Ch 2 · Fertilisers · O Level Chemistry 5070
Topic 2 of 3 in Chemistry of the Environment — O Level Chemistry 5070 — 47 lessons.
Introduction
Plants remove nitrogen, phosphorus and potassium from the soil as they grow, and harvesting the crop carries those elements away. Fertilisers put them back, and the nitrogen in most of them comes from ammonia made by the Haber process. This lesson connects three earlier ideas, neutralisation, salt formation and reversible reactions, to one of the largest industries in the world. It also includes the percentage-by-mass calculation that regularly appears alongside fertiliser questions.
Key ideas — Ammonium salts and nitrates as fertilisers
Ammonium salts and nitrates are used as fertilisers because they are soluble in water and supply nitrogen in a form that plant roots can absorb. Common examples are ammonium nitrate (NH_4NO_3), ammonium sulfate ((NH_4)_2SO_4), ammonium phosphate and potassium nitrate (KNO_3). Ammonium nitrate is especially valuable because both its cation and its anion contain nitrogen.
🧑🏫 Teacher's explanation
Think of plants like professional athletes. To build muscle (proteins) and maintain energy (chlorophyll), they need specific building blocks. Nitrogen is the most critical 'building block' for these biological molecules. While nitrogen gas (N_2) is abundant in the air, plants cannot 'breathe' it in; they need it delivered in a 'pre-digested' form that they can absorb through their roots.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of ammonium sulfate, (NH_4)_2SO_4, required to provide 2.0 moles of nitrogen atoms.
🔍 Worked example — Standard
Write the balanced chemical equation for the reaction between ammonia (NH_3) and sulfuric acid (H_2SO_4) to produce ammonium sulfate.
🔍 Worked example — Exam-style
A farmer uses NPK fertiliser containing 25\% Nitrogen by mass. How many grams of fertiliser are needed to supply 5.0 moles of Nitrogen?
⚠️ Common errors
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🎯 Exam question
Key ideas — NPK fertilisers
An NPK fertiliser is a mixture that supplies all three essential elements:
🧑🏫 Teacher's explanation
Think of a plant like a high-performance athlete. To grow strong muscles (leaves), develop a sturdy skeletal system (roots), and have the energy to compete (flowers/fruit), the athlete needs a balanced diet of proteins, minerals, and vitamins. If they only eat protein, they might get big but won't have the energy to run. Similarly, plants need a balanced 'diet' of nutrients to thrive.
📖 Definitions
🔍 Worked example — Easy
A farmer uses a fertiliser with a ratio of 10:10:10. If the bag contains 50 kg of nitrogen, what is the total mass of the fertiliser bag?
🔍 Worked example — Standard
Write the balanced equation for the reaction between ammonia (NH_3) and sulfuric acid (H_2SO_4) to produce ammonium sulfate.
🔍 Worked example — Exam-style
A fertilizer contains 25% Nitrogen, 10% Phosphorus, and 5% Potassium. Calculate the mass of Phosphorus in a 200 kg bag.
⚠️ Common errors
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🎯 Exam question
Key ideas — Making ammonium salts by neutralisation
Ammonia solution is a weak alkali, so it neutralises acids to form ammonium salts. These are the equations expected:
🧑🏫 Teacher's explanation
Think of plant nutrition like a balanced diet for humans. Just as we need specific vitamins and minerals to grow, plants need specific elements—Nitrogen (N), Phosphorus (P), and Potassium (K)—to build proteins, DNA, and regulate water. When soil lacks these, we add NPK fertilisers.
📖 Definitions
🔍 Worked example — Easy
Write a balanced chemical equation for the reaction between ammonia solution and sulfuric acid to produce ammonium sulfate.
🔍 Worked example — Exam-style
In the Haber process, N_2(g) + 3H_2(g) leftharpoons 2NH_3(g) is an exothermic reaction. Explain why a temperature of 450 °C is used instead of a much lower temperature.
⚠️ Common errors
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🎯 Exam question
Key ideas — Calculating the percentage of nitrogen
Fertiliser questions often ask for the percentage by mass of nitrogen so that fertilisers can be compared. Use
🧑🏫 Teacher's explanation
Think of a fertiliser like a nutritional supplement for plants. Just as a protein bar might tell you it is '20% protein' by mass, a fertiliser tells you how much of its total weight is actually the essential nutrient. If a bag of fertiliser is 100g, and it is 35% nitrogen, you know you are getting exactly 35g of nitrogen to feed your plants.
📖 Definitions
🔍 Worked example — Easy
Calculate the percentage by mass of nitrogen in ammonium sulfate, (NH_4)_2SO_4. (Use A_r: H=1, N=14, S=32, O=16)
🔍 Worked example — Standard
Calculate the percentage by mass of nitrogen in ammonium nitrate, NH_4NO_3. (Use A_r: H=1, N=14, O=16)
🔍 Worked example — Exam-style
A fertiliser contains 40% nitrogen by mass. If a farmer uses 500 kg of this fertiliser, calculate the total mass of nitrogen provided to the soil.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Ammonium sulfate is made by reacting ammonia with sulfuric acid. (a) Write a balanced symbol equation for this reaction. [2] (b) Calculate the percentage by mass of nitrogen in ammonium sulfate, (NH_4)_2SO_4. [3] (c) Explain why farmers use fertilisers containing nitrogen. [1]
🔍 Worked example — Easy
Write a balanced symbol equation for the reaction between ammonia and sulfuric acid to produce ammonium sulfate. Use state symbols (aq).
🔍 Worked example — Standard
Calculate the percentage by mass of nitrogen in ammonium sulfate, (NH_4)_2SO_4. (A_r: H=1, N=14, S=32, O=16)
🔍 Worked example — Exam-style
A farmer uses a fertilizer containing 30% nitrogen by mass. If the farmer applies 500 kg of this fertilizer to a field, calculate the total mass of nitrogen added to the soil.
⚠️ Common errors
Common mistakes
Practice
Answers Ammonium salts [B1]; nitrates [B1]. Nitrogen for leaf growth and protein synthesis [B1]; phosphorus for root growth [B1]; potassium for flowers, fruit and disease resistance [B1]. 3NH_3(aq) + H_3PO_4(aq) arrow (NH_4)_3PO_4(aq) [M1 species, A1 balanced]. M_r = 12 + 16 + 2(14 + 2) = 60 [M1]; nitrogen = 28 [M1]; 28/60 × 100 = 46.7\% [A1]. Nitrogen from air and hydrogen from methane (natural gas) and steam [B1]; about 450 °C and 200 atm [B1]; iron catalyst [B1]; the ammonia is neutralised with acids to make ammonium salt fertilisers (or oxidised to nitric acid for nitrates) [B1].