ChemistryICSE Class 10 · Ammonia

Haber's Process: Manufacture of Ammonia (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

In Haber's process ammonia is made by combining nitrogen and hydrogen in the ratio 1 : 3 by volume: N₂ + 3H₂ ⇌ 2NH₃ + heat. The gases are passed over finely divided iron (catalyst) with molybdenum (promoter) at about 450–500 °C and 200–900 atm. Ammonia is liquefied by cooling and unreacted gases are recycled.

Notice the one-way flow to liquid ammonia and the purple loop sending unreacted nitrogen and hydrogen back to the compressor.

Illustration drawn for QuestionBuddy.

Understand Haber's Process

The reaction. Nitrogen and hydrogen combine directly to form ammonia:

$N_{2} + 3 H_{2} ⇌ 2 N H_{3} + heat$

Three features of this equation explain every condition used in the plant:
1. It is reversible, so the gases never react completely; only part of the mixture becomes ammonia in each pass.
2. It is exothermic (gives out heat).
3. 4 volumes of reacting gases (1 N₂ + 3 H₂) give only 2 volumes of ammonia.

Raw materials. Nitrogen (obtained from liquefied air) and hydrogen, mixed in the ratio 1 : 3 by volume, the ratio in the equation. The gases are purified first, because impurities would poison the catalyst.

Conditions.

Pressure: about 200–900 atmospheres. A high pressure favours the side with fewer gas molecules (2 volumes rather than 4), so more ammonia forms.
Temperature: about 450–500 °C. A lower temperature would give a higher proportion of ammonia (the forward reaction is exothermic) but the reaction would be far too slow. This temperature is a compromise between yield and rate.
Catalyst: finely divided iron, with molybdenum (or alumina) as a promoter that makes the iron catalyst more effective. The catalyst speeds up the reaction but does not change the amount of ammonia at equilibrium.

Separating the ammonia. The gas mixture leaving the catalyst chamber is cooled. Ammonia liquefies much more easily than nitrogen and hydrogen, so it condenses and is drawn off as liquid. The unreacted nitrogen and hydrogen are recycled through the converter, so nothing is wasted.

Why it matters. Most ammonia goes to make fertilisers (ammonium sulphate, ammonium nitrate, urea), and to make nitric acid by Ostwald's process, explosives and cleansing agents; liquid ammonia is also used as a refrigerant that does not deplete the ozone layer.

Calculations. Because all three substances are gases, Gay-Lussac's law gives volume ratios 1 : 3 : 2 directly; mass questions use moles in the usual way.

Haber's Process: formulas and key facts

Haber's process: $N_{2} + 3 H_{2} ⇌ 2 N H_{3} + heat$
Volume ratio (same T and P): $V_{N_{2}} : V_{H_{2}} : V_{N H_{3}} = 1 : 3 : 2$
Conditions: Ratio of nitrogen to hydrogen 1 to 3 by volume
Temperature about 450 to 500 °C
Pressure about 200 to 900 atm
Catalyst finely divided iron
Promoter molybdenum
Separation cooling liquefies ammonia; unreacted gases recycled
Catalysts in the ICSE syllabus: Haber's process iron, with molybdenum as promoter
Catalytic oxidation of ammonia platinum
Contact process vanadium pentoxide
Hydrogenation of ethene nickel

Ratio of nitrogen to hydrogen	1 to 3 by volume
Temperature	about 450 to 500 °C
Pressure	about 200 to 900 atm
Catalyst	finely divided iron
Promoter	molybdenum
Separation	cooling liquefies ammonia; unreacted gases recycled

Haber's process	iron, with molybdenum as promoter
Catalytic oxidation of ammonia	platinum
Contact process	vanadium pentoxide
Hydrogenation of ethene	nickel

How to solve Haber's Process questions

Haber's process questions test the conditions and the reasons for them. Learn each condition with its 'because'.

1Write the balanced, reversible equation with '+ heat' on the right.
2List the conditions: N₂ : H₂ = 1 : 3, about 450–500 °C, about 200–900 atm, iron catalyst with molybdenum promoter.
3Link pressure to volumes: 4 volumes go to 2, so high pressure helps.
4Link temperature to rate and yield: lower is better for yield but too slow, so a moderate temperature with a catalyst.
5Explain separation: cooling liquefies ammonia; unreacted gases are recycled.
6For numericals, use the 1 : 3 : 2 volume ratio or moles.

Haber's Process in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Raw gas needed for a day's output

Suppose a fertiliser plant aims to make 3.4 tonnes (3 400 000 g) of ammonia in a day. Assume all the gas fed in is eventually converted, thanks to recycling. (N = 14, H = 1; volumes at S.T.P.)

What volumes of nitrogen and hydrogen at S.T.P. must be supplied?

Try it first, then show the solutionSolution

Moles of NH₃ $= 3400 000/17 = 200000$ mol.
From $N_{2} + 3 H_{2} ⇌ 2 N H_{3}$ : N₂ $= 100000$ mol; H₂ $= 300000$ mol.
N₂ $= 100000 \times 22.4$ L $= 2240000$ L $= 2240$ m³; H₂ $= 6720$ m³.

Answer: 2240 m³ of nitrogen and 6720 m³ of hydrogen at S.T.P., in the 1 : 3 ratio.

Memory hook: One of nitrogen, three of hydrogen, two of ammonia: the 1-3-2 recipe.

Tricks to remember Haber's Process

Short memory aids. Say each one aloud once, then use it on a question the same day.

Iron Man's Moly-sidekick
Iron is the catalyst (hero); molybdenum is the promoter (sidekick that makes the hero stronger).
Use it when: Catalyst versus promoter.
1-3-2 at 450 and 200
Ratio 1 : 3 gives 2; about 450 °C and at least 200 atm.
Use it when: Recalling the conditions together.
Cool to collect, recycle the rest
Ammonia condenses on cooling; N₂ and H₂ go round again.
Use it when: Separation step.

Haber's Process: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: The promoter · 1 mark · MCQBoard level

In Haber's process, molybdenum is used as:
(a) the catalyst
(b) a promoter
(c) a reactant
(d) a drying agent

Try it first, then show the solutionSolution

Iron is the catalyst.
Molybdenum improves the efficiency of the iron catalyst.

Answer: (b) a promoter.

Example 2: Equation and conditions · 2 marks · Short answerBoard level

Write the balanced equation for Haber's process and state (a) the catalyst, (b) the approximate temperature.

Try it first, then show the solutionSolution

$N_{2} + 3 H_{2} ⇌ 2 N H_{3} + heat$ .
(a) Finely divided iron (with molybdenum as promoter). (b) About 450–500 °C.

Answer: Equation as above; iron catalyst; about 450–500 °C.

Example 3: Reasons for the conditions · 3 marks · Short answerHarder board level

Give a reason for each: (a) a high pressure is used, (b) a temperature of about 450–500 °C is used rather than a lower one, (c) the catalyst is finely divided.

Try it first, then show the solutionSolution

(a) 4 volumes of reactants give 2 volumes of ammonia; high pressure favours the side with fewer molecules, increasing the yield.
(b) The reaction is exothermic, so lower temperatures give more ammonia, but the rate would be too slow; 450–500 °C is a compromise.
(c) A fine powder has a large surface area, so the reaction on its surface is faster.

Answer: (a) Fewer gas molecules on the product side. (b) Compromise between yield and rate. (c) Larger surface area.

Example 4: Limiting gas · 3 marks · Short answerHarder board level

50 L of nitrogen and 120 L of hydrogen are mixed and, in a test, the reaction is driven to completion of the limiting gas. Find (a) the maximum volume of ammonia formed and (b) the volume of the gas left over (same temperature and pressure).

Try it first, then show the solutionSolution

50 L N₂ would need 150 L H₂, but only 120 L is present: hydrogen is limiting.
(a) NH₃ $= 120 \times \frac{2}{3} = 80$ L.
(b) N₂ used $= 120/3 = 40$ L; N₂ left $= 50 - 40 = 10$ L.

Answer: (a) 80 L of ammonia (b) 10 L of nitrogen left.

Example 5: Masses from nitrogen · 3 marks · Short answerHOTS / competency

Calculate the mass of ammonia that can be made from 56 kg of nitrogen, the mass of hydrogen needed, and the percentage of nitrogen in ammonia. (N = 14, H = 1)

Try it first, then show the solutionSolution

Moles of N₂ $= 56 000/28 = 2000$ mol.
NH₃ $= 4000$ mol $= 68000$ g $= 68$ kg; H₂ $= 6000$ mol $= 12$ kg.
% N in NH₃ $= \frac{14}{17} \times 100 = 82.35%$ .

Answer: 68 kg NH₃; 12 kg H₂; 82.35% nitrogen.

Example 6: Partial conversion · 4 marks · Long answerHOTS / competency

In one pass through a converter, 30 L of nitrogen and 90 L of hydrogen enter, and 20% of the nitrogen is converted to ammonia. (a) What volume of ammonia forms? (b) What is the total volume of gas leaving the converter? (c) Why is the leaving mixture cooled and the remaining gas recycled? (Same temperature and pressure.)

Try it first, then show the solutionSolution

N₂ reacted $= 0.20 \times 30 = 6$ L; H₂ reacted $= 18$ L.
(a) NH₃ formed $= 2 \times 6 = 12$ L.
(b) Left: N₂ 24 L + H₂ 72 L + NH₃ 12 L $= 108$ L (12 L less than 120 L, as 4 volumes become 2).
(c) Cooling liquefies the ammonia so it can be removed; the unreacted N₂ and H₂ (96 L) are recycled because the reaction is reversible and only part reacts in each pass.

Answer: (a) 12 L (b) 108 L (c) To liquefy and remove NH₃ and reuse the unreacted gases.

Where students lose marks in Haber's Process

The slips examiners see most often, and what to write instead.

Mistake: Confusing catalysts between processes
In 2025 (Q2(iv)) examiners noted that candidates confused platinum with nickel and mixed up the catalysts of different processes.
Do this: Keep a four-line list: iron (Haber), platinum (ammonia to NO), V₂O₅ (Contact), nickel (hydrogenation).
Usually costs: 1 mark per wrong match.
Mistake: Writing a one-way arrow
Students write N₂ + 3H₂ → 2NH₃, missing that the reaction is reversible.
Do this: Use ⇌ and add '+ heat' to show it is exothermic.
Usually costs: ½ to 1 mark.
Mistake: Calling molybdenum the catalyst
Students swap catalyst and promoter.
Do this: Iron catalyses; molybdenum promotes.
Usually costs: 1 mark.
Mistake: No 'because' for a condition
Reason questions lose marks when only the condition is restated.
Wrong working
High pressure is used because it is needed.
Right working
High pressure is used because 4 volumes of reactants give 2 volumes of product, so the yield rises.
Do this: Tie pressure to volumes and temperature to rate and yield.
Usually costs: 1 mark.

Exam tips

The syllabus asks for Haber's process in terms of reactants, products, conditions and equations; diagrams of the plant are not needed.
Give temperature and pressure as approximate ranges (about 450–500 °C; about 200–900 atm); a single sensible value within the range is accepted.
If asked for uses of ammonia, give fertilisers, nitric acid, explosives, refrigerant and cleansing agent.
Use the 1 : 3 : 2 ratio for any gas-volume question on ammonia synthesis.

Practice questions on Haber's Process

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The catalyst used in Haber's process is:
(a) platinum
(b) vanadium pentoxide
(c) finely divided iron
(d) nickel
Hint
Platinum is used for oxidising ammonia, not making it.
Answer
(c) finely divided iron.
Q2 · 1 mark · Very short answerBoard level
In what ratio by volume are nitrogen and hydrogen mixed in Haber's process?
Hint
Read the equation.
Answer
1 : 3.
Q3 · 2 marks · Short answerBoard level
How is ammonia separated from the unreacted gases in Haber's process?
Hint
Which gas liquefies most easily?
Answer
The mixture is cooled; ammonia, which liquefies far more easily than nitrogen and hydrogen, condenses to a liquid and is removed. The unreacted gases are recycled.
Q4 · 2 marks · Short answerHarder board level
What is a promoter? Name the promoter in Haber's process.
Hint
It helps the catalyst.
Answer
A substance that increases the efficiency (activity) of a catalyst. Molybdenum.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A high pressure increases the yield of ammonia in Haber's process.
Reason (R): The reaction is exothermic.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Pressure is about volumes, not heat.
Answer
(b). Both are true, but pressure helps because 4 volumes of gas form 2 volumes of ammonia.
Q6 · 2 marks · Short answerHarder board level
Ammonia made by Haber's process is oxidised to nitric acid by Ostwald's process. Name the catalyst used in each of the two processes, and the promoter used with one of them.
Hint
Two different metals; the promoter helps the first.
Answer
Haber's process: finely divided iron, with molybdenum as promoter. Ostwald's process (catalytic oxidation of ammonia): platinum gauze.
Q7 · 2 marks · Short answerHarder board level
What volume of hydrogen is needed to make 60 cm³ of ammonia, and what volume of nitrogen reacts with it (same temperature and pressure)?
Hint
1 : 3 : 2.
Answer
H₂ $= 60 \times \frac{3}{2} = 90$ cm³; N₂ $= 60 \times \frac{1}{2} = 30$ cm³.
Q8 · 2 marks · Short answerHOTS / competency
Suppose an engineer proposes running the converter at 200 °C to get more ammonia, without changing the catalyst. Explain why the plant output per hour would probably fall, even though a higher percentage of ammonia is possible at that temperature.
Hint
Yield versus rate.
Answer
At 200 °C the equilibrium favours ammonia (the reaction is exothermic), but the reaction becomes extremely slow even with the iron catalyst, so much less ammonia is produced per hour. 450–500 °C balances a reasonable yield with a fast rate.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Promoter in Haber's process?Show answer
Molybdenum.
2.Is the Haber reaction exothermic or endothermic?Show answer
Exothermic.
3.Source of nitrogen?Show answer
Liquefied air.
4.Volume of NH₃ from 10 L N₂ and 30 L H₂ (complete)?Show answer
20 L.
5.Why are unreacted gases recycled?Show answer
The reaction is reversible; only part reacts in one pass, so reuse avoids waste.

Haber's Process: frequently asked questions

What are the conditions of Haber's process?

N₂ and H₂ in a 1 : 3 ratio, about 450–500 °C, about 200–900 atm, finely divided iron as catalyst with molybdenum as promoter.

Why is a moderate temperature used in Haber's process?

The reaction is exothermic, so lower temperatures give more ammonia but react too slowly; about 450–500 °C gives a reasonable yield at a useful rate.

What is the role of the iron catalyst?

It speeds up the reaction so that equilibrium is reached quickly; it does not change the proportion of ammonia at equilibrium.

How is ammonia removed from the mixture?

By cooling: ammonia liquefies easily and is drawn off, while nitrogen and hydrogen stay gaseous and are recycled.