Haber Process & Ammonia
Summary: The Haber process manufactures ammonia (NH3) from nitrogen (N2) and hydrogen (H2) at 450°C and 200 atm with an iron catalyst. Ammonia is used mainly for NPK fertilisers and nitric acid production. The conditions represent a compromise between rate of reaction and equilibrium yield. Tags: igcse chemistry haber-process equilibrium industrial-chemistry Created: 2026-07-14 Last Updated: 2026-07-16
Learning Objectives
By the end of this topic, you should be able to:
- Write the balanced equation for the Haber process, including the equilibrium arrow and enthalpy change
- State the conditions used (450°C, 200 atm, iron catalyst) and explain why each is chosen
- Explain why the conditions represent a compromise between rate and yield
- Describe the sources of nitrogen (air) and hydrogen (natural gas / steam reforming)
- Outline the uses of ammonia, particularly for NPK fertilisers and nitric acid production
- Explain why unreacted gases are recycled in the process
Content
1. The Reaction
N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = −92 kJ/mol
- The forward reaction is exothermic (releases heat)
- 4 moles of gas → 2 moles of gas (fewer molecules on the product side)
- The reaction is reversible — ammonia decomposes back into N2 and H2 at high temperatures
2. Sources of Raw Materials
| Raw Material | Source | How It’s Obtained |
|---|---|---|
| Nitrogen (N2) | Air (78% N2) | Fractional distillation of liquid air — air is cooled to −200°C; N2 boils at −196°C and is separated from O2 (boils at −183°C) |
| Hydrogen (H2) | Natural gas (CH4) + Water | Steam reforming: CH4 + H2O → CO + 3H2, then CO + H2O → CO2 + H2 |
3. Conditions — The Compromise
This is the most heavily examined part of the topic. Examiners ask WHY each condition is chosen — you must discuss both yield AND rate.
| Condition | Value | Favours Yield? | Favours Rate? | Why This Value? |
|---|---|---|---|---|
| Temperature | 450°C | Low temp (forward reaction is exothermic — equilibrium shifts right to release heat) | High temp (particles have more KE, more collisions exceed Ea) | Compromise: Low temp gives better yield but too slow. 450°C gives a reasonable rate with acceptable yield |
| Pressure | 200 atm | High pressure (4 mol gas → 2 mol gas — fewer molecules on product side, equilibrium shifts right) | High pressure (more particles per volume, more frequent collisions) | Compromise: Higher pressure increases yield AND rate, but beyond 200 atm is very expensive (stronger pipes, more energy to compress, safety risks) |
| Catalyst | Iron (Fe) | No effect on yield (speeds up forward and backward reactions equally) | Speeds up both directions equally | Iron catalyst allows equilibrium to be reached faster at the chosen temperature. Without it, the reaction would be impractically slow at 450°C |
The Yield vs Rate Trade-Off (Key Exam Concept)
- Low temperature → HIGHER yield of NH3 (equilibrium shifts right, exothermic) but SLOWER rate
- High temperature → LOWER yield of NH3 but FASTER rate
- High pressure → HIGHER yield AND faster rate (equilibrium shifts to side with fewer gas molecules)
- So why not use maximum possible pressure? Cost and safety — high-pressure equipment is expensive to build and maintain
4. The Industrial Process (Step by Step)
- Purification: N2 and H2 are purified to remove impurities — especially sulfur compounds which would poison (deactivate) the iron catalyst
- Compression: Gases are compressed to 200 atm
- Reaction: Mixed gases (N2 : H2 in 1:3 ratio) pass over iron catalyst at 450°C in the catalytic converter
- Cooling and Separation: Gases leave the converter containing ~15% NH3 plus unreacted N2 and H2. The mixture is cooled; ammonia liquefies (NH3 boils at −33°C) while N2 (bp −196°C) and H2 (bp −253°C) remain as gases
- Recycling: Unreacted N2 and H2 are recycled back into the converter — this improves the overall yield and is more economical
5. Why Only ~15% Yield Per Pass?
At 450°C, the equilibrium position favours the reactants (the forward reaction is exothermic, so the relatively high temperature shifts equilibrium left). Only about 15% of the N2/H2 mixture converts to NH3 per pass through the converter. However, recycling unreacted gases means the overall conversion approaches 100% — very little raw material is wasted.
6. Uses of Ammonia
| Use | % of Production | Details |
|---|---|---|
| NPK Fertilisers | ~80% | Ammonia provides nitrogen for plant growth. Used to make ammonium nitrate (NH4NO3), ammonium sulfate ((NH4)2SO4), and urea |
| Nitric Acid (HNO3) | ~10% | Ammonia oxidised via the Ostwald process: NH3 → NO → NO2 → HNO3 |
| Explosives | — | Ammonium nitrate (NH4NO3) from NH3 + HNO3 |
| Cleaning Products | — | Household ammonia solutions (ammonium hydroxide) |
| Refrigeration | — | Anhydrous ammonia as industrial refrigerant |
| Nylon and Plastics | — | Caprolactam (for nylon-6) made from ammonia derivatives |
7. NPK Fertilisers
NPK = Nitrogen, Phosphorus, Kalium (potassium). These are the three essential elements for plant growth.
| Element | Role in Plants | Deficiency Symptoms | Common Fertiliser Source |
|---|---|---|---|
| N (Nitrogen) | Making proteins and chlorophyll; leaf growth | Yellow leaves, stunted growth | NH4NO3 (ammonium nitrate), (NH4)2SO4, urea CO(NH2)2 |
| P (Phosphorus) | Root development, energy transfer (ATP), DNA | Poor root growth, purple-tinged leaves | Ca3(PO4)2 (rock phosphate) treated with sulfuric/nitric acid to make soluble phosphates |
| K (Potassium) | Enzyme activation, water regulation, disease resistance | Yellow edges on leaves (scorching), weak stems | KCl (potassium chloride / muriate of potash), K2SO4 (potassium sulfate) |
Eutrophication: Excess nitrate fertilisers wash into rivers and lakes by rain → algae grow rapidly (algal bloom) → algae block sunlight → underwater plants die → aerobic bacteria decompose dead plants using dissolved O2 → fish and other aquatic life die from lack of oxygen (hypoxia).
8. Comparison with the Contact Process
The IGCSE syllabus covers two key industrial equilibrium processes. Know the differences:
| Feature | Haber Process | Contact Process |
|---|---|---|
| Product | NH3 (ammonia) | H2SO4 (sulfuric acid) |
| Equation | N2 + 3H2 ⇌ 2NH3 | 2SO2 + O2 ⇌ 2SO3 |
| Temperature | 450°C | 450°C |
| Pressure | 200 atm | 1-2 atm (atmospheric) |
| Catalyst | Iron | Vanadium(V) oxide (V2O5) |
| ΔH | −92 kJ/mol (exothermic) | −197 kJ/mol (exothermic) |
Worked Examples (Exam-Style)
Example 1: Explaining the Temperature
Question: Explain why a temperature of 450°C is used in the Haber process rather than a lower temperature. [3 marks]
Solution (model answer):
- The forward reaction is exothermic, so a lower temperature would give a higher equilibrium yield of ammonia ✓
- However, at a lower temperature the rate of reaction would be too slow ✓
- 450°C is a compromise temperature that gives a reasonable rate of reaction while still producing an acceptable yield ✓
Example 2: Effect of Pressure on Yield
Question: State and explain the effect of increasing the pressure on the yield of ammonia in the Haber process. [3 marks]
Solution (model answer):
- Increasing pressure increases the yield of ammonia ✓
- There are 4 moles of gas on the reactant side and 2 moles of gas on the product side ✓
- Increasing pressure shifts the equilibrium to the side with fewer gas molecules (the product side) to reduce the pressure ✓
Example 3: Recycling
Question: Explain why unreacted nitrogen and hydrogen are recycled in the Haber process. [2 marks]
Solution (model answer):
- Recycling improves the overall yield/conversion of raw materials to ammonia ✓
- It is more economical / reduces waste / conserves raw materials ✓
Practice Questions
-
Write the balanced equation for the Haber process, including state symbols. [2 marks]
-
State the three conditions used in the Haber process. [3 marks]
-
Explain why iron is used in the Haber process and state what effect, if any, it has on the equilibrium yield. [3 marks]
-
Nitrogen for the Haber process is obtained from the air. Describe how. [2 marks]
-
State two uses of ammonia. [2 marks]
-
Explain why the Haber process is carried out at 200 atm rather than at atmospheric pressure. [3 marks]
Key Facts to Memorise
- Equation: N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92 kJ/mol (exothermic)
- Conditions: 450°C, 200 atm, iron catalyst
- N2 source: Fractional distillation of liquid air
- H2 source: Steam reforming of natural gas (methane, CH4)
- Iron catalyst: Speeds up rate; does NOT affect equilibrium position or yield
- Recycling: Unreacted N2 and H2 are recycled to improve overall yield
- Ammonia separated by cooling/liquefaction: NH3 boils at −33°C; N2 (−196°C) and H2 (−253°C) remain as gases
- NPK: Nitrogen (leaf growth), Phosphorus (roots), Potassium (disease resistance)
- Eutrophication: excess nitrate → algal bloom → O2 depletion → aquatic life dies
Common Misconceptions
| Students often think… | But the correct understanding is… |
|---|---|
| ”Low temperature increases yield because the reaction is endothermic” | The forward reaction is EXOTHERMIC. Low temperature favours the exothermic direction. High temperature would favour the endothermic (reverse) direction |
| ”The catalyst increases the yield of ammonia” | The catalyst only speeds up the RATE — it has NO effect on equilibrium position or yield |
| ”Ammonia is separated by filtration” | Ammonia is separated by cooling (liquefaction). NH3 condenses at −33°C; N2 and H2 remain as gases |
| ”The Haber process is the same as the Contact process” | Haber makes NH3 (N2 + H2); Contact makes H2SO4 (SO2 + O2 → SO3 then + H2O). Different catalysts (Fe vs V2O5), different pressures (200 atm vs 1-2 atm) |
| “Higher pressure always increases yield in any equilibrium” | Higher pressure only shifts equilibrium if there are different numbers of gas molecules on each side. If the numbers are equal, pressure has no effect on yield |
| ”All the N2 and H2 react to form NH3 in a single pass” | Only ~15% converts per pass. Unreacted gases are recycled |
Key Concepts from Past Papers
Definitions You MUST Know (Exact Mark Scheme Wording)
- The Haber process: the industrial manufacture of ammonia from nitrogen and hydrogen
- Equilibrium: the state in a reversible reaction where the rates of the forward and backward reactions are equal
- Catalyst: a substance that speeds up a reaction without being used up; provides an alternative pathway with lower activation energy
Recurring Mark Scheme Answers
- “450°C is a compromise between rate (faster at high temp) and yield (higher at low temp because exothermic)”
- “200 atm is a compromise — high pressure increases yield AND rate, but beyond 200 atm is too expensive”
- “Iron catalyst speeds up the rate but does not affect equilibrium position or yield”
- “Unreacted N2 and H2 are recycled to improve yield / to save money / to avoid wasting raw materials”
- “N2 is obtained from the air by fractional distillation of liquid air”
- “H2 is obtained from natural gas (methane) by reacting with steam”
Common Question Types
Type 1: Explain the Conditions (3-4 marks)
- Frequency: Appears in ~70% of papers on this topic
- Model answer format: “[Temperature/Pressure] is a compromise between [yield] and [rate/cost]”
- Must mention: low temp = good yield but too slow; high pressure = good yield + rate but expensive; catalyst = no effect on yield
Type 2: Effect of Changing Conditions on Yield (2-3 marks)
- “What happens to the yield of ammonia if temperature is increased?”
- Answer: Yield DECREASES because forward reaction is exothermic; equilibrium shifts left (endothermic direction) to absorb added heat
Type 3: Why is Iron Used? (1-2 marks)
- Iron acts as a catalyst — speeds up BOTH forward and backward reactions equally
- Allows reaction to reach equilibrium faster at a lower temperature than would otherwise be needed
- Does NOT change the position of equilibrium or the yield
Type 4: Source of Raw Materials (1-2 marks)
- N2: from the air by fractional distillation of liquid air
- H2: from natural gas (CH4) by reacting with steam (steam reforming)
Type 5: Diagram / Flowchart (3-4 marks)
- Reactor → Cooler → NH3(l) collected → Unreacted N2 + H2 recycled back to reactor
Type 6: Eutrophication (2-3 marks)
- Describe the sequence: fertiliser runoff → algal bloom → blocks sunlight → plants die → bacteria use O2 → fish die
Exam Tips
- The phrase “compromise between rate and yield” is the single most important exam phrase for this topic
- Always state that the forward reaction is exothermic when explaining temperature effects (this is the key marking point students miss)
- Remember that the catalyst has NO effect on yield — this is a classic exam trap
- The diagram of the Haber process showing the recycling loop is frequently tested — practise drawing it
- Be careful to distinguish between “rate” and “yield” in your answers — they are different concepts tested with different mark scheme points
- For eutrophication, the mark scheme expects the complete sequence, not just “algae grow”
Keywords from Past Papers
equilibrium, yield, compromise, exothermic, catalyst, iron, pressure, temperature, recycled, nitrogen, hydrogen, ammonia, fertiliser, eutrophication, reversible
Related Notes
- Reversible Reactions — Dynamic equilibrium and Le Chatelier’s principle
- Sulfur and Compounds — Contact Process for H2SO4 (often compared to Haber in exams)
- Uses of Sulfur Compounds — NPK fertilisers from sulfuric acid
- Electrolysis Applications — Hydrogen production alternative
- Rates of Reaction — Factors affecting reaction rate (temperature, pressure, catalyst)
- Composition of Air — N2 makes up 78% of air
- Transition Metals — Iron as a catalyst
- Climate Change and Greenhouse Gases — Nitrous oxide (N2O) from fertilisers
- Organic Chemistry Fundamentals — Nitrogen-containing organic compounds
- IGCSE-Chem-Index
Past Paper Sources
Based on 0620/0971 past papers:
- 0620/32 May/June 2018 Q8(c): Haber process conditions explanation (3 marks)
- 0620/33 Oct/Nov 2017 Q8(a)(i): Write equation for Haber process (1 mark)
- 0971/42 May/June 2023 Q7(d): Explain why a temperature of 450°C is used (3 marks)
- 0620/42 Feb/March 2020 Q8(c): Effect of increasing pressure on equilibrium yield (2 marks)
- 0620/43 May/June 2019 Q7(e): Explain why unreacted gases are recycled (2 marks)
- 0971/32 Oct/Nov 2022 Q6(d): Identify the catalyst used in the Haber process (1 mark)
IGCSE Chemistry (0620/0971) wiki. Key industrial chemistry topic — expect it in most exam papers.