Definition of chlorine and important facts. Assessment of the impact of the chlor-alkali industry on the environment. Description of the chemical process involved in the electrolysis of brine using the Diaphragm cell. Discussion on the economic advantages of chlorine production by the diaphragm cell method. Discussion on the industrial importance of the halogens and their compounds.
•CHLORINE, symbol Cl, greenish-yellow gaseous element. Theelement’s name comes from theGreek word chloros, which means“pale green.” In group 17 (or VIIa)of the periodic table, chlorine isone of the halogens. The atomicnumber of chlorine is 17.•Elementary chlorine was firstisolated in 1774 by the Swedishchemist Carl Wilhelm Scheele,who thought that the gas was acompound; it was not until 1810that the British chemist SirHumphry Davy proved thatchlorine was an element and gaveit its present name.
About 100 to 200 g mercury was lost for each 1000 kg chlorine produced-apparently a small quantity until one realizes that 2 500 000 kg chlorine was produced by mercury cells every day during 1960 in the United States. Thus every 2 to 4 days 1000 kg mercury entered the environment, and by 1970 sizable quantities were being found in fish.
Since 1970 adequate controls have been installed on mercury cells and most new alkali plants use diaphragm cells, but the very large quantities of mercury introduced into rivers and lakes prior to 1970 are expected to remain for a century or more.
Three important chemicals, NaOH, Cl2 and H2, can be obtained by electrolyzing an aqueous NaCl solution (brine). This forms the basis of the chlor- alkali industry, in which the electrolysis is carried out (see fig. 1).
At the cathode, water is reduced: 2H2O + 2e– → H2 + 2OH– (eqn 1a) Chlorine is produced at the anode: 2Cl– → Cl2 + 2e– (eqn 1b) Thus the overall reaction is: 2H2O(l) + 2Cl–(aq) → H2(g) + Cl2(g) + 2OH–(aq) (eqn 1)
Since the H2(g) and Cl2(g) might recombine explosively should they come in contact, the cathode must be entirely surrounded by a porous diaphragm of asbestos. Hence the name of this type of cell.
Cl2(g) produced in Eq. (1) are dried, purified, and compressed into cylinders. Fresh brine is continually pumped into the cell, and the solution which is forced out contains about 10% NaOH together with a good deal of NaCl. [Remember that the spectator ions, Na+(aq), are not included in a net ionic equation such as Eq. (1).] H2O is allowed to evaporate from this solution until the concentration of the solution reaches 50% NaOH, by which time most of the NaCl has crystallized out and can be recycled to the electrolysis.
The NaOH is sold as a 50% solution or further dried to give crystals whose approximate formula is NaOH•H2O. The considerable effort required to concentrate the NaOH solution obtained from diaphragm cells can be avoided by using mercury cells. The cathode in such a cell is mercury, and the cathode reaction is: Na+(aq) + e– + xHg(l) → NaHgx(l)
Halogens and their compounds are very important in the chemical industry. Halogens are used in the chemical, water and sanitation, plastics, pharmaceutical, pulp and paper, textile, military and oil industries.
They are used in a wide range ofproduction including themanufacturing ofchlorofluorocarbons, disinfectants,halogen lamps, oxidants, Teflonand some hydrocarbons.
Bromine, chlorine, fluorine and iodine are chemical intermediates, bleaching agents and disinfectants. Both bromine and chlorine are used in the textile industry for bleaching and shrink- proofing wool.
Bromine is also used in goldmining extraction processes andin oil- and gas-well drilling. It is afire retardant in the plasticsindustry and an intermediate inthe manufacture of hydraulicfluids.
Which Greek word does chlorine come from and what does it mean? How much kg of chlorine was produced by mercury cells every day and in what year was it recorded in the United States? What are the three important chemicals which can be obtained by electrolyzing an aqueous NaCl solution (brine)? What’s another name for the Diaphragm cell? State one industrial importance of halogens and their compounds.
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