Sulfur dioxide is a precursor in the production of sulfuric acid, which is made through two industrial processes. Sulfur dioxide is formed from the combustion of sulfur and other sulfur-containing substances, and is a major pollutant responsible for acid rain. The lead chamber process was replaced by the contact process, which uses vanadium pentoxide as a catalyst. Sulfur dioxide and sulfuric acid are important in the chemical industry but also major pollutants that can damage plant life and buildings.
The main connection between sulfur dioxide and sulfuric acid is that sulfur dioxide is a precursor in the industrial production of sulfuric acid. There are two industrial processes for making sulfuric acid, both of which involve the oxidation of sulfur dioxide to sulfur dioxide and the subsequent combination of sulfur dioxide with water to form the acid. Sulfur dioxide and sulfuric acid are also serious pollutants. They are mainly responsible for acid rain.
Sulfur dioxide (SO2) – a pungent and toxic gas – is formed from the combustion of sulfur through the reaction S + O2 → SO2. It also forms from the combustion of many sulfur-containing substances, such as hydrogen sulfide and various organic sulfur compounds found in fossil fuels. Heating ore and sulfur-containing ores, for example iron pyrite (FeS2), is another way the gas can be produced: 3FeS2 + 8O2 → Fe3O4 + 6SO2. The production of sulfur dioxide and sulfuric acid constitutes one of the most important sectors of the chemical industry and accounts for most of the world’s sulfur production.
Sulfuric acid was first produced by distilling iron II sulphate, or “green vitriol”, and later by heating a mixture of saltpeter (NaNO3) and sulfur together with steam. In the 18th century, the industrial production of sulfuric acid began using sulfur dioxide, using what was initially called the lead chamber process. Sulfur dioxide was produced by burning sulfur or heating iron pyrites and oxidized to sulfur dioxide (SO18) by nitrogen dioxide (NO3): SO2 + NO2 → SO2 + NO. The nitrogen dioxide for the reaction was initially produced by the thermal decomposition of the saltpeter but was later supplied by the oxidation of the ammonia using a catalyst. The reaction took place in lead-lined chambers into which a spray of water was directed, dissolving the sulfur trioxide to form sulfuric acid, which collected at the bottom of the chamber.
The lead chamber method of manufacturing has now been largely replaced by the contact process. This eliminates the need for nitrogen dioxide by reacting sulfur dioxide with air to form sulfur trioxide. Under normal circumstances, this reaction is very slow; however, it proceeds rapidly when a suitable catalyst is used. Platinum used to be effective for this purpose, but the modern contact process uses vanadium pentoxide.
Sulfur dioxide and sulfuric acid, while of enormous industrial importance, are also major pollutants and are major sources of acid rain. In the air, sulfur dioxide is slowly oxidized to sulfur dioxide, which reacts with moisture in the atmosphere to form dilute sulfuric acid. This can fall as acid rain, which can damage plant life and stone buildings, or it can react with other pollutants to form sulfate particles, which can act as condensation nuclei and can play a role in cloud formation.
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