Acids are substances that tend to react with bases and metals to form salts and water (with bases) or hydrogen (with metals). Common acids are protonic acids—their molecules can dissociate to release protons (hydrogen ions, H+), which form hydronium ions (H3O+) with water.
Protonic acids include hydrochloric, nitric, and sulfuric acids (HCl, HNO3, and H2SO4) and organic (carbon-based) acids, such as ethanoic acid (acetic acid, CH3COOH). Some organic acids occur in nature; others are synthetic.
Some substances, called Lewis acids, react with bases even though they contain no hydrogen; such substances include aluminum chloride (AlCl3), and they are characterized by their ability to form bonds by accepting electron pairs from Lewis bases, which include ammonia (NH3).
Alkalis are water-soluble substances that release hydroxide ions (OH–) when they dissolve in water and react with acids to form salts and water. Alkalis include the hydroxides of certain metals, notably sodium and potassium hydroxides (NaOH and KOH), and ammonia. They are a subgroup of bases; nonalkaline bases react with acids in the same way as alkalis do, but they do not dissolve in water to any appreciable extent.
Acids and alkalis are important products and raw materials of the chemical industry. The manufacture of caustic soda (sodium hydroxide) was one of the earliest industrial chemical processes, and caustic soda is used to manufacture a wide range of products, including bleach, paper, and soap. Acids, such as phosphoric acid (H3PO4) and sulfuric acid, are important industrial catalysts.
Acid and base strengths
When some acids dissolve in water, they completely dissociate into hydrogen ions and anions. Hydrochloric acid is a strong acid, for example, so it forms a solution of oxonium ions and chloride ions (Cl–) in water (H2O):
Thus a 1 mol/l solution of hydrochloric acid is really a solution of 1 mol/l hydrogen ions and 1 mol/l chloride ions. Acids that dissociate completely in this way are called strong acids.
Not all acids are strong, however. Weak acids remain largely in molecular form when they dissolve in water, and the concentration of hydrogen ions they produce is much less than that produced by strong acids. Ethanoic acid is an example of a weak acid. When it dissolves in water, only a small proportion of ethanoic acid molecules react to form hydronium ions and ethanoate ions according to the following equation:
Approximately four in every thousand molecules dissociate in water at room temperature. In fact, a 1 mol/l solution of ethanoic acid contains only 0.0042 mol/l of hydrogen ions (and the same concentration of ethanoate ions).
Just as acids may be strong or weak, bases (and therefore alkalis) can be classified as strong or weak. Sodium hydroxide is an example of a strong alkali, whereas ammonia is weak.
2SO4
Concentrated sulfuric acid is a clear, oily liquid that is corrosive to skin and to metals. Apart from being fiercely acidic, it is a powerful oxidant.
The principal starting material for sulfuric acid is elemental sulfur—sulfur is one of the few elements that occur uncombined in nature. Sulfur burns in air to produce sulfur dioxide (SO2) in the first stage of manufacture. Alternative starting materials include metal sulfides, which occur as ores or are by-products of industrial processes, such as the desulfurization of natural gas and petroleum. Roasting a metal sulfide in air produces sulfur dioxide and the metal oxide.
The two industrial processes for making sulfuric acid from sulfur dioxide are the chamber process and the contact process. The original and traditional method is the chamber process, so called because sulfur dioxide reacts with air and steam in a lead-lined chamber. Oxides of nitrogen are also present as catalysts. The reaction is complex, on account of the involvement of catalysts, but is basically an oxidation reaction that produces chamber acid—an impure solution of around 65 percent sulfuric acid in water:
The chamber process now produces less than 20 percent of all sulfuric acid, since the process has been largely supplanted by the more efficient contact process (see the box below).
The contact process can produce pure sulfuric acid at any concentration by the addition of the appropriate amount of water. An intermediate product is oleum (H2S2O7), a derivative of sulfuric acid that is intensely corrosive and oxidizing and a useful substance in its own right.
Of the sulfuric acid produced, about one-third goes to make fertilizers. Other important uses are the production of paints, pigments, fibers, detergents, and plastics, as well as pickling—the preparation of metal surfaces by oxide removal.
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SULFURIC ACID—THE CONTACT PROCESS |
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The vast majority of sulfuric acid produced by the chemical industry is made by the contact process. The process is a fine example of how the heat produced in a chemical process can be used to save energy from external sources and of how an appropriate catalyst can improve the efficiency of a reaction—in this case, the oxidation of sulfur dioxide. In the first stage of the contact process, sulfur is melted using steam. The molten sulfur then passes to burners, where it ignites in air that has been dried using concentrated sulfuric acid. The product of the combustion of sulfur in air is sulfur dioxide (SO2). This reaction produces heat, which is used to produce steam in waste-heat boilers. That steam then serves to melt more sulfur and to drive turbines that power compressors that supply the same plant with compressed air. Sulfur dioxide from the first stage passes with air over a catalyst—vanadium pentoxide (V2O5)—in a converter reactor. Much of the sulfur dioxide reacts with oxygen from air in this reactor, producing sulfur trioxide (SO3) and releasing heat. The hot gases produce more steam in a waste-heat boiler before passing to a second converter, where more sulfur trioxide and heat are produced. The gases give up some heat in an economizer boiler before passing through an absorbing tower, where the sulfur trioxide forms oleum (H2S2O7) with sulfuric acid (H2SO4). The oleum can then react with water to form sulfuric acid.
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