Industrial processesIB MYP Chemistry: Revision notes
Section 1
The Haber process
The Haber process makes ammonia, NH₃, from nitrogen and hydrogen. Nitrogen comes from the air, and hydrogen is usually made from natural gas.
N₂ + 3H₂ ⇌ 2NH₃
The reaction is reversible: ammonia can break back down into nitrogen and hydrogen. When the forward and backward reactions go at the same rate, the mixture is at equilibrium.
Conditions: a temperature of about 450 °C, a pressure of about 200 atmospheres and an iron catalyst.
The mixture is cooled so that ammonia turns into a liquid and is removed. The unreacted nitrogen and hydrogen are recycled to the reactor.
A catalyst makes the reaction faster, but it does not give a higher yield. It only makes equilibrium reached sooner.
Section 2
Compromise between rate and yield
The forward reaction is exothermic, so a low temperature gives a higher yield of ammonia, but the reaction is then too slow. A high temperature gives a fast rate but a low yield. About 450 °C is a compromise.
The forward reaction turns 4 gas molecules into 2, so a high pressure gives a higher yield. High pressure also costs more, because the pipes and vessels must be stronger and pumps use more energy, and it is more dangerous. About 200 atmospheres is a compromise.
The yield in one pass is low (about 15% to 20%), but recycling the unreacted gases gives a high overall yield.
Section 3
Fertilisers
Crops take nitrogen compounds out of the soil. Fertilisers replace these so plants can make proteins and grow well. Fertilisers often contain nitrogen, phosphorus and potassium.
Ammonia is used to make fertilisers. For example, ammonia reacts with nitric acid in a neutralisation reaction:
NH₃ + HNO₃ → NH₄NO₃
ammonium nitrate is a nitrogen fertiliser. Fertilisers increase crop yield and help feed more people, but excess fertiliser can wash into rivers and cause algal blooms that use up oxygen and kill fish.
Section 4
The Contact process (outline)
The Contact process makes sulfuric acid, H₂SO₄, in stages:
- Sulfur is burned in air: S + O₂ → SO₂
- Sulfur dioxide is converted to sulfur trioxide: 2SO₂ + O₂ ⇌ 2SO₃, using a vanadium(V) oxide catalyst at about 450 °C
- Sulfur trioxide is absorbed in concentrated sulfuric acid and then diluted with water to make more sulfuric acid
Step 2 is reversible and exothermic, so, as in the Haber process, 450 °C is a compromise between rate and yield.
Section 5
Choosing industrial conditions
Industrial chemists choose conditions by balancing five factors:
- Cost: energy, raw materials, catalysts and equipment
- Rate: the faster the reaction, the more product per hour
- Yield: the more product obtained from the reactants, the less waste
- Sustainability: recycling unreacted materials, using less energy, avoiding pollution
- Safety: high temperatures, high pressures and toxic gases need strong equipment and careful control
In an evaluation, say what each condition gains and what it costs, and then reach a justified conclusion.
For any condition, say what it changes (rate or yield) and what it costs, for example: higher pressure raises the yield but needs stronger, more expensive equipment.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Industrial processes
- Ammonia is made on a large scale in the Haber process. Nitrogen from the air reacts with hydrogen at about 450 °C and 200 atmospheres pressure, over an iron catalyst: N₂ + 3H₂ ⇌ 2NH₃. The mixture leaving the reactor contains ammonia, nitrogen and hydrogen.Explain why the unreacted nitrogen and hydrogen leaving the reactor are recycled.2 marks
- A fertiliser factory in Brazil uses ammonia from the Haber process to make ammonium nitrate, NH₄NO₃, which is sold to farmers who grow maize.Explain why farmers add nitrogen fertilisers to the soil.2 marks
- Sulfuric acid is made on a large scale by the Contact process. Sulfur is burned in air to make sulfur dioxide. The sulfur dioxide is then converted to sulfur trioxide, SO₃, in a reversible reaction using a vanadium(V) oxide catalyst at about 450 °C. Finally the sulfur trioxide is absorbed to make concentrated sulfuric acid.Write balanced symbol equations for the burning of sulfur and for the conversion of sulfur dioxide to sulfur trioxide.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).