Sulfur dioxide to sulfur trioxide?

Question

why in the production of sulfur trioxide an excess of oxygen is required when their is already dry air?

Answer 1

The mixture of sulfur dioxide and air is heated to 450oC and subjected to a pressure of 101.3 - 202.6 kPa (1 -2 atmospheres) in the presence of a vanadium catalyst (vanadium (V) oxide) to produce sulfur trioxide, SO3(g), with a yield of 98%.

2SO2(g) + O2(g) -----> 2SO3(g)

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As a catalyst it is less active and requires high oxygen or sulfur dioxide to give economic conversions

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Answer 2

Sulfur dioxide is a very stable gaseous compound, readily made by just burning solid Sulfur in air (as the Oxygen source).

To convert this stable dioxide to the trioxide using the following equation:

2 SO2 (g) + O2 (g)  --->  2 SO3 (g)

requires a catalyst, excess Oxygen gas and elevated temperatures and pressure for the reaction to proceed to the trioxide.