Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Temperature
The forward reaction is exothermic, so lower temperature gives higher sulfur trioxide equilibrium yield. About 450 °C is used as a compromise between high yield and fast rate.
Pressure
Higher pressure favours sulfur trioxide because three gas moles form two. However, the equilibrium yield is already high at modest pressure, so the small extra benefit does not justify expensive high-pressure equipment. A typical pressure is about 200 kPa or 2 atm.
Catalyst
Vanadium(V) oxide, V2O5, increases rate by lowering activation energy. It does not alter equilibrium yield. The catalyst enables an adequate rate at the moderate temperature.
Economics and environment
Efficient heat exchange can use energy released by the exothermic reaction. High conversion and recycling reduce sulfur dioxide emissions. Safe containment is important because sulfur oxides and concentrated sulfuric acid are hazardous.
Practical or data skill
Compare hypothetical Contact-process conditions and recommend an operating set using yield, rate, energy, pressure cost and emissions.
Examination tip
Do not copy Haber pressure into the Contact process: Contact uses only about 2 atm.
Review questions and suggested answers
Question 1
What catalyst is used?
Suggested answer
Vanadium(V) oxide.
Question 2
Why is very high pressure not used?
Suggested answer
The small extra yield does not justify the equipment and energy cost.
Question 3
State the typical temperature and pressure.
Suggested answer
About 450 °C and 200 kPa or 2 atm.
