NCEA Level 3 · Year 13 · Chemistry · 2 minutes
The Haber process: bread from air
Fewer moles of gas, an exothermic reaction, a catalyst — and a compromise that feeds billions.
Key terms
- Dynamic equilibrium
- The state of a reversible reaction where the forward and reverse reactions run at the same rate.
- Le Chatelier's principle
- When an equilibrium is disturbed, it shifts to oppose the change.
- Exothermic
- A reaction that releases heat, so a lower temperature shifts the equilibrium towards the products.
- Catalyst
- A substance that speeds up the forward and reverse reactions equally, so equilibrium is reached faster but the yield does not change.
- Equilibrium constant (K)
- A value describing the balance of products and reactants at equilibrium, which increases when a lower temperature favours ammonia.
- Ammonia
- NH3, the compound made from nitrogen and hydrogen in the Haber process and used to make fertiliser.
Transcript
In eighteen ninety eight, a famous British chemist warned that the world would soon run short of food. The missing ingredient? Nitrogen. Strange, since air is about seventy eight per cent nitrogen.
But nitrogen gas, N two, has a triple bond, one of the strongest there is, so most plants can't use it. Farms relied on manure, and nitrate dug up in Chile.
In nineteen oh nine, German chemist Fritz Haber found a way: nitrogen and hydrogen, squeezed together over a catalyst, made ammonia. Engineer Carl Bosch scaled it up, and the first factory opened in nineteen thirteen.
Here's the reaction: N two plus three H two makes two N H three. See the double arrow? It's reversible, so it reaches dynamic equilibrium, with the forward and reverse reactions running at the same rate.
So how do you get more ammonia? Le Chatelier's principle: disturb an equilibrium, and it shifts to oppose the change. Four moles of gas on the left, only two on the right, so high pressure shifts it right. But pressure costs money, so plants settle for about two hundred atmospheres.
Temperature is the trickier one. Making ammonia is exothermic, so a lower temperature shifts the equilibrium right, and increases the equilibrium constant, K. But cold reactions are painfully slow. The compromise: around four hundred and fifty degrees Celsius.
And the iron catalyst? It speeds up the forward and reverse reactions equally, so equilibrium arrives faster, but the yield doesn't change.
The payoff was enormous. Today, roughly half the people on Earth are fed by crops grown with nitrogen fertiliser made this way. Bread, quite literally, from air.
But there's a dark side. In the First World War, the process supplied nitrates for Germany's explosives, and Haber led its poison gas program. He still won the Nobel Prize in Chemistry for nineteen eighteen.
So, for your exam: fewer moles of gas on the right, so high pressure favours ammonia. Exothermic forward reaction, so low temperature favours ammonia, but it's slow. And the catalyst gets you to equilibrium faster, without changing the yield.
Sources
Every fact in the video, and where it comes from.
- 1archive.spectator.co.uk/article/10th-september-1898/3/sir-william-croo…
- In 1898 Sir William Crookes, as President of the British Association, warned that world wheat supplies would fall short as population grew unless chemists found a way to fix nitrogen from the air.
- 2newsletter.rootsofprogress.org/p/solutionism-part-1
- In 1898 Sir William Crookes, as President of the British Association, warned that world wheat supplies would fall short as population grew unless chemists found a way to fix nitrogen from the air.
- 3britannica.com/science/air
- Air is about 78% nitrogen by volume.
- 4britannica.com/science/nitrogen-fixation
- Nitrogen gas (N2) has a very strong triple bond, which makes it unreactive; most plants cannot use it directly.
- 5sciencehistory.org/education/scientific-biographies/fritz-haber
- Before synthetic ammonia, nitrogen fertiliser came largely from manure and from sodium nitrate (saltpetre) mined in Chile.
- In the First World War the process supplied Germany with ammonia for nitric acid and nitrates for explosives (and fertiliser) after the Allied blockade cut off Chilean nitrate; Haber developed poison gas and supervised the first chlorine attack at Ypres in 1915.
- 6en.wikipedia.org/wiki/Haber_process
- In 1909 Fritz Haber (with Robert Le Rossignol) demonstrated a laboratory apparatus producing ammonia from nitrogen and hydrogen at high pressure over a catalyst; Carl Bosch led BASF's scale-up; the first plant opened at Oppau in 1913.
- In the First World War the process supplied Germany with ammonia for nitric acid and nitrates for explosives (and fertiliser) after the Allied blockade cut off Chilean nitrate; Haber developed poison gas and supervised the first chlorine attack at Ypres in 1915.
- 7basf.com/global/en/who-we-are/history/chronology/1902-1…
- In 1909 Fritz Haber (with Robert Le Rossignol) demonstrated a laboratory apparatus producing ammonia from nitrogen and hydrogen at high pressure over a catalyst; Carl Bosch led BASF's scale-up; the first plant opened at Oppau in 1913.
- 8chemguide.co.uk/physical/equilibria/haber.html
- N2 + 3H2 ⇌ 2NH3 is reversible and the forward reaction is exothermic (ΔH ≈ −92 kJ per mole of N2).
- Four molecules of gas on the left, two on the right: higher pressure shifts the equilibrium towards ammonia; plants use about 200 atmospheres as an economic compromise (very high pressure needs costly, strong equipment).
- Lower temperature gives a higher equilibrium yield of ammonia (exothermic forward reaction) but a slower rate; 400–450 °C is a compromise.
- An iron catalyst speeds up the forward and reverse reactions equally; it has no effect on the position of equilibrium, so equilibrium is reached faster. About 15% conversion per pass; unreacted gases are recycled (about 98% overall).
- 9chemguide.co.uk/physical/equilibria/introduction.html
- At dynamic equilibrium the forward and reverse reactions continue at equal rates, so concentrations stay constant.
- 10chemguide.co.uk/physical/equilibria/lechatelier.html
- Le Chatelier's principle: if a system at equilibrium is disturbed, the position of equilibrium shifts to counteract the change.
- 11chemguide.co.uk/physical/equilibria/kchange.html
- For an exothermic reaction, the equilibrium constant K increases as temperature decreases; changing pressure or adding a catalyst does not change K.
- 12ourworldindata.org/how-many-people-does-synthetic-fertilizer-feed
- Synthetic nitrogen fertiliser supports roughly half of the world's population (Erisman et al. 2008 estimate 48%).
- 13nobelprize.org/prizes/chemistry/1918/summary
- Haber received the Nobel Prize in Chemistry for 1918 for the synthesis of ammonia from its elements; Bosch shared the 1931 prize.
Image and sound credits
- Sir William Crookes: unidentified photographer, public domain, via Wikimedia Commons
- Harvesting wheat, Canterbury, 1933; wheat stooks; gas masks, 1915 and 1916: Auckland Libraries Heritage Collections, no known copyright restrictions
- Fritz Haber and Carl Bosch portraits: Nobel Foundation, public domain, via Wikimedia Commons
- Replica of Haber's ammonia apparatus, 1911: BASF, public domain, via Wikimedia Commons
- Ammonia tanks, Oppau, 1927: Nationaal Archief / Spaarnestad, public domain, via Wikimedia Commons
- Ammonia plant, Kapuni, Taranaki: Archives New Zealand Te Rua Mahara o te Kāwanatanga, CC BY
- New Zealand Field Bakery, Rouen: Alexander Turnbull Library, no known copyright restrictions
- Fritz Haber's Nobel diploma, 1918: public domain, via Wikimedia Commons
- Map data: Natural Earth (public domain)
- Molecules, equilibrium graphs and charts: drawn in code
- Paper textures: ambientCG (CC0)
- Sound effects: Freesound contributors (CC0)
- Music: generated with ACE-Step · Voice: Gemini TTS
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