5.19CThe Haber process
Chemistry only — not on the Combined Science specification
The Haber process is used to manufacture ammonia, which can be used to produce nitrogen-based fertilisers.
The raw materials for the Haber process are nitrogen and hydrogen. You should be able to recall a source for each: the nitrogen is obtained from the air, which is about 80% nitrogen, and the hydrogen is obtained from natural gas — mainly methane — or from other hydrocarbons.
The purified gases are passed over a catalyst of iron at a high temperature, about 450 °C, and a high pressure, about 200 atmospheres. Some of the hydrogen and nitrogen reacts to form ammonia. The reaction is reversible, so some of the ammonia produced breaks down into nitrogen and hydrogen:
nitrogen + hydrogen ⇌ ammonia N₂ + 3H₂ ⇌ 2NH₃
On cooling, the ammonia liquefies and is removed. The remaining hydrogen and nitrogen are recycled.
Two features of the process are there to make it economic rather than to change the equilibrium. Cooling to liquefy the ammonia removes the product as it forms, which by the concentration rule keeps the equilibrium shifting towards more ammonia. And recycling the unreacted nitrogen and hydrogen means that although only some of the gas reacts on each pass, almost none of it is wasted overall.
Common mistake
Ammonia is removed by cooling because it liquefies at a higher temperature than nitrogen and hydrogen do, so it condenses while they stay as gases. It is not filtered off.
- Haber process
- the manufacture of ammonia from nitrogen obtained from the air and hydrogen obtained from natural gas, passed over an iron catalyst at about 450 °C and about 200 atmospheres in a reversible reaction
5.20C–5.21CWhy these conditions are usedHigher tier
Chemistry only — not on the Combined Science specification
Higher tier students should be able to interpret graphs of reaction conditions versus rate, apply the principles of dynamic equilibrium to the Haber process, explain the trade-off between rate of production and position of equilibrium, and explain how the commercially used conditions are related to the availability and cost of raw materials and energy supplies, control of equilibrium position and rate.
Start from what the equilibrium rules predict. The forward reaction is exothermic, so a lower temperature would give a higher yield. There are 4 molecules of gas on the left and 2 on the right, so a higher pressure gives a higher yield.
So the trade-off is really about temperature. A low temperature would give the best yield at equilibrium but take far too long to reach it; a high temperature is fast but gives little ammonia. About 450 °C is chosen because it gives a reasonable yield in a reasonable time — and because the unreacted gases are recycled, a modest yield per pass is acceptable.
The wider commercial argument brings in the other factors. Nitrogen from the air is free, so the cost of raw materials is dominated by the hydrogen and the natural gas it comes from. Energy costs money, so both the heating and the compression are kept no higher than they need to be. And the catalyst is worth its cost because it raises the rate without any energy at all.
Worked example
A graph shows that the yield of ammonia at 200 atmospheres falls from 52% at 350 °C to 28% at 450 °C. Explain why 450 °C is used commercially.
- Read the trend: the yield falls as the temperature rises, which is what is expected for an exothermic forward reaction.
- So on yield alone, 350 °C would be better.
- But rate rises with temperature, and at 350 °C the reaction would be too slow — the equilibrium yield is only reached after a long time.
- 450 °C is a compromise that gives a lower yield per pass but produces ammonia much faster.
- Add the point that makes the compromise work: the unreacted gases are recycled, so the lower yield per pass is not wasted.
Answer: 450 °C gives a lower equilibrium yield than 350 °C, but a much faster rate, so more ammonia is made per day. Because unreacted nitrogen and hydrogen are recycled, the lower yield per pass costs little, and the compromise is worth making.
5.22C–5.24CProduction and uses of NPK fertilisers
Chemistry only — not on the Combined Science specification
Compounds of nitrogen, phosphorus and potassium are used as fertilisers to improve agricultural productivity. NPK fertilisers contain compounds of all three elements.
Industrial production of NPK fertilisers can be achieved using a variety of raw materials in several integrated processes. NPK fertilisers are formulations of various salts containing appropriate percentages of the elements — which is what makes them a formulation: the salts are mixed in measured quantities to give a product with the properties the crop needs.
Ammonia can be used to manufacture ammonium salts and nitric acid, which is where the nitrogen in an NPK fertiliser comes from.
Potassium chloride, potassium sulfate and phosphate rock are obtained by mining, but phosphate rock cannot be used directly as a fertiliser, because it is insoluble — a plant can only take up a compound dissolved in water.
Phosphate rock is treated with nitric acid or sulfuric acid to produce soluble salts that can be used as fertilisers.
You should be able to recall the names of the salts produced when phosphate rock is treated with nitric acid, sulfuric acid and phosphoric acid.
You should also be able to compare the industrial production of fertilisers with laboratory preparations of the same compounds, given appropriate information.
The comparison follows a pattern. In the laboratory an ammonium salt is made in a single batch by titrating ammonia solution with the acid, then crystallising — a small quantity, made by hand, using pure dilute reagents, and taking minutes. Industrially the same salt is made continuously on a very large scale, using concentrated reagents made on site from raw materials, with the heat given out by the reaction recovered rather than wasted, and with the whole process running without interruption.
Common mistake
A fertiliser compound must be soluble in water, or the plant cannot take it up through its roots. That single fact is why phosphate rock has to be treated with acid before it is any use.
- NPK fertiliser
- a formulation of salts containing appropriate percentages of nitrogen, phosphorus and potassium, used to improve agricultural productivity
Before you practise
- Nitrogen comes from the air and hydrogen from natural gas; both are purified before use.
- 450 °C is a compromise: a lower temperature gives a higher yield, but far too slowly to be worth it.
- 200 atmospheres favours ammonia because there are fewer gas molecules on that side, but higher pressures cost more and are less safe.
- The iron catalyst speeds the reaction up without changing the yield.
- Ammonia is condensed out and the unreacted gases are recycled, so nothing is wasted.
- Fertilisers supply nitrogen, phosphorus and potassium — an NPK fertiliser is a formulation containing all three.
- Ammonia + nitric acid → ammonium nitrate, a fertiliser with nitrogen from both reactants.
- The laboratory preparation is a small batch by titration and crystallisation; industry runs continuously, on a huge scale, with several stages.