Chlorofluorocarbons decomposition

Note Depletion of the ozone layer in the stratosphere, which acts as a shield against penetration of UV light in the sun s rays, is beheved to be caused by light-induced chlorofluorocarbon decomposition resulting from increased use of halocarbon aerosol propellants. Their manufacture and use were prohibited in 1979, except for a few specialized items. 

Most chlorofluorocarbons are hydrolytically stable, CCI2F2 being considerably more stable than either CCl F or CHCI2F. Chlorofluoromethanes and ethanes disproportionate in the presence of aluminum chloride. For example, CCl F and CCI2F2 give CCIF and CCl CHCIF2 disproportionates to CHF and CHCl. The carbon—chlorine bond in most chlorofluorocarbons can be homolyticaHy cleaved under photolytic conditions (185—225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

The chlorine atoms that catalyze the decomposition of ozone come from chlorofluorocarbons (CFCs) used in many refrigerators and air conditioners. A major culprit is CF2CI2, Freon, which forms Cl atoms when exposed to ultraviolet radiation at 200 nm ... 

C15-0130. If chlorofluorocarbons catalyze O3 decomposition, which is exothermic by 392 kJ/mol, they must also catalyze O3 production from O2. Using the energies shown in Figure 15-20. explain why catalysis results in net O3 destruction even though reactions in both directions are accelerated. 

You probably know that compounds called chlorofluorocarbons (CFCs) are responsible for depleting the ozone layer in Earth s stratosphere. Did you know, however, that CFCs do their destructive work by acting as homogeneous catalysts Use the Internet to find out how CFCs catalyze the decomposition of ozone in the stratosphere. To start your research, go to the web site above and click on Web Links. Communicate your findings as a two-page press release. 

The chlorofluorocarbons (CFCs) have very long lifetimes in the troposphere. This is a consequence of the fact that they do not absorb light of wavelengths above 290 nm and do not react at significant rates with 03, OH, or N03. In addition to the lack of chemical sinks, there do not appear to be substantial physical sinks thus they are not very soluble in water and hence are not removed rapidly by rainout. While laboratory studies have shown that some of the CFCs decompose on exposure to visible and near-UV present in the troposphere when the compounds are adsorbed on siliceous materials such as sand (Ausloos et al., 1977 Gab et al., 1977, 1978), the lifetimes for CFC-11 and CFC-12 with respect to these processes have been estimated to be 540 and 1800 years, respectively (National Research Council, 1979). Similarly, an observed thermal decomposition when adsorbed on sand appears to be an insignificant loss process under atmospheric conditions. 

An explanation begins with chlorofluorocarbons such as Freon-12 (CC12F2), formerly used in refrigerators and air conditioners. These long-lived compounds, which are not found in nature,2 diffuse to the stratosphere, where they catalyze ozone decomposition. 

R. S. Stolarski, The Antarctic Ozone Hole, Scientific American, January 1988. The 1995 Nobel Prize in Chemistry was shared by Paul Crutzen, Mario Molina, and F. Sherwood Rowland for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone. Their Nobel lectures can be found in P. J. Crutzen, My Life with 03, NO, and Other YZO Compounds, Angew. Chem. lnt. Ed. Engl. 1996,35, 1759 M. J. Molina, Polar Ozone Depletion, ibid., 1779 F. S. Rowland, Stratospheric Ozone Depletion by Chlorofluorocarbons, ibid., 1787. 

Chlorofluorocarbons, such as CF3C1, catalyze this reaction and are responsible for the formation of the ozone hole. The decomposition is a chain reaction involving chlorine atoms as the chain-carrying species. Suggest a mechanism for this reaction. 

The compounds called chlorofluorocarbons, or Freons, were found to be very valuable as refrigerants and as cleaning agents for circuit boards. Unfortunately, in the atmosphere these compounds produce chlorine atoms that catalyze the decomposition of the ozone that protects the earth from ultraviolet radiation. Two of these compounds have the following mass percentages ... 

Nitrous oxide, N2O, is commonly used as a mild dental anesthetic and propellant for aerosols on atmospheric decomposition, it yields its innocuous parent gases and is therefore an environmentally acceptable substitute for chlorofluorocarbons. On the other hand, N2O contributes to the greenhouse effect and is increasing in the atmosphere. Nitric oxide, NO, is an effective coordinating ligand its function in this context is discussed in Chapter 13. It also has many biological functions, discussed in Chapter 16. 

Absorption of ultraviolet radiation by O3 causes it to decompose to O2. In the upper atmosphere, therefore, a steady-state concentration of ozone is achieved, a concentration ordinarily sufficient to provide significant ultraviolet protection of the Earth s surface. However, pollutants in the upper atmosphere such as nitrogen oxides (some of which occur in trace amounts naturally) from high-flying aircraft and chlorine atoms from photolytic decomposition of chlorofluorocarbons (from aerosols, refrigerants, and other sources) catalyze the decomposition of ozone. The overall processes governing the concentration of ozone in the atmosphere are extremely complex. The following reactions can be studied in the laboratory and are examples of the processes believed to be involved in the atmosphere ... 

Although nonflammable, when heated to decomposition chlorofluorocarbons emit toxic fumes containing phosgene and fluorides. Although not as volatile as dichlorodifluoroethane or dichlorotetrafluoroethane, trichloromonofluoromethane should be handled as indicated above. Since it is a liquid at room temperature, caution should be exercised in handling this material to prevent spillage onto the skin. It is an irritant to the eyes. 

Exposure limits have been defined for land, sea and air the National Academy of Sciences gives a 90-day atmospheric limit of 0.05 p.p.m. in submarines, and a Douglas aircraft continuous atmospheric limit of 0.04 p.p.m. [40]. These recommendations exist because of the possibility of producing phosgene as a result of the decomposition of chlorofluorocarbon refrigerants used in air-conditioning systems. 

Although many refrigerants, propellants and anaesthetics are non-toxic, in the presence of a flame or on contact with very hot surfaces, phosgene may be produced as one of the toxic decomposition products. The chlorofluorocarbons, CCI3F (CFC-11), CQjF (CFC-12) and CjCljF, (CFC-114), when exposed to a gas flame, all generate phosgene as one of the... 

The carbon-chlorine bond in most chlorofluorocarbons can be homolytically cleaved under photolytic conditions (185-225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

Plasma Decomposition of Freons (Chlorofluorocarbons) and Other Waste Treatment Processes Organized in Thermal and Transitional Discharges... 

Ozone (O3) exists in the atmosphere, 20-40 km above sea level. Ozone absorbs UV radiation which would otherwise harm living things. The effects of ozone loss could include increased human cataracts and skin cancer, reduction of plankton in ocean waters and destruction of plants, including crops. Ozone layer destruction in the Antarctic was reported in 1985 a major cause of this was believed to be the release of chlorofluorocarbon compounds, such as CCI2F2 (CFCs). These compounds are chemically unreactive, non-toxic and odourless, properties which at one time caused them to be used as solvents, aerosol propellants, refrigerant fluids and blowing agents for expanded plastic foams. They are so stable, however, that they persist in the atmosphere for years and eventually enter its upper layers, where they are broken down by the powerful UV radiation emitted by the sun. Their decomposition products can then destroy ozone ... 

The decomposition of the chlorofluorocarbon products a highly reactive chlorine atom (free radical). This reacts with ozone ... 

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