Contact Process
Definition and meaning of Contact Process in chemistry.
Contact process is the industrial method for manufacturing sulfuric acid by catalytically oxidizing sulfur dioxide to sulfur trioxide, which is then converted to the acid.
In more detail
Sulfur (or pyrite) is burned in air to form sulfur dioxide, which is passed over a vanadium(V) oxide catalyst at about 450°C and 1-2 atm to establish the equilibrium 2SO2 + O2 ⇌ 2SO3. This moderate temperature balances reaction rate against the exothermic equilibrium's yield, per Le Chatelier's principle, while excess air pushes conversion above 99%. The SO3 is not dissolved directly in water, since that reaction is so violently exothermic it produces a corrosive acid mist; instead it is absorbed into concentrated sulfuric acid to form oleum (fuming sulfuric acid), which is then carefully diluted to give sulfuric acid of the desired concentration.
Key facts
| Product | H2SO4 (sulfuric acid) |
|---|---|
| Catalyst | V2O5 (vanadium(V) oxide) |
| Typical conditions | ~450°C, 1-2 atm |
| Field | Inorganic Chemistry |
A chemical plant burns sulfur to produce SO2, oxidizes it over V2O5 catalyst to SO3, absorbs the SO3 into 98% H2SO4 to form oleum, and dilutes the oleum with water to yield concentrated sulfuric acid for fertilizer manufacture.
Frequently asked questions
Why is SO3 absorbed into sulfuric acid instead of water?
Direct reaction of SO3 with water is extremely exothermic and forms a fine, hard-to-condense acid mist. Absorbing SO3 into concentrated H2SO4 instead forms oleum (H2S2O7), which is then safely diluted with water to give sulfuric acid.
Why use V2O5 instead of platinum as the catalyst?
Vanadium(V) oxide is far cheaper and more resistant to poisoning by arsenic and other impurities in the feed gas, even though platinum is intrinsically more active.