250 | The History and Use of Our Earth’s Chemical Elements Chlorine is produced commercially by the electrolysis of a liquid solution of sodium chlo-
250 | The History and Use of Our Earth’s Chemical Elements Chlorine is produced commercially by the electrolysis of a liquid solution of sodium chlo-
ride (or seawater), through which process an electric current is passed though the solution (electrolyte).
History In 1774 Carl Wilhelm Scheele experimented with the mineral pyrolusite (manganese
dioxide), which was the major source of manganese metal. He mixed pyrolusite powder with what he called muriatic acid (HCl), which is a form of hydrochloric acid. A pungent, green- ish-yellow gas was produced, but Scheele did not know what it was since most of the gases he worked with were colorless. Scheele is credited with naming the gas chlorine after the Greek word khlôros, which means “green.” Scheele isolated several other elements and compounds, including oxygen, hydrogen fluoride, hydrogen sulfide, and hydrogen cyanide.
Sometime later, Sir Humphry Davy believed that when acid reacted with a metal, the acid was the source of this unknown gas. This belief was at odds with the positions of most of the chemists of the day who believed that the source of the gas was the metals themselves—not the acid. In 1810 Davy declared that the new element was chlorine. He is generally recognized as the discoverer of chlorine because he correctly identified it as a new element. Some scientists of the day claimed that Davy believed the new element was a compound of oxygen and thus misdiagnosed the new element. Most accepted his identification of the new element and used the name proposed by Scheele.
CommonUses In addition to the use of chlorine as an antiseptic for swimming pools and drinking water,
large amounts are used during industrial processes that produce paper, plastics, textiles, dyes, medicines, insecticides, solvents, and some paints. Following are some of the more important
compounds of chlorine used in industries: hydrochloric acid (HCl + H 2 O), table salt (NaCl), chloroform (CHCL 3 ), carbon tetrachloride (CCl 4 ), magnesium chloride (MgCl 2 ), chlorine dioxide (ClO 2 ), potassium chloride (KCl), and lithium chloride (LiCl). Chlorine is used to make plastics such as neoprene and polyvinyl chloride (vinyl). It is used to make insecticides, fireworks, explosives, and paint pigments; pharmaceuticals, chloroform, and chlorofluorocarbons (ClFCs); and chlorohydrocarbons (ClHCs).
ExamplesofCompounds Chlorine forms ionic bonds with almost all the metals and molecular bonds with the
semimetals and nonmetals. With group 1 metals it produces well-known salts when chlorine’s –1 ion combines with this group’s +1 ions (e.g, NaCl, LiCl, and KCl). Group 2 metals have +2 ions and thus, when combined with –1 ions of chlorine, form salts such as magnesium
chloride (MgCl 2 ), calcium chloride (CaCl 2 ), and barium chloride (BaCl 2 ). Chlorine combines with oxygen, producing four different types of oxides:
1. Dichlorine monoxide: Cl 2 O is a reddish gas that, at subzero temperatures, is a dark brown liquid. 2. Chlorine dioxide: ClO 2 is a very explosive yellowish gas. It is so unstable that it must be diluted with a noble gas such as argon. It is used to bleach foods and as antiseptic for swimming pools.
Guide to the Elements |
3. Dichlorine hexoxide: Cl 2 O 6 is an unstable reddish liquid that decomposes into chlorine and chlorine dioxide at room temperatures. 4. Dichlorine heptoxide: Cl 2 O 7 is a colorless liquid that is an excellent oxidizer. It is perchloric acid with the water removed.
Chlorine combines with numerous other elements as well. Hydrogen and chlorine gases are extremely explosive when mixed, and they produce hydrogen chloride (HCl) that, when
dissolved in water, results in hydrochloric acid: H 2 + Cl 2→ 2HCl.
Carbon tetrachloride (CCl 4 ): Decades ago, this compound was mixed with ether and sold as Carbona, a dry-cleaning fluid for clothes. It is no longer permissible to sell or
buy CCL 4 for household use. It is classed as a carcinogen by the U.S. government and is toxic if ingested, inhaled, or absorbed by the skin. Carbon tetrachloride is used to manufacture CFHCs, to fumigate grains to kill insects, and in the production of semi- conductors.
Sodium hypochlorite: NaOCl is a strong oxidizer used in swimming pools, and when diluted to 5.25%, it is known as the laundry bleach Clorox. Phosgene: COCl 2 is a very poisonous gas that was used in combat in the early twentieth century. When not concentrated, it smells like newly cut hay or grass. Chlorinated hydrocarbons: One example, DDT, is an insecticide. It was extensively used in World War II to delouse personnel and to prevent the spread of plague and other insect- borne diseases. Today, its use is restricted because of its toxicity and its very long life. Even though DDT is extremely effective, it is difficult to eliminate in nature. Its restricted use in some third-world countries has resulted in large increases in deaths due to malaria and other insect-borne diseases.
Chloroform (CHCl 3 ): Toxic and carcinogenic if ingested or inhaled over a long period of time. Formerly used as an anesthetic during surgical procedures, it is currently banned for use in cosmetics and items such as toothpaste and cough syrup.
Hazards
A series of chlorofluorohydrocarbons that are used as refrigerants are being phased out of manufacture and use, because of their possible deleterious effects on the ozone layer of the atmosphere. (See the entry on oxygen for more on the ozone layer.)
From time to time, railroad tank cars are involved in accidents that will leak liquid or gaseous chlorine that, when escaping into the air, forms toxic chlorine compounds. This is extremely dangerous, both as a fire hazard and for human health. When water is used to flush away the escaping chlorine, it may end up as hydrochloric acid, which can be hazardous to the water supply and to aquatic life.
Concentrated chlorine gas and many chlorine compounds will oxidize powdered metals, hydrogen, and numerous organic materials and release enough heat to generate fires or explo- sions. Chlorine is constantly evaporating from the oceans and drifting into the atmosphere where it causes a natural depletion of the ozone.
Warning: One should never mix, or use together, chlorine cleaners, such as Clorox, with other cleaning substances containing ammonia. It is a deadly mixture.
252 | The History and Use of Our Earth’s Chemical Elements BROMINE
SYMBOL:BrPERIOD:4 GROUP:17(VIIA) ATOMICNO:35 ATOMICMASS:79.904amu VALENCE:1,3,5,and7 OXIDATIONSTATE:+1,–1,and
+5 NATURALSTATE:Liquid ORIGINOFNAME: NamedfortheGreekwordbromos,whichmeans“stench.” ISOTOPES:Thereareatotalof40isotopesofbromine.Ofthese,onlytwoarestable:Br-79
constitutes50.69%ofthestablebrominefoundonEarth,andBr-81makesup49.31% ofthenaturallyoccurringabundance.Alltheotherisotopesofbromineareradioactive withhalf-livesrangingfrom1.2nanosecondsto16.2hours.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=7
s2,p5
Properties Bromine is a thick, dark-red liquid with a high density. It is the only nonmetallic element
that is a liquid at normal room temperatures. (The other element that is liquid at room temper- atures is the metal mercury.) Bromine’s density is 3.12g/cm 3 , which is three times the density of water. Its vapor is much denser than air, and when it is poured into a beaker, the fumes hug the bottom of the container. Bromine’s melting point is –72°C, and its boiling point is 58.8°C.
Characteristics Bromine is a very reactive nonmetallic element, located between chlorine and iodine in the
periodic table. Bromine gas fumes are very irritating and toxic and will cause severe burns if spilled on the skin.
Bromine is soluble in most organic solvents and only slightly soluble in H 2 O. Liquid bro- mine will attack most metals, even platinum.
AbundanceandSource Bromine is the 62nd most abundant element found on Earth. Although it is not found
uncombined in nature, it is widely distributed over the Earth in low concentrations. It is found in seawater at a concentration of 65 ppm. This concentration is too low for the bromine to be extracted directly, so the salt water must be concentrated, along with chlorine and other salts, by solar evaporation, distillation, or both.
Most of the commercial bromine that is recovered comes from underground salt mines and deep brine wells. A major source is the deep brine wells found in the state of Arkansas
253 and Great Salt Lake of Utah in the United States. This brine contains about one-half percent
Guide to the Elements |
bromine. Chlorine gas is added to hot brine that oxidizes the bromine ions in solution, which is then collected as elemental bromine. It is also commercially produced, along with potash, from evaporation of the high-salt-content water of the Dead Sea, which is 1290 feet below sea level and is located on the borders of the Middle Eastern countries of Israel and Jordan.
History Bromine was used centuries before it was identified. A sea mussel known as the murex
secretes a liquid that was made into an expensive dye known as Tyrian purple. However, the dyemakers were unaware that a compound of bromine was the main ingredient of the dye until the 1900s.
Carl Lowig (1803–1890), a freshman chemistry student in 1825, produced a smelly, dark- reddish liquid substance. He was encouraged to produce more of this substance and to study it. However, his studies delayed his ability to do so. In the meantime, another young chemist, Antoine-Jerome Balard (1802–1876), wrote and presented a paper on his work with bromine in 1826. Therefore, Balard is credited with the discovery of bromine and was given the right to name the new element. Since his new discovery had such a strong odor, he named it “bro- mine” from the Greek word bromos, which means “stench or smell.” Both young chemists went on to distinguished careers in science.
CommonUses
A major use is as a gasoline additive called ethylene dibromide, an antiknock agent that removes lead additives from automobile engines after the combustion of leaded gasoline, thus preventing the lead from forming deposits in the engine. The lead prevents premature burning of the gas/air mixture in the engine’s cylinders that causes the “knocking” sound in the engine. The lead combines with the bromine to form lead bromide, a volatile gas, which is expelled through the car’s exhaust system. Ethylene dibromide is a potent carcinogen. Since the intro- duction of lead free gasoline, this use has declined in importance; recently, other chemicals have been substituted for the lead as an additive in gasoline.
Methyl bromide is used as a pesticide, which is very effective against parasitic nematodes (e.g., hookworms). Silver bromide is used in photography. Compounds of bromide are used as flame-retardants, water purifiers, dyes, and pharmaceuticals. Bromine has many applica- tions when it is combined with organic compounds. For example, it is used as a reagent to study the organic reactions of many other compounds. It is also used as a disinfectant, a fumigant, and a sedative. In the nineteenth century, many people took a “bromide” to ease tension.
ExamplesofCompounds The main oxidation state of bromine is –1, and thus it reacts with group 1 alkali metals,
which have an oxidation state of +1 to form common salt compounds as follows: Sodium (I) bromide (NaBr): Na 1+ + Br 1- → NaBr. Potassium(I) bromide (KBr): K 1+ + Br 1- → KBr. If a metal has more than one oxidation state, bromine tends to react with the lowest value,
as in the following example: