Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Contact Process
What is Contact Process?
The industrial manufacture of sulfur trioxide, used to make sulfuric acid, by the reversible oxidation of sulfur dioxide over a vanadium(V) oxide catalyst. Sulfur dioxide is obtained by burning sulfur or by roasting sulfide ores, oxygen comes from the air, and the reaction is run at about 450 degrees Celsius and only 200 kilopascals, which is 2 atmospheres.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Contact Process in context
A reversible reaction, shown with the symbol \(\rightleftharpoons\), can run in both directions at once, and in a closed system it reaches dynamic equilibrium when the forward and reverse rates are equal, so the concentrations of reactants and products stop changing. Changing temperature, pressure or concentration shifts the position of equilibrium, but a catalyst never does — it speeds up both directions equally, reaching the same equilibrium sooner rather than reaching a different one. The Haber and Contact processes each pick operating conditions that trade off rate, yield, safety and cost rather than chasing the highest possible yield alone. In redox terms, oxidation is loss of electrons and reduction is gain of electrons, so an oxidising agent is itself reduced and a reducing agent is itself oxidised.
Common mistakes with Contact Process
- 12. “At equilibrium the amounts of reactants and products are equal.” Why tempting“Equilibrium” and “balance” suggest equality. RepairThe rates are equal; the concentrations are constant. The Contact process sits at equilibrium with about \(98\%\) product. TransferFill the gap: “the concentrations remain ______.” Constant.
- 17. “The Contact process runs at 200 atmospheres.” Why temptingThe two processes share a temperature, so the pressures get shared too. RepairContact runs at \(200\ \mathrm{kPa}\), which is \(2\) atmospheres. Haber runs at \(20\,000\ \mathrm{kPa}\), which is \(200\) atmospheres. Both numbers contain “200”, and the unit is the whole difference. TransferGive both pressures with units, then say why they differ.
Examiner tips on Contact Process
- If you have only one hour before the exam. Read the seven danger zones, the nine comparison matrices, and the five-line specifications of the Haber and Contact processes. Those three pages are the densest summary of the topic in the chapter: the recall facts, the distinctions that are easiest to merge, and the errors that turn correct chemistry into a wrong answer.
Questions students ask about Contact Process
Why do the Haber and Contact processes not use the temperature that gives the highest yield?
Both forward reactions are exothermic, so the temperature that maximises equilibrium yield is a low one, and at a low temperature the rate is too slow for a viable plant. The chosen operating temperature is a compromise between rate, yield, safety and cost, giving the most product per day at an acceptable cost rather than the largest possible percentage yield.