In the Contact process, sulfur trioxide is made from sulfur dioxide and oxygen. The forward reaction is exothermic.
2SO2(g)+O2(g)⇌2SO3(g) \mathrm{2SO_{2}(g) + O_{2}(g) \rightleftharpoons 2SO_{3}(g)} 2SO2(g)+O2(g)⇌2SO3(g)The process uses a pressure of only 1 to 2 atmospheres. At this pressure, more than 99% of the sulfur dioxide is converted into sulfur trioxide.
Why is a much higher pressure not used?
A higher pressure would decrease the equilibrium yield of sulfur trioxide.
The yield is already very high, so the small increase would not cover the extra cost of high-pressure equipment and energy.
The catalyst stops working at high pressure, so the reaction would become too slow.
A higher pressure would decrease the rate of reaction, because the molecules would have less room to move.
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.