hard to name a single chemical reaction that changed more lives. By making fertiliser cheap and abundant, Haber–Bosch let farms feed billions of extra people; a common estimate is that around half the people alive today are fed thanks to it. And the very same power over nitrogen armed a century of war. It is the clearest case we have of a discovery that is both indispensable and dangerous.
A way to picture it
Imagine two dancers who are perfectly happy alone and refuse to take a partner — that's nitrogen's tight N₂ pair. To get them to swap partners with hydrogen, you crowd the dance floor (high pressure pushes them together) and you hire a matchmaker who introduces them without being used up (the catalyst). Heat speeds everyone up, but too much heat and the new couples split again — so you settle on a careful middle temperature. Crowd them, warm them just enough, and keep recycling the wallflowers, and a steady stream of new pairs — ammonia — walks off the floor.
Where it sits
This is industrial chemistry's first great triumph, standing on the equilibrium ideas of Gibbs and on Le Chatelier's principle, and on the atomic chemistry of Dalton, Avogadro and Lewis already in this Library. It answered the nitrogen-famine fear of its day — and it set the template for the high-pressure catalytic processes that the twentieth-century chemical industry was built on.
Gaseous nitrogen combines with gaseous hydrogen in simple quantitative proportions to produce gaseous ammonia.