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15.5–15.6 The Haber Process and Its Economics

GCSE Chemistry · AQA · Chapter 15: Using Our Resources

A free GCSE Chemistry lesson for AQA (8462) from OBA School: making ammonia from nitrogen and hydrogen, the conditions used, and why they are a compromise (Higher tier). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the Haber process: raw materials, conditions, and how ammonia is separated and unreacted gases recycled
  • Write the balanced equation for the reversible reaction and state that the forward reaction is exothermic
  • (Higher tier) explain the effect of temperature, pressure and a catalyst on the rate and the equilibrium yield of ammonia
  • (Higher tier) explain why the conditions used are a compromise based on yield, rate, energy, equipment and safety costs
  • Explain why nitrogen fertilisers are needed, and calculate reacting masses, percentage yield, atom economy and gas volumes for the Haber process

Key words

  • Haber process: The industrial process that makes ammonia from nitrogen and hydrogen.
  • Reversible reaction: A reaction in which the products can react to re-form the reactants, shown by ⇌.
  • Dynamic equilibrium: In a closed system, the forward and reverse reactions happen at the same rate, so amounts stay constant.
  • Yield: The amount of product made. The equilibrium yield is the percentage of ammonia in the mixture at equilibrium.
  • Catalyst: A substance that speeds up a reaction without being used up. Iron is used in the Haber process.
  • Compromise conditions: Conditions chosen to give a reasonable yield at a reasonable rate and an acceptable cost.

Why make ammonia?

Ammonia, NH₃, is used mainly to make nitrogen-based fertilisers such as ammonium nitrate. Ammonia is also used to make nitric acid, explosives, dyes and cleaning products.

Plants need nitrogen to make proteins. They take it in through their roots as soluble nitrate ions, NO₃−(aq), dissolved in soil water. When a crop is harvested, the nitrogen in it is carried away instead of rotting back into the soil, so each harvest leaves the soil with less. Farmers replace it with fertilisers.

Why fix N₂?: The air is about 78% nitrogen, but most plants cannot use it: nitrogen gas, N₂, is very unreactive and hardly dissolves in water. Turning N₂ into compounds that plants can absorb is called fixing nitrogen. The Haber process is the first step.

Raw materials: nitrogen comes from the air (about 78% nitrogen), separated by fractional distillation of liquid air. Hydrogen comes mainly from natural gas (methane) reacted with steam: CH₄ + H₂O → CO + 3H₂. It can also be made by electrolysis of water.

The Haber process

nitrogen + hydrogen ⇌ ammonia

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) the forward reaction is exothermic

  • The purified gases are mixed in the ratio 1 volume of nitrogen to 3 volumes of hydrogen and compressed to about 200 atmospheres.
  • They pass over an iron catalyst at about 450 °C. Only some of the nitrogen and hydrogen react, about 15% on each pass, because the reaction is reversible and some ammonia breaks down again.
  • The mixture is cooled. Ammonia has a much higher boiling point (−33 °C) than nitrogen or hydrogen, so it liquefies and is removed.
  • The unreacted nitrogen and hydrogen are recycled back to the reactor, so almost none is wasted.

Choosing the conditions (Higher tier)

Le Chatelier’s principle predicts how the conditions affect the equilibrium yield, shown in the graph.

  • Pressure; Effect on equilibrium yield higher pressure moves the equilibrium to the side with fewer molecules of gas (4 on the left, 2 on the right), so the yield increases; Effect on rate higher pressure: gas particles closer, more frequent collisions, faster; Compromise 200 atm: high pressure is expensive and dangerous
  • Temperature; Effect on equilibrium yield the forward reaction is exothermic, so a lower temperature moves the equilibrium to the right and the yield increases; Effect on rate lower temperature: slower, as fewer collisions have the activation energy; Compromise 450 °C: fast enough with an acceptable yield
  • Iron catalyst; Effect on equilibrium yield no effect on the position of equilibrium; Effect on rate much faster; equilibrium is reached sooner; Compromise lets a lower temperature be used

Exam tip: 450 °C is not the temperature for the best yield. A lower temperature would give a higher yield, but the reaction would be far too slow. 450 °C is a compromise between yield and rate.

The economics of the process (Higher tier)

The company wants the cheapest ammonia, not the highest yield per pass, so the conditions balance several costs.

  • Energy for compressing; How it affects the choice Pressures above 200 atm need much more energy, so running costs rise.
  • Equipment and safety; How it affects the choice Higher pressures need thicker, more expensive pipes and reactors and increase the risk of leaks or explosions.
  • Energy for heating; How it affects the choice The reaction is exothermic: its heat warms the incoming gases, saving fuel.
  • Raw materials; How it affects the choice Unreacted gases are recycled, so a low yield per pass wastes very little.
  • Rate; How it affects the choice A faster rate means more ammonia each day from the same plant.
  • Natural gas; How it affects the choice The biggest single cost. Methane is bought as natural gas, a finite fossil fuel, and making hydrogen from it also gives carbon monoxide (which ends up as carbon dioxide).
  • Nitrogen from air; How it affects the choice Air is free, but cooling and compressing it to about −200 °C to liquefy it for fractional distillation uses a lot of energy.

Worked example: atom economy of the Haber process

percentage atom economy = (Mr of desired product ÷ total Mr of reactants) × 100
N₂ + 3H₂ → 2NH₃: reactants 28 + (3 × 2) = 34; desired product 2 × 17 = 34
Atom economy = 34 ÷ 34 × 100 = 100%. Ammonia is the only product, so no atoms are wasted; with recycling, this is part of what makes the process sustainable.

Answer: 100%

Haber process calculations

Worked example: reacting masses and percentage yield

Ar: N = 14, H = 1. Mr: N₂ = 28, NH₃ = 17.
N₂ + 3H₂ → 2NH₃: 1 mol N₂ (28 g) makes 2 mol NH₃ (2 × 17 = 34 g).
So 1.4 tonnes of nitrogen can make 1.4 × 34 ÷ 28 = 1.7 tonnes of ammonia.
If the plant actually collects 1.53 tonnes: percentage yield = 1.53 ÷ 1.7 × 100 = 90%

Answer: 1.7 tonnes theoretical; 90% yield

Worked example: gas volumes (Higher tier)

Volume ratio N₂ : H₂ : NH₃ = 1 : 3 : 2 (equal volumes of gas contain equal numbers of molecules). So 50 dm³ of nitrogen reacts with 150 dm³ of hydrogen to make 100 dm³ of ammonia.
1.7 kg of ammonia = 1700 ÷ 17 = 100 mol, which occupies 100 × 24 = 2400 dm³ at RTP.

Answer: 150 dm³ hydrogen, 100 dm³ ammonia; 2400 dm³

Worked example: from a gas volume to a percentage yield (Higher tier)

A test reactor is fed 1800 dm³ of hydrogen (at RTP) with plenty of nitrogen. It collects 680 g of ammonia.
Volume of NH₃ possible = 1800 × 2 ÷ 3 = 1200 dm³ → moles = 1200 ÷ 24 = 50 mol
Theoretical mass = 50 × 17 = 850 g. Percentage yield = 680 ÷ 850 × 100 = 80%

Answer: 850 g theoretical; 80% yield

Common mistakes

  • Saying the catalyst increases the yield. A catalyst speeds up both forward and reverse reactions equally; it does not change the position of equilibrium.
  • Saying 450 °C gives the highest yield. A lower temperature gives a higher yield, but too slowly.
  • Saying higher pressure increases the yield “because there are more collisions”. That is why the rate increases; the yield increases because there are fewer gas molecules on the right.
  • Forgetting that unreacted gases are recycled, so the overall conversion is high even though each pass converts only about 15%.
  • Writing the equation as N + H₃ → NH₃. Nitrogen and hydrogen are diatomic: N₂ + 3H₂ ⇌ 2NH₃.

Revision questions

Try these, then check your answers below each one.

Question 1. How is ammonia separated from the unreacted nitrogen and hydrogen? (Choose from: The mixture is cooled so ammonia liquefies, The mixture is filtered, Ammonia is dissolved in oil, The mixture is heated so ammonia boils off.)

Answer: The mixture is cooled so ammonia liquefies

Working: Cooling condenses the ammonia to a liquid; nitrogen and hydrogen stay as gases and are recycled.

Question 2. True or false? The iron catalyst increases the equilibrium yield of ammonia.

Answer: False

Working: False. The catalyst only increases the rate, so equilibrium is reached faster. It does not change the yield.

Question 3. Air is about 78% nitrogen. Why do crops still need nitrogen fertilisers? (Choose from: Nitrogen gas is unreactive and insoluble, so roots cannot absorb it, Plants do not need nitrogen to grow, Nitrogen gas is poisonous to plants, Roots can only absorb nitrogen gas, not nitrates.)

Answer: Nitrogen gas is unreactive and insoluble, so roots cannot absorb it

Working: Plants absorb nitrogen as soluble nitrate (or ammonium) ions to make proteins. N₂ must first be “fixed”, e.g. made into ammonia by the Haber process.

Related lessons

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This lesson is section 15.5–15.6 (AGCCH1505) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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