Experiments involving fuming sulfuric acid are usually done inside sulfuric acid.
Fuming sulfuric acid, more commonly called oleum, is produced by dissolving sulfur trioxide (SO 3 ) in concentrated sulfuric acid (H 2 SO 4 ). This results in a mixture of compounds including sulfuric acid, disulfuric acid (H 2 S 2 O 7), and free sulfur trioxide. Being volatile, sulfur trioxide causes the acid to smoke by absorbing moisture from the air, creating a cloud of tiny droplets of sulfuric acid. Fuming sulfuric acid is produced by the normal industrial process used to make sulfuric acid, and most of it is converted to this chemical. However, a relatively small proportion is retained as oleum and used in the manufacture of explosives, drugs, and dyes.
The industrial process used to make sulfuric acid begins with the oxidation of sulfur dioxide (SO 2 ) to produce sulfur trioxide; this substance can react with water (H 2 O) to produce sulfuric acid. However, this reaction is too violent to be easily controlled, and therefore the sulfur trioxide dissolves in the existing concentrated sulfuric acid to form fuming sulfuric acid. Most of this is then converted to sulfuric acid by carefully adding it to the appropriate volume of water. As long as the oleum is added to the water, and not the other way around, the reaction, although exothermic, is controllable. The remaining oleum can be used for other industrial uses.
Oleum is available in different grades depending on the amount of dissolved sulfur trioxide. Containers will indicate the grade by indicating the proportion of free sulfur trioxide, eg 20%, 30% or 65%. Oleum is normally a vaporous, oily liquid, but some forms are solid at room temperature.
Sulfur trioxide reacts with sulfuric acid to produce disulfuric acid, also called pyrosulfuric acid: SO 3 + H 2 SO 4 → H 2 S 2 O 7 . Pure disulfuric acid is solid at room temperature, but is rarely used industrially or in the laboratory. In the oleum, it exists along with sulfuric acid, free sulfur trioxide, and possibly some more complex molecules.
Fuming sulfuric acid is an even more powerful dehydrating agent than sulfuric acid. Reacts violently with water, releasing a lot of heat and acid unless added to water slowly. Like sulfuric acid, it removes water from carbohydrates and leaves behind carbon, thereby charring paper, wood, and many other organic materials. This reaction releases so much heat that it can cause combustion.
One of the main applications of fuming sulfuric acid is in nitration reactions. It mixes with nitric acid to produce nitronium ions (NO 2 + ) that add nitro groups (NO 2 ) to organic compounds. For many of these reactions, it is essential that no water be present. This is usually achieved by mixing commercial nitric acid -68.5%- with fuming sulfuric acid so that the latter absorbs all the water. Nitration reactions are important in the production of explosives and dyes.
Oleum is also used as a sulfonating agent in organic chemistry. This means that you can add a sulfonic acid group (SO 3 H) to an organic compound. Sulfonated compounds include important drugs such as sulfonamides, as well as detergents and dyes.
Due to its dehydrating properties, violent reaction with water, and volatility, fuming sulfuric acid is a very dangerous chemical to work with. Causes severe burns on contact with skin and inhalation of vapors can cause severe damage to the respiratory system. In laboratories, experiments with this acid are usually carried out inside a fume hood.