Sulfur dioxide is an inorganic, heavy, colourless, and poisonous gas. It is produced in huge quantities in the intermediate steps of sulfuric acid manufacturing. Sulfur dioxide contains an irritating, pungent odour, familiar as the just-struck match smell. Occurring in nature in solution in the waters of some warm springs and volcanic gases, sulfur dioxide can usually be industrially prepared by the burning in the oxygen of sulfur or air or such compounds of sulfur as copper pyrite or iron pyrite. It has the chemical formula as SO2.
Structure and Bonding
SO2 is a bent molecule with the C2v symmetry point group. A valence bond theory approach by considering simply s and p orbitals would define the bonding in terms of resonance between the two resonance structures.
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The sulfur–oxygen bond holds a bond order of 1.5. There is support for this simple approach that does not invoke the participation of d-orbital. In terms of electron-counting formalism, sulfur atoms contain a formal charge of +1 and an oxidation state of +4.
This is found on Earth and exists in the atmosphere and very smaller concentrations at about 1 ppm.
On the other planets, this compound can be found in different concentrations, the most significant being the Venus atmosphere, which is the third-most significant atmospheric gas at 150 ppm. There, it condenses in the formation of clouds, and is a key component of chemical reactions in the atmosphere of the planet, and contributes to global warming. It also has been implicated as a key agent in the early Mars warming, with concentration estimates in the lower atmosphere as high as 100 ppm, though it exists only in trace amounts. As on Earth, on both Mars and Venus, its major source is thought to be volcanic. The Io-atmosphere, a natural satellite of Jupiter, is 90% sulfur dioxide, and the trace amounts are also thought to exist in the Jupiter atmosphere.
It is thought to exist as a block of ice in abundance on the Galilean moons—as subliming frost or ice on the Io’s trailing hemisphere, and in the crust and mantle of Europa, Callisto, and Ganymede, also possibly in liquid form and reacting readily with water.
Primarily, sulfur dioxide is produced for the manufacturing of sulfuric acid. In the United States, in the year 1979, 23.6 million tonnes of sulfur dioxide were used in the same way, compared to 150 thousand tonnes, which is used for other purposes. Most of the sulfur dioxide is produced by elemental sulfur combustion. Some quantity of sulfur dioxide can also be produced by roasting pyrite and other sulfide ores in the air.
Sulfur dioxide is a reducing agent, featuring sulfur in the oxidation state of +4. It is oxidized by halogens to form sulfuryl halides, like sulfuryl chloride. The chemical reaction is given as follows.
SO2 + Cl2 → SO2Cl2
Sulfur dioxide is considered one of the few common acidic yet reducing gases. Being acidic, this compound turns moist litmus pink, then white (because of its bleaching effect). It can be identified by bubbling it through the dichromate solution and turning the solution to the green from orange (Cr3+ (aq)). It also reduces ferric ions to ferrous.
Sulfur dioxide reacts with certain 1,3-dienes in a cheletropic reaction to produce cyclic sulfones. On an industrial scale, this reaction is exploited for sulfolane synthesis, which is an essential solvent in the petrochemical industry.
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The overarching and dominant use of sulfur dioxide is in the formation of sulfuric acid.
Precursor to Sulfuric Acid
Sulfur dioxide acts as an intermediate in the formation of sulfuric acid, being converted to sulfur trioxide, and then to the oleum, which can be made into the sulfuric acid. For this purpose, sulfur dioxide is made when the sulfur combines with oxygen. The conversion of sulfur dioxide to the sulfuric acid method is known as the contact process. Many billion kilograms are produced for this purpose annually.
As a Reducing Agent
Sulfur dioxide can also be a good reductant. It is also able to decolourize substances in the presence of water. Particularly, it is useful to reduce bleach for delicate materials such as clothes and papers. Normally, this bleaching effect does not last much longer. Oxygen reoxidizes the reduced dyes in the atmosphere by restoring the colour. Sulfur dioxide is used in the municipal wastewater treatment to treat chlorinated wastewater before release. It also reduces combined and free chlorine to chloride.
Sulfur dioxide injections in the stratosphere have been proposed in climate engineering. The sulfur dioxide cooling effects would be the same as what has been observed after the large explosive eruption of Mount Pinatubo in 1991. However, this geo-engineering form would have uncertain regional consequences on the patterns of rainfall, for example, in the monsoon regions.
As a Refrigerant
Being condensed and possessing a high heat of evaporation easily, sulfur dioxide can be a candidate material for refrigerants. Before the chlorofluorocarbons development, sulfur dioxide was used as a refrigerant in home refrigerators.