Ammonia is made from nitrogen and hydrogen in a reversible reaction using an iron catalyst. Four moles of gaseous reactants form two moles of gaseous ammonia.
A plant operates at 200 atmospheres and recycles unreacted nitrogen and hydrogen. A manager proposes reducing the pressure to 100 atmospheres to use less energy for compression.
Why might operating at 100 atmospheres not reduce the cost per tonne of ammonia?
At 100 atmospheres, the equilibrium yield would be higher but the reaction rate would be lower, so less gas would need recycling while ammonia formed more slowly.
At 100 atmospheres, the equilibrium yield and reaction rate would be unchanged, so only the amount of gas supplied would affect the cost.
At 100 atmospheres, using more iron catalyst would restore the equilibrium yield, but the extra catalyst would cost more than the energy saved.
At 100 atmospheres, both the equilibrium yield and reaction rate would be lower, so more gas would need recycling and less ammonia would be made each hour, which could outweigh the saving on compression.
136 exam-style questions on OCR GCSE Chemistry 6.1 Improving processes and products, covering 6.1.1 Extracting metals industrially and by electrolysis, 6.1.2 Life cycle assessment and recycling, 6.1.3 Biological methods of metal extraction, 6.1.4 The Haber process and the production of fertilisers, 6.1.5 Alloys and their uses, 6.1.6 Corrosion and its prevention, 6.1.7 Comparing and selecting materials, and 6.1.8 Industrial conditions, rate and position of equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.