The Haber Process

📖 In-Depth Theory

The HABER PROCESS manufactures AMMONIA (NH₃) — used to make fertilisers, explosives and industrial chemicals.

RAW MATERIALS:

NITROGEN: from the air (78% nitrogen) — separated from air by fractional distillation.

HYDROGEN: from NATURAL GAS (methane) by steam reforming: CH₄ + H₂O → CO + 3H₂ (then CO removed).

CHEMICAL EQUATION:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol (exothermic)

The reaction is REVERSIBLE — equilibrium is established.

At equilibrium, only about 15% of reactants convert to ammonia under typical conditions.

Unreacted N₂ and H₂ are RECYCLED back into the reactor — overall yield approaches ~98%.

Industrial Conditions and Reasons

TEMPERATURE: approximately 450°C

Lower temperature favours more ammonia (exothermic forward reaction).

But lower temperature = slower rate — not economically viable.

450°C is a COMPROMISE — fast enough rate, acceptable equilibrium yield.

PRESSURE: approximately 200 atmospheres

Lower volume side of equation: 4 mol gas → 2 mol gas.

Higher pressure favours MORE AMMONIA (fewer moles of gas).

200 atm used — higher pressure too expensive and dangerous.

Again a compromise.

CATALYST: iron (with promoters Al₂O₃ and K₂O)

Iron catalyst INCREASES THE RATE — allows the equilibrium to be reached faster.

Does NOT change the equilibrium position or yield.

Allows lower temperature to be used while maintaining acceptable rate.

COOLING AND SEPARATING:

Gases leave the reactor and are COOLED — ammonia liquefies and is separated.

Unreacted N₂ and H₂ remain gaseous and are recycled back to the reactor.

Why These Conditions Are Chosen

The Haber process illustrates the principles of INDUSTRIAL CHEMISTRY:

COMPROMISE BETWEEN RATE AND YIELD:

High temperature: fast rate but low equilibrium yield.

Low temperature: high yield but too slow — not economically viable.

450°C balances rate and yield.

ECONOMIC CONSIDERATIONS:

Very high pressure is expensive (strong reactors needed, pumps, energy).

200 atm balances yield improvement against cost.

Iron catalyst is cheap and abundant — reduces costs.

Recycling unreacted gases improves overall efficiency dramatically.

ENVIRONMENTAL CONSIDERATIONS:

Natural gas (CH₄) as hydrogen source releases CO₂ — carbon footprint.

Research into 'green ammonia' using hydrogen from electrolysis (renewable energy).

Ammonia production accounts for ~2% of global energy use.

FRITZ HABER and CARL BOSCH:

Developed the process in the early 1900s.

Haber process revolutionised agriculture — nitrogen fertilisers from ammonia enabled massive increase in food production.

Estimated to sustain about half the world's current population through increased crop yields.

⚠️ Common Mistake

The catalyst does NOT change the equilibrium yield — it only speeds up how quickly equilibrium is reached. The temperature (450°C) is a compromise — lower temperature gives higher yield but too slow; higher temperature gives faster rate but lower yield. BOTH nitrogen and hydrogen are needed — nitrogen from air, hydrogen from natural gas.

📐 Key Equations

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (450°C, 200 atm, iron catalyst)

📌 Key Note

N₂ + 3H₂ ⇌ 2NH₃. N₂ from air, H₂ from natural gas. Conditions: 450°C (rate/yield compromise), 200 atm (yield, cost compromise), iron catalyst (rate only — no effect on yield). Unreacted gases recycled. Overall yield ~98% with recycling. Used to make fertilisers — feeds ~half the world.

🎯 Matching Activity — Haber Process Conditions

Match each condition to the reason it is chosen. — drag the symbols on the right to match the component names on the left.

~450°C

Drop here

~200 atmospheres

Drop here

Iron catalyst

Drop here

Recycling unreacted gases

Drop here

Higher pressure favours more ammonia (4 mol → 2 mol gas) but very high pressure is too expensive

Increases rate of reaction — allows equilibrium to be reached faster without changing the yield

Compromise — lower temperature gives higher yield but too slow; this temperature balances rate and yield

N₂ and H₂ not converted are returned to reactor — improves overall yield from ~15% to ~98%

⚽ FIFA Worked Examples

Haber Process

Explain why a temperature of 450°C is used in the Haber process rather than a lower temperature.

F

Consider the effect of temperature on RATE and on EQUILIBRIUM YIELD separately

I

Lower T → higher equilibrium yield (exothermic forward reaction). Lower T → slower rate.

F

A very low temperature gives high yield but the rate is too slow to be economically viable.

A

450°C is a compromise — fast enough rate to be economically viable while maintaining an acceptable yield

⭐ Higher Tier Only

Apply Le Chatelier's principle to predict how changing temperature, pressure and concentration affect the equilibrium position. Explain why 450°C is an economic compromise. Calculate atom economy of the Haber process. Evaluate modifications to improve sustainability (green hydrogen, lower-temperature catalysts). Explain why unreacted gases are recycled rather than discarded.

🔬 Triple Science Only

The Haber process (4.10.4.1) is chemistry-only — not in Combined Science.

🎯 Test Yourself

Question 1 of 2

1. The Haber process uses a pressure of about 200 atmospheres. Why is an even higher pressure not used?

2. How does the iron catalyst improve the Haber process without affecting the equilibrium yield?

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🧪 The Haber Process

The Haber Process

Industrial Conditions and Reasons

Why These Conditions Are Chosen

Mr. Badmus AI