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Grade 12 · Chemical systems · Paper 2

The fertiliser industry

Why plants need N, P and K, how the Haber, Ostwald and Contact processes make the raw materials, how to read and calculate NPK ratios, and how fertiliser runoff causes eutrophication.

Plant nutrients

NutrientRole in the plant
Nitrogen (N)Leaf and stem growth; part of proteins and chlorophyll
Phosphorus (P)Root growth, flowering, energy transfer (ATP)
Potassium (K)Flowers and fruit, resistance to disease

Plants get C, H and O from air and water. N, P and K are taken up from the soil as soluble ions, and intensive farming removes them faster than nature replaces them — so fertilisers are needed to feed a growing population.

Making the raw materials

📖 Haber process (ammonia)

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH < 0; iron catalyst, about 450 °C and 200 atm. N₂ comes from fractional distillation of liquid air; H₂ from methane or coal.

📖 Ostwald process (nitric acid)

4NH₃ + 5O₂ → 4NO + 6H₂O (platinum catalyst)

2NO + O₂ → 2NO₂

3NO₂ + H₂O → 2HNO₃ + NO

📖 Contact process (sulfuric acid)

S + O₂ → SO₂; 2SO₂ + O₂ ⇌ 2SO₃ (V₂O₅ catalyst)

SO₃ + H₂SO₄ → H₂S₂O₇ (oleum); H₂S₂O₇ + H₂O → 2H₂SO₄

SO₃ is not added directly to water because the reaction is so exothermic that it forms an acid mist.

Fertiliser salts: NH₃ + HNO₃ → NH₄NO₃ (ammonium nitrate); 2NH₃ + H₂SO₄ → (NH₄)₂SO₄ (ammonium sulfate); phosphate rock + H₂SO₄ → superphosphate; phosphoric acid + NH₃ → ammonium phosphate.

NPK ratios and percentages

A label such as 3 : 2 : 1 (25) means the ratio N : P : K is 3 : 2 : 1 and 25% of the bag’s mass is these three nutrients.

✏️ Worked example: a 50 kg bag of 3 : 2 : 1 (25)
1Total nutrients = 25% × 50 kg = 12,5 kg.
2N = 3/6 × 12,5 = 6,25 kg; P = 2/6 × 12,5 = 4,17 kg; K = 1/6 × 12,5 = 2,08 kg.
3The other 37,5 kg is filler (sand, etc.) that makes spreading even.
✏️ Worked example: % N in ammonium nitrate
1M(NH₄NO₃) = 80 g·mol⁻¹; it contains 2 N atoms = 28 g.
2% N = 28 ÷ 80 × 100 = 35%.

Eutrophication and alternatives

Eutrophication is the enrichment of water with nutrients (especially nitrates and phosphates) that causes excessive growth of algae.

  1. Excess fertiliser washes into rivers and dams.
  2. Algae grow rapidly (algal bloom) and cover the surface, blocking sunlight from water plants.
  3. Plants and algae die; bacteria decompose them and use up the dissolved oxygen.
  4. Fish and other aquatic animals die from lack of oxygen; biodiversity falls. Some algae also produce toxins.

Reducing the impact: apply the correct amount at the right time; avoid fertilising before heavy rain; leave buffer strips of vegetation near water; use organic fertilisers (compost, manure) that release nutrients slowly.

0/8
Review the explanations above.
Answer in your exercise book, then open each memo to mark your work.
Question 1 · (3 marks)
Name the primary nutrients N, P and K and give ONE function of each in a plant.
Show memo
N: leaf/stem growth (proteins, chlorophyll) ✓; P: root growth/flowering ✓; K: fruit and flower formation/disease resistance ✓
Question 2 · (4 marks)
Write balanced equations for the three steps of the Ostwald process.
Show memo
4NH₃ + 5O₂ → 4NO + 6H₂O ✓
2NO + O₂ → 2NO₂ ✓
3NO₂ + H₂O → 2HNO₃ + NO ✓ (balancing ✓)
Question 3 · (4 marks)
A 20 kg bag of fertiliser is labelled 2 : 3 : 2 (21). Calculate the mass of phosphorus in the bag.
Show memo
Nutrients = 21% × 20 = 4,2 kg ✓; P fraction = 3/7 ✓; m(P) = 3/7 × 4,2 ✓ = 1,8 kg ✓
Question 4 · (4 marks)
Explain how excessive use of fertiliser can lead to the death of fish in a dam.
Show memo
Runoff carries nitrates/phosphates into the dam ✓; algae multiply rapidly (algal bloom) ✓; algae and plants die and are decomposed by bacteria that use up dissolved oxygen ✓; fish die from lack of oxygen ✓.