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6.5Chemical Reactions

The Haber process — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 11 parts Lesson 6 of 7 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Lesson summary

6.5 The Haber process — Notes — IGCSE Chemistry notes: Describe how ammonia is made in the Haber process, including raw materials, catalyst and conditions, and explain the compromise between rate and yield.Purpose — the Haber process manufactures ammonia, mostly used to make nitr…

  • Enthalpy change
  • Conversion per pass
  • Temperature
  • High pressure
  • Iron catalyst
  • Without a catalyst

Section 1 of 11Purpose and equation

Purpose
  • Purpose the Haber process manufactures ammonia, mostly used to make nitrogen-containing fertilisers.
Equation
  • N2(g) + 3H2(g) ⇌ 2NH3(g).
The reaction is reversible and the forward reaction is exothermic.
Gas moles
  • 4 mol of gas on the left become 2 mol on the right.
Enthalpy change
  • ΔH = −92 kJ/mol for the equation as written.

Section 2 of 11Raw materials

Nitrogen
  • Nitrogen from the air, by fractional distillation of liquid air.
Hydrogen
  • Hydrogen from methane (natural gas) reacting with steam.
Hydrogen can also be made by electrolysis of water, but this uses a lot of electricity.
Ratio
  • Ratio nitrogen and hydrogen are mixed in a 1 : 3 ratio by volume.

Section 3 of 11Stages of the process

1
Nitrogen from air and hydrogen from methane are pumped into a compressor.
2
The gases are compressed to about 20 000 kPa (200 atm).
3
They pass over layers of an iron catalyst at about 450 °C, where some react to form ammonia.
4
The mixture passes into a cooling tank, where the ammonia liquefies and is removed to storage.
5
Unreacted nitrogen and hydrogen are recycled to the reactor.
6
Recycling
  • Recycling only a fraction of the gases reacts on each pass.
7
Conversion per pass
  • Conversion per pass only about 15–20% of the mixture is converted to ammonia in one pass, but recycling gives a high overall conversion.

Section 4 of 11Conditions

Catalyst
  • Catalyst finely divided iron increases the rate.
Temperature
  • Temperature about 450 °C.
Pressure
  • Pressure about 20 000 kPa (200 atm).

Section 5 of 11Effect of temperature

Low temperature
  • Low temperature the forward reaction is exothermic, so a lower temperature moves the equilibrium to the right and increases the yield of NH3.
Drawback
  • Drawback a low temperature makes the reaction too slow and the iron catalyst less effective.
Compromise
  • 450 °C gives a reasonable yield at an acceptable rate.

Section 6 of 11Effect of pressure

High pressure
  • High pressure shifts the equilibrium towards fewer gas molecules, so the yield of NH3 increases.
It also increases the rate because particles are closer and collide more often.
Drawback
  • Drawback very high pressures are costly and dangerous.
    • (thick pipes, powerful compressors)
Compromise
  • 20 000 kPa is chosen.

Section 7 of 11Catalyst and uses

Iron catalyst
  • Iron catalyst lowers the activation energy, so equilibrium is reached faster.
It does not change the position of equilibrium or the yield.
Without a catalyst
  • Without a catalyst a higher temperature would be needed, which costs more energy and decomposes more NH3.
Uses of ammonia
  • Uses of ammonia fertilisers such as ammonium nitrate and ammonium sulfate, and making nitric acid.
Ammonia is also used to make explosives.

Section 8 of 11Evaluating industrial conditions

Factors
  • Factors yield, rate, cost of energy and equipment, safety and recycling are all weighed together.
The best conditions give the most economical output per day.
Recycling
  • Recycling separating the product and returning unreacted gases means little reactant is wasted.
Catalyst
  • Catalyst it lowers the activation energy so that equilibrium is reached quickly at a moderate temperature, which saves energy.

Section 9 of 11More from the specification

1
Le Chatelier's Principle and Equilibrium Shifts
  • If a dynamic equilibrium system is perturbed by altering temperature, pressure,
  • or reactant concentration, the system shifts its equilibrium position to counteract that change and restore equilibrium.
2
Effect of Temperature on Endothermic and Exothermic Reactions — Increasing temperature favours the endothermic direction because heat absorption lowers system temperature,
  • whereas cooling favours the exothermic reaction.
3
Reversible Hydration Reactions
  • Heating blue hydrated copper(II) sulfate drives off water of crystallisation endothermically to form white anhydrous copper(II) sulfate,
  • while adding water is exothermic and restores the blue hydrated form.
4
Contact Process Equilibria
  • In the manufacturing stage of sulfuric acid from sulfur, sulfur dioxide reacts reversibly with oxygen to produce sulfur trioxide,
    • (2SO2 + O2 2SO3, Delta H < 0)
  • where high pressure and moderate temperature favour product yield.
5
Concentration
  • Moles ÷ volume in dm³, or g/dm³.
6
Endothermic
  • A reaction that takes in thermal energy from the surroundings.
7
Hydrated
  • Containing water of crystallisation.
8
Anhydrous
  • Containing no water of crystallisation.
9
Sulfur dioxide
  • An acidic gas made when sulfur-containing fuels burn.
10
Endothermic reaction
  • Reaction that takes in thermal energy.

Section 10 of 11Key terms

TermMeaning
Haber processIndustrial manufacture of ammonia from nitrogen and hydrogen.
Reversible reactionA reaction that can go in both directions (⇌).
Iron catalystSpeeds up the reaction without changing the equilibrium position.
Compromise conditionsConditions balancing rate, yield, cost and safety.
RecyclingReturning unreacted nitrogen and hydrogen to the reactor.
FertiliserA substance, such as an ammonium salt, that supplies nitrogen to plants.

Section 11 of 11Exam technique

1
Saying the catalyst increases the yield — it only makes equilibrium be reached faster.
2
Saying a low temperature is used — 450 °C is a compromise, because lower temperatures are too slow.
3
Saying nitrogen comes from methane — nitrogen comes from air and hydrogen from methane.
4
Forgetting that unreacted gases are recycled.
5
Writing that ammonia is made with a nickel catalyst — the catalyst is iron

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