Revision notes · Using resources
The Haber process and the use of NPK fertilisers (chemistry only)
The Haber process4.10.4.1
Definition: The Haber process is an industrial process used to manufacture ammonia from nitrogen and hydrogen gas.
The Haber process produces the ammonia needed to make synthetic nitrogen-based fertilisers.
- •Nitrogen is obtained from the air, and hydrogen is obtained mainly from natural gas (methane).
- •The reaction conditions are a temperature of 450 °C, a pressure of 200 atmospheres, and an iron catalyst.
- •The reaction is reversible, so unreacted nitrogen and hydrogen gases are recycled back into the reactor.
- •A compromise temperature of 450 °C is used to balance a fast rate of reaction with a reasonable yield of ammonia from the exothermic reaction.
- •A compromise pressure of 200 atmospheres is used: a higher pressure would increase the yield further, but the energy cost of generating it makes higher pressures too expensive.
- •On cooling, ammonia gas condenses into a liquid and is removed from the reactor mixture.
⚠️ Common mistake: Thinking a lower temperature decreases yield; lowering temperature actually increases the yield of ammonia because the forward reaction is exothermic, but it slows the reaction rate down too much.
🧠 Remember: Remember 450, 200, Fe: 450 °C, 200 atm, and iron catalyst.
Production and uses of NPK fertilisers4.10.4.2
Definition: NPK fertilisers are formulation mixtures containing compounds of nitrogen, phosphorus, and potassium to improve agricultural crop growth.
NPK fertilisers supply plants with essential nutrients to increase crop yields.
- •The symbols N, P, and K stand for the plant nutrient elements nitrogen, phosphorus, and potassium.
- •Ammonia made in the Haber process is reacted with acids to produce soluble nitrogen compounds such as ammonium nitrate.
- •Potassium chloride and potassium sulfate are mined directly from the ground and used without further chemical processing.
- •Insoluble phosphate rock must be treated with acids to produce soluble salts.
- •Industrial fertiliser production uses continuous high-concentration processes, whereas laboratory preparation uses batch titration and crystallisation.
| Acid used on phosphate rock | Product |
|---|---|
| Nitric acid | Calcium nitrate |
| Sulfuric acid | Single superphosphate |
| Phosphoric acid | Triple superphosphate |
⚠️ Common mistake: Believing that phosphate rock can be applied directly to soil as a fertiliser; it is insoluble and must first be reacted with acids to make soluble phosphate salts.
🧠 Remember: Acid treats phosphate rock: Nitric makes nitrate, Sulfuric makes single superphosphate, Phosphoric makes triple superphosphate.
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