Where the raw materials come from, the conditions used and why each is a compromise, and how ammonia is turned into NPK fertilisers. Chemistry only content.
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1.What does the Haber process make?
Ammonia, which is used to make nitrogen based fertilisers.
2.Where does the nitrogen come from?
From the air, which is about four fifths nitrogen.
3.Where does the hydrogen come from?
Mainly from natural gas.
4.Write the equation for the Haber process.
N2 + 3H2 gives 2NH3, and the reaction is reversible.
5.What temperature is used?
About 450 degrees Celsius.
6.What pressure is used?
About 200 atmospheres.
7.What catalyst is used?
Iron.
8.Is the forward reaction exothermic or endothermic?
Exothermic.
9.Why is the temperature a compromise?
A lower temperature would give a higher yield because the forward reaction is exothermic, but the reaction would be too slow to be economic.
10.Why is the pressure a compromise?
A higher pressure gives a higher yield because there are fewer gas molecules on the product side, but very high pressures need expensive, strong equipment and are dangerous.
11.Why does high pressure favour ammonia?
There are 4 molecules of gas on the left and only 2 on the right, so the equilibrium shifts right to oppose the increase in pressure.
12.What does the catalyst do to the yield?
Nothing. It only speeds up the rate at which equilibrium is reached.
13.How is the ammonia separated from the mixture?
The mixture is cooled so the ammonia liquefies and is removed, while the unreacted gases remain as gases.
14.What happens to the unreacted nitrogen and hydrogen?
They are recycled back into the reactor so nothing is wasted.
15.Why does removing the ammonia improve the process?
It shifts the equilibrium to the right to replace what was removed, so more reactant is converted overall.
16.What three elements do NPK fertilisers provide?
Nitrogen, phosphorus and potassium.
17.Why do plants need these elements?
They are essential for growth. Nitrogen in particular is needed to make proteins.
18.What is an NPK fertiliser?
A formulation containing compounds of all three elements in measured proportions.
19.How are ammonium salts made for fertiliser?
By reacting ammonia with an acid. Ammonia and nitric acid give ammonium nitrate.
20.How is potassium supplied?
Potassium chloride and potassium sulfate are mined and used directly.
21.Why can phosphate rock not be used directly?
It is insoluble, so plants cannot take it up. It must be reacted with an acid first.
22.What does phosphate rock and nitric acid produce?
Phosphoric acid and calcium nitrate, which are then treated further.
23.What does phosphate rock and sulfuric acid produce?
Single superphosphate, a mixture of calcium phosphate and calcium sulfate.
24.What does phosphate rock and phosphoric acid produce?
Triple superphosphate, which is calcium phosphate.
25.How does industrial production differ from a school preparation?
Industry works continuously on an enormous scale with recycling of unreacted material, whereas a school preparation is a small single batch.
26.Why is the Haber process so important?
It allows fertilisers to be made on a scale that supports food production for the world population.
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