Chemical changes · GCSE Chemistry
The Haber process
GCSE Chemistry revision on the Haber process: N₂ + 3H₂ ⇌ 2NH₃, iron catalyst, 450 °C and 200 atmospheres as compromise conditions, recycling, and ammonia for fertilisers.
N₂ + 3H₂ ⇌ 2NH₃, exothermic forward, iron catalyst. 450 °C and about 200 atm are a compromise: yield versus rate versus cost. Cool the ammonia out as a liquid and recycle the unreacted gases.
The important bits
What you need to know
- 1
The Haber process makes ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. Nitrogen comes from the air; hydrogen is usually made from natural gas (methane) and steam.
- 2
Typical conditions: about 450 °C, about 200 atmospheres (20 MPa), and an iron catalyst. Unreacted nitrogen and hydrogen are recycled over the catalyst.
- 3
Ammonia is cooled so that it liquefies and is removed. Removing the product shifts the equilibrium to the right (Higher: Le Chatelier), increasing the amount of ammonia obtained from each pass in effect.
- 4
A high pressure favours the side with fewer gas molecules (4 volumes of gas → 2), so equilibrium yield of ammonia rises. High pressure also increases rate. Plant cost and safety limit how high you go — hence ~200 atm.
- 5
A low temperature favours the exothermic forward reaction, so equilibrium yield would be higher, but rate would be too slow. 450 °C is a compromise: a reasonable yield in a short time.
- 6
The iron catalyst increases rate by providing a lower activation-energy pathway. It does not change the equilibrium yield. It lets the mixture reach equilibrium faster so the plant can recycle sooner.
- 7
Ammonia is used to make fertilisers (ammonium nitrate, ammonium sulfate), and also nitric acid and some explosives. Food production on the current scale depends on Haber ammonia.
- 8
Evaluation questions want rate, yield and cost together. The chosen conditions are not the theoretical maximum yield; they are the economically useful set.
Quotations worth analysing
Short evidence. Real method.
“N₂ + 3H₂ ⇌ 2NH₃ iron catalyst, 450 °C, 200 atm”
Learn the equation with the reversible arrow, the mole ratio 1:3:2, the catalyst, and the two numbers. Missing the reversible sign loses a mark.
“The temperature is a compromise between yield and rate.”
Low temperature: higher yield (exothermic forward) but slow. High temperature: fast but lower yield. 450 °C sits in the middle. Do not say 450 °C “gives the best yield”.
“The catalyst does not change the yield of ammonia; it increases the rate of reaching equilibrium.”
This sentence separates Haber from a rates-only answer. Iron is not there to “produce more ammonia” in the equilibrium sense.
Go deeper
Compromise means you can name what you sacrificed
If you only wanted yield, you would run cold and at extremely high pressure, and wait. Industry cannot wait, and ultra-high-pressure vessels are expensive and dangerous. So temperature is raised until the rate is acceptable, even though the equilibrium mixture then contains less ammonia. Pressure is raised until the extra yield and rate justify the engineering cost, commonly quoted as 200 atmospheres. The catalyst restores rate without needing an even higher temperature. Cooling ammonia out of the mixture, then recycling N₂ and H₂, means unused reactants are not wasted. A six-mark answer that lists 450 °C and 200 atm without “compromise”, “yield”, “rate” and “recycle” is under-selling the topic.
Go deeper
Count gas molecules before you quote Le Chatelier
Left: 1 + 3 = 4 gas molecules. Right: 2. Increasing pressure shifts equilibrium to the right, more NH₃. Decreasing pressure would favour N₂ and H₂. Temperature: forward exothermic, so raising temperature shifts left, less NH₃ at equilibrium, but every collision is more likely to succeed, so rate up. Concentration: adding more H₂ or N₂ shifts right; removing NH₃ (by liquefying it) also shifts right. Catalyst: both directions faster, position unchanged. If a question does not mention closed systems or equilibrium, you can still describe Haber as a reversible reaction with those conditions. On Higher papers, use Le Chatelier by name and state the opposing change.
Go deeper
Fertiliser is the reason the plant exists
Ammonia itself is a gas, alkaline in water, and not spread on fields as the pure gas. It is converted into solids such as ammonium nitrate that supply nitrogen to plants. Without this industrial nitrogen fixation, yields of wheat and maize would collapse relative to modern agriculture. That is the social and economic evaluation: high pressure plants cost money and energy (often from fossil methane for hydrogen), but they feed billions. A life-cycle or atmosphere-linked question may note that making hydrogen from methane produces carbon dioxide, and that excess fertiliser can pollute water. Stay specific: name ammonia, NPK fertilisers, and one cost or environmental drawback of the process conditions.
See the idea in action
Explain why 450 °C is used rather than 100 °C or 800 °C. Forward reaction exothermic, so 100 °C would give a higher equilibrium yield of NH₃, but the rate would be too low for a commercial plant. 800 °C would be fast, but the equilibrium would lie further left and the yield of ammonia would be poor. 450 °C with an iron catalyst gives a moderate yield quickly. Pressure is kept high (~200 atm) because that favours the side with fewer gas molecules and also speeds the reaction. Liquefied NH₃ is removed and N₂/H₂ are recycled.
Exam technique
Turn knowledge into marks
Write the reversible equation first. Then treat temperature, pressure and catalyst as three separate sentences: effect on yield, effect on rate, and the compromise. Mention recycling and liquefying ammonia. Never say the catalyst increases yield.
Common mistakes
Do not give these marks away
- 01
Saying the iron catalyst increases the equilibrium yield of ammonia.
- 02
Claiming 450 °C is used because it gives the maximum possible yield.
- 03
Forgetting that four gas molecules become two, so high pressure favours ammonia.
Why is an iron catalyst used in the Haber process?
AIt increases the equilibrium yield of ammonia
BIt provides an alternative pathway with a lower activation energy so equilibrium is reached faster
CIt makes the forward reaction endothermic
DIt removes ammonia from the mixture
Show the answer
It provides an alternative pathway with a lower activation energy so equilibrium is reached faster. A catalyst changes rate, not the position of equilibrium. Iron lets the plant get to equilibrium quickly; temperature and pressure control the yield.
Quick questions
If this is the bit you searched
What is the Haber process equation and conditions?
N₂ + 3H₂ ⇌ 2NH₃, iron catalyst, about 450 °C and 200 atmospheres. Nitrogen from air, hydrogen from natural gas, ammonia cooled and unreacted gases recycled.
Why is 450 °C a compromise in the Haber process?
The forward reaction is exothermic, so a low temperature would give a higher yield but a slow rate. 450 °C gives a worthwhile yield in a short time.
Why is high pressure used in the Haber process?
There are fewer gas molecules on the product side (2 versus 4), so high pressure increases the equilibrium yield of ammonia. It also increases rate. Cost and safety limit the pressure to about 200 atm.
What is ammonia from the Haber process used for?
Making nitrogen fertilisers such as ammonium nitrate, and also nitric acid. Modern large-scale food production depends on this ammonia.