Sufuric Acid

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SULFURIC ACID, H₂SO₄

Sulfuric acid is the product of the U.S. chemical industry produced in largest quantity in terms of mass. About 40 million tons are produced annually. There are two major processes used in the production of H₂SO₄, lead chamber process and contact process. The lead-chamber process is the older of the two processes, and its product is aqueous sulfuric acid containing 62% to 78% H₂SO₄. The contact process yields pure sulfuric acid. In both processes, sulfur dioxide, SO₂, is oxidized to sulfur trioxide, SO₃, and the SO₃ is dissolved in water.

Sulfur dioxide is obtained by burning sulfur,

S(s) + O₂(g) SO₂(g)

by roasting pyrite (iron sulfide) or other metal sulfides prior to smelting,

4 FeS(s) + 7 O₂(g) 2 Fe₂O₃(s) + 4 SO₂(g)

or by burning hydrogen sulfide,

H₂S(g) + O₂(g) SO₂(g) + H₂O(g)

The sulfur dioxide is oxidized to sulfur trioxide catalytically.

2 SO₂(g) + O₂(g) 2 SO₃(g)

Without the catalyst the oxidation of SO₂ is quite slow. In the old lead-chamber process, the catalyst is nitrogen dioxide gas. In the contact process, the catalyst is vanadium(V) oxide, V₂O₅, mixed with an alkali metal sulfate. The mixture is supported on small silica beads, and at the high temperature inside the reactor, the mixture is a liquid. The product SO₃ is dissolved in 98% sulfuric acid. The dissolved SO₃ reacts with the 2% water, forming H₂SO₄.

SO₃(g) + H₂O(l) ₂SO₄(l)

Pure sulfuric acid is a colorless, odorless, oily liquid. It freezes at 10.5 °C. It fumes when heated, because some of the H₂SO₄ decomposes to H₂O and SO₃. The H₂O is retained in the liquid, while SO₃ gas is released. Therefore, the concentration of H₂SO₄ decreases, reaching a concentration of 98.33%. This solution boils at 338 °C and is the material sold as "concentrated sulfuric acid." Concentrated sulfuric acid, which is 18M, has a strong affinity for water and is sometimes used as a drying agent. It can be used to chemically remove water from many compounds. It dehydrates sucrose (table sugar), C₁₂H₂₂O₁₁, leaving a spongy black mass of carbon and diluted sulfuric acid. Concentrated sulfuric acid reacts similarly with skin, paper, and other animal and plant matter. When it is mixed with water, a highly exothermic reaction occurs, and the energy released can be enough to heat the mixture to boiling. Therefore, concentrated sulfuric acid must be diluted by adding the acid slowly to cold water while the mixture is stirred to dissipate the heat.

Sulfuric acid has a wide range of uses and plays a part in the production of nearly all manufactured goods. About 65% of the H₂SO₄ produced annually is used in the production of agricultural fertilizers.

TOP 20 CHEMICALS

This list includes the most recent figures for the United States chemical industry. These figures are obtained from government, trade associations reports, and industry estimates. The list includes chemically homogeneous finished products. It does not include minerals which do not require processing, such as salt and sulfur, and petrochemical feedstocks, such as ethane and butane, which are considered products of oil companies.

EXERCISE: For the first ten chemicals in the table, write the formula of the substance in the fourth column. (Look up in the textbook the formulas you don't know. Ethylene dichloride is the trade name for 1,2-dichloroethane.) Then, calculate the number of moles of each of the top ten chemicals. Record these amounts in the fifth column. Then, rank the top ten in order of moles produced, writing this rank in the last column.

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