C38: Using Materials
The Haber process for producing ammonia, compromise conditions in industrial processes, and the production and composition of NPK fertilisers for plant growth.
The Haber Process
The Haber process is the industrial method for producing ammonia (NH₃) from nitrogen and hydrogen. It is one of the most important industrial processes in the world because ammonia is used to make fertilisers and many other chemicals.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
The reaction is reversible — ammonia decomposes back into nitrogen and hydrogen. The conditions must be carefully chosen to maximise the yield of ammonia.
Raw Materials
- Nitrogen — obtained from the air (air is 78% nitrogen). It is separated from air by fractional distillation.
- Hydrogen — obtained from natural gas (methane) or from the electrolysis of water. When methane is used:
CH₄(g) + H₂O(g) → CO(g) + 3H₂(g)
Compromise Conditions in the Haber Process
The conditions used in the Haber process are a compromise between the ideal conditions for maximum yield and the practical and economic considerations of running an industrial process.
Temperature: 450°C
The forward reaction to make ammonia is exothermic (releases heat). According to Le Chatelier's principle, a low temperature would give the highest yield of ammonia. However, at low temperatures the reaction is very slow, meaning less ammonia is produced per unit time. A compromise temperature of 450°C is used — this gives a reasonable yield at an acceptable rate.
Pressure: 200 atmospheres
The forward reaction produces fewer moles of gas (4 moles of reactants → 2 moles of product). According to Le Chatelier's principle, a high pressure would favour the forward reaction and give a higher yield of ammonia. Very high pressures would give the best yield but are expensive and dangerous to maintain (stronger equipment, more energy for compression). A compromise pressure of 200 atm is used — high enough for a good yield but not so high as to be uneconomic or unsafe.
Catalyst: Iron
An iron catalyst is used to speed up the reaction. The catalyst does not change the position of equilibrium or the yield — it simply helps the reaction reach equilibrium faster. This means more ammonia is produced per unit time. The catalyst allows a lower temperature to be used while still achieving a reasonable reaction rate.
Haber process conditions: 450°C, 200 atm, iron catalyst. These are compromise conditions — they balance yield, rate and cost.
Never say the catalyst increases the yield of ammonia. A catalyst increases the rate of reaction but does not change the position of equilibrium. It helps reach equilibrium faster.
How the Haber Process Works
The industrial process operates as follows:
- Nitrogen and hydrogen are mixed in a 1:3 ratio
- The gas mixture is compressed to 200 atm
- The mixture is passed over an iron catalyst at 450°C in a reaction vessel
- The reaction reaches equilibrium — only about 15-20% of the nitrogen and hydrogen convert to ammonia
- The gas mixture is cooled — ammonia condenses into a liquid (it has a higher boiling point than nitrogen and hydrogen) and is removed
- The unreacted nitrogen and hydrogen are recycled back into the reaction vessel
Recycling unreacted gases means that eventually nearly all the nitrogen and hydrogen are converted to ammonia, even though each pass through the reactor only gives a 15-20% yield.
Uses of Ammonia
Ammonia produced by the Haber process has many important uses:
- Fertilisers — the largest use, including ammonium nitrate, ammonium sulfate and ammonium phosphate
- Nitric acid — ammonia is oxidised to make nitric acid, which is used to make more fertilisers and explosives
- Cleaning products — ammonia solution is used in household cleaners
- Nylon — ammonia is used in the production of nylon and other polymers
- Explosives — nitric acid from ammonia is used to make TNT and other explosives
NPK Fertilisers
NPK fertilisers contain the three essential elements that plants need for healthy growth: Nitrogen (N), Phosphorus (P) and Potassium (K). These elements are needed in large quantities and are often in short supply in soil.
Why Plants Need N, P and K
- Nitrogen (N) — needed for making amino acids and proteins, which are essential for growth. Nitrogen promotes healthy leaf and stem growth. A deficiency causes stunted growth and yellowing leaves.
- Phosphorus (P) — needed for root development and for making DNA and ATP (energy transfer molecules). A deficiency causes poor root growth and discoloured leaves.
- Potassium (K) — needed for flower and fruit formation, and for regulating water movement in cells. A deficiency causes poor flower and fruit development and yellowing leaves.
Common Fertiliser Compounds
| Fertiliser | Formula | Elements Provided |
|---|---|---|
| Ammonium nitrate | NH₄NO₃ | Nitrogen |
| Ammonium sulfate | (NH₄)₂SO₄ | Nitrogen |
| Ammonium phosphate | (NH₄)₃PO₄ | Nitrogen, Phosphorus |
| Superphosphate | Ca(H₂PO₄)₂ | Phosphorus |
| Potassium chloride | KCl | Potassium |
| Potassium sulfate | K₂SO₄ | Potassium |
Production of Fertilisers
Making Ammonium Nitrate
Ammonium nitrate is made by reacting ammonia with nitric acid. This is a neutralisation reaction:
NH₃ + HNO₃ → NH₄NO₃
The nitric acid itself is made from ammonia through the Ostwald process:
4NH₃ + 5O₂ → 4NO + 6H₂O (catalytic oxidation with platinum)
2NO + O₂ → 2NO₂
4NO₂ + O₂ + 2H₂O → 4HNO₃
Making Ammonium Sulfate
Ammonium sulfate can be made by reacting ammonia with sulfuric acid:
2NH₃ + H₂SO₄ → (NH₄)₂SO₄
Making Superphosphate
Phosphate rock (calcium phosphate) is insoluble in water, so plants cannot absorb it. It is treated with sulfuric acid to produce soluble calcium dihydrogen phosphate (superphosphate):
Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄
Alternatively, phosphate rock can be treated with nitric acid to produce a fertiliser that also contains nitrogen.
Fertiliser compounds must be soluble in water so that plants can absorb the nutrients through their roots. Raw phosphate rock is insoluble and must be converted to a soluble form before use.
Calculating the Percentage of Nitrogen in Fertilisers
You may be asked to calculate the percentage by mass of an element in a fertiliser compound.
Worked Example: Percentage of Nitrogen in Ammonium Nitrate (NH₄NO₃)
Step 1: Calculate the relative formula mass of NH₄NO₃
Mᵣ = 14 + (4 × 1) + 14 + (3 × 16) = 14 + 4 + 14 + 48 = 80
Step 2: Calculate the total mass of nitrogen in the formula
Mass of N = 14 + 14 = 28 (two nitrogen atoms)
Step 3: Calculate the percentage
% N = (28 ÷ 80) × 100 = 35%
Ammonium nitrate is 35% nitrogen by mass.
Worked Example: Percentage of Nitrogen in Ammonium Sulfate ((NH₄)₂SO₄)
Step 1: Calculate the relative formula mass of (NH₄)₂SO₄
Mᵣ = 2 × (14 + 4) + 32 + (4 × 16) = 2 × 18 + 32 + 64 = 36 + 32 + 64 = 132
Step 2: Calculate the total mass of nitrogen
Mass of N = 2 × 14 = 28 (two nitrogen atoms)
Step 3: Calculate the percentage
% N = (28 ÷ 132) × 100 = 21.2%
Ammonium sulfate is 21.2% nitrogen by mass.
When calculating percentage composition, always check how many atoms of the element are in the formula. In NH₄NO₃, there are TWO nitrogen atoms — one in the ammonium ion and one in the nitrate ion.
Comparing Fertilisers
| Fertiliser | Formula | % Nitrogen | Advantages | Disadvantages |
|---|---|---|---|---|
| Ammonium nitrate | NH₄NO₃ | 35% | High N content, good for leafy growth | Can be explosive if contaminated |
| Ammonium sulfate | (NH₄)₂SO₄ | 21.2% | Also supplies sulfur, lowers soil pH | Lower N content than ammonium nitrate |
| Urea | CO(NH₂)₂ | 46.7% | Highest N content of solid fertilisers | Needs bacteria in soil to convert to usable form |
Environmental Impact of Fertilisers
While fertilisers are essential for modern agriculture, they can cause environmental problems:
- Eutrophication — excess fertiliser washed into rivers and lakes causes algal blooms, which deplete oxygen and kill aquatic life
- Water pollution — nitrates leaching into drinking water can be a health concern
- Soil degradation — long-term use can reduce soil quality and organic matter
Fertilisers must be applied in the correct quantities and at the right time to minimise environmental damage. Overuse is both wasteful and harmful.
Practice Questions
1. Write the balanced equation for the Haber process and state the conditions used.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The conditions are 450°C, 200 atmospheres pressure, and an iron catalyst.
2. Explain why the Haber process uses a compromise temperature of 450°C rather than a lower temperature.
The forward reaction is exothermic, so a lower temperature would give a higher yield of ammonia. However, at low temperatures the reaction rate is very slow, producing less ammonia per unit time. The compromise temperature of 450°C gives a reasonable yield at an acceptable reaction rate.
3. What are the three essential elements provided by NPK fertilisers and what does each do for the plant?
Nitrogen (N) — needed for amino acids and proteins, promotes leaf and stem growth. Phosphorus (P) — needed for root development, DNA and ATP. Potassium (K) — needed for flower and fruit formation and regulating water movement in cells.
4. Calculate the percentage of nitrogen in urea, CO(NH₂)₂. (Relative atomic masses: C=12, O=16, N=14, H=1)
Mᵣ of CO(NH₂)₂ = 12 + 16 + 2 × (14 + 2) = 12 + 16 + 2 × 16 = 12 + 16 + 32 = 60. Mass of N = 2 × 14 = 28. %N = (28 ÷ 60) × 100 = 46.7%.
5. Explain why unreacted nitrogen and hydrogen are recycled in the Haber process.
Only about 15-20% of the reactants convert to ammonia on each pass through the reactor because the reaction reaches equilibrium. Recycling the unreacted nitrogen and hydrogen means they can react again, so that eventually nearly all the raw materials are converted to ammonia. This improves the overall yield and reduces waste and cost.
Maths Skills
Calculating Ammonia Yield from Haber Process Conditions
The percentage yield of ammonia depends on the conditions used. At different temperatures and pressures, different percentages of the nitrogen and hydrogen react to form ammonia before equilibrium is reached.
Example data:
- At 200 atm and 450 degrees C: approximately 15-20% of reactants form NH3 per pass
- At 300 atm and 450 degrees C: approximately 25% yield per pass
- At 200 atm and 350 degrees C: approximately 30% yield per pass (but slower rate)
Calculation: If 100 mol of N2 and 300 mol of H2 enter the reactor and the yield per pass is 15%, then 15 mol of N2 reacts to form 30 mol of NH3 (since 1 N2 produces 2 NH3). The unreacted 85 mol N2 and 255 mol H2 are recycled.
Overall yield with recycling: Because unreacted gases are recycled, the overall conversion approaches 97-98%, even though each pass only converts 15-20%. This is why recycling is economically essential.
Percentage atom economy for the Haber process: atom economy = (M_r of desired product / sum of M_r of all products) x 100 = (2 x 17 / 2 x 17) x 100 = 100% (since NH3 is the only product).
Common Misconceptions
Misconceptions About the Haber Process and Industrial Conditions
The Haber process uses the highest possible pressure and temperature to maximise yield.
The Haber process uses compromise conditions of 450 degrees C and 200 atm. A higher pressure would give a higher yield (fewer gas moles on the product side) but is expensive and dangerous. A lower temperature would give a higher yield (exothermic forward reaction) but would make the rate unacceptably slow. These conditions balance a reasonable yield with a reasonable rate while keeping costs and safety risks manageable.
The catalyst in the Haber process increases the yield of ammonia.
The iron catalyst increases the rate of the reaction — it helps the system reach equilibrium faster — but it does NOT change the position of equilibrium or increase the yield. The same yield would be achieved without the catalyst, but it would take much longer.
NPK fertilisers contain nitrogen, phosphorus and potassium as elements.
NPK fertilisers contain nitrogen, phosphorus and potassium in the form of compounds — not as the pure elements. Nitrogen is supplied as ammonium nitrate or urea, phosphorus as phosphate compounds, and potassium as potassium chloride or potassium sulfate. The elements would be useless or dangerous in their pure form.
6-Mark Question
Explain why compromise conditions are used in the Haber process. Discuss the economic and environmental factors.
The Haber process uses compromise conditions of 450 degrees C, 200 atm and an iron catalyst. The forward reaction (N2 + 3H2 to 2NH3) is exothermic, so a low temperature would give the highest yield of ammonia. However, at low temperatures the rate is too slow to be economically viable — the reaction would take an unacceptably long time to reach equilibrium. A high temperature gives a fast rate but shifts equilibrium to the left, reducing the yield. 450 degrees C is a compromise that gives a reasonable yield at a reasonable rate. The reaction produces fewer moles of gas on the product side (4 moles to 2 moles), so high pressure increases yield. However, very high pressures are expensive (requiring stronger, thicker equipment), consume more energy for compression, and pose safety risks from potential leaks or explosions. 200 atm is a compromise that gives a good yield without excessive costs or dangers. An iron catalyst speeds up the rate without affecting the equilibrium position, allowing the compromise temperature to work effectively. Unreacted N2 and H2 are recycled, giving an overall conversion of about 97%. Environmentally, the Haber process uses methane (from natural gas) as the source of hydrogen, which is a finite fossil fuel and produces CO2. Economically, the process is essential for producing fertilisers that support global food production for billions of people.
AO3: Analysis and Evaluation
Evaluating Fertiliser Use and Environmental Impact
NPK fertilisers increase crop yields but can cause environmental damage.
Evaluate the benefits and problems of using artificial fertilisers, and discuss whether organic farming is a viable alternative for feeding a growing global population.
Answer: Benefits of artificial NPK fertilisers: they provide readily available nutrients (nitrogen for leaf growth, phosphorus for root development, potassium for flowers and fruit), significantly increasing crop yields to feed a growing population; they are relatively cheap to produce; and they can be formulated for specific crops and soils. Problems: overuse leads to eutrophication — excess nitrate and phosphate wash into rivers and lakes, causing excessive algal growth. When the algae die, bacteria decompose them using up dissolved oxygen, killing fish and other aquatic organisms. Nitrates can also leach into drinking water supplies. Fertiliser production itself uses energy and raw materials (the Haber process requires natural gas). Organic farming uses natural fertilisers (manure, compost) and avoids synthetic chemicals, reducing water pollution. However, organic crop yields are typically 20-40% lower than conventional farming, meaning more land would be needed to produce the same amount of food — leading to deforestation and habitat loss. For a growing global population of 8 billion, organic farming alone cannot produce enough food without converting vast areas of natural habitat to farmland. A balanced approach — using fertilisers efficiently (precision farming, applying only what is needed, avoiding over-application) — is the most sustainable solution.
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