The contact process is the industrial method used for producing large amounts of sulphuric acid. The first stage is to convert sulphur to sulphur dioxide by reacting it with oxygen: S + O2 → SO2 Then, more oxygen is added to turn it to sulphur trioxide: 2SO2 + O2 ⇌ 2SO3 In order to receive to maximum reaction rate with this reversible reaction, certain conditions are required: * 450°C * 1-2 atm * Vanadium Penta-oxide (V2O5) catalyst. (Platinum would be more effective, but it is very expensive) The sulphur trioxide is then reacted with sulphuric acid to produce oleum: SO3 + H2SO4 → H2S2O7 .