Chlorofluorocarbon
Chlorofluorocarbons (CFC) is an organic compounds that contain carbon, chlorine, and fluorine atoms. CFCs are highly effective refrigerants that were developed in response to the pressing need to eliminate toxic and flammable substances, such as sulfur dioxide and ammonia, in refrigeration units and air conditioners. The most common commercial CFCs, marketed under the trade name Freon, are trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12).
Halons are organic compounds that are similar to CFCs. They contain carbon, fluorine, and bromine and may contain chlorine. Halons have been used primarily as propellants in fire extinguishers. Because of their bromine content they are even more destructive to ozone than CFCs, and an amendment to the Montreal Protocol banned their use by 1994.
Hydrochlorofluorocarbons (HCFCs) are organic compounds that are similar to CFCs but less destructive to ozone. HCFCs consist of carbon, hydrogen, chlorine and fluorine. They are used as replacements for CFCs, but are to be phased out by the year 2020, as specified by the Montreal Protocol as amended, when they are expected to be replaced by hydrofluorocarbons. (HFCs). HFCs are organic compounds that contain hydrogen, carbon and fluorine. HFCs, which do not contain chlorine, do not have any potential for the destruction of ozone, and so are suitable replacements for CFCs.
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[edit] Uses and applications
Commercial CFCs are nonflammable, noncorrosive, nontoxic, and odorless, and their vapor pressures and heats of vaporization made them very suitable for refrigeration applications. They were also widely used as aerosol propellants, cleansing agents for electrical and electronic components, and foaming agents in shipping-plastics manufacturing.
[edit] Effects on Ozone
In the mid-1970s, scientists identified CFCs as the major cause of ozone depletion in the upper atmosphere; this was later confirmed by satellite studies. When CFCs are released into the atmosphere, they move via air currents to altitudes ranging from 15 to 25 miles (25–40 km). There, they are dissociated by ultraviolet light as given by the reaction: CF2Cl2 → CF2Cl + Cl. The resulting free chlorine atoms (Cl) decompose ozone (O3) into oxygen (O2), Cl + O3 → ClO + O2, and are regenerated by interaction with free oxygen atoms (O), ClO + O → Cl + O2. When chlorine is regenerated, it is free to continue to break down other ozone molecules. This process continues for the atmospheric lifetime of the chlorine atom (one to two years), during which it destroys an average of 100,000 ozone molecules.
In other words, when CFCs are released into the atmosphere, they drift up slowly into the stratosphere, where, under the influence of ultraviolet radiation from the Sun, they react with ozone (O3) to form free chlorine (Cl) atoms and molecular oxygen (O2), thereby destroying the ozone layer which protects the Earth's surface from the Sun's harmful ultraviolet rays. The chlorine liberated during ozone breakdown can react with still more ozone, making the CFCs particularly dangerous to the environment. CFCs can remain in the atmosphere for more than a hundred years. Replacements for CFCs are being developed, and research into safe methods for destroying existing CFCs is being carried out.
[edit] Montreal Protocol
The Montreal Protocol on Substances That Deplete the Ozone Layer is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances believed to be responsible for ozone depletion. It is believed that if the international agreement is adhered to, the ozone layer is expected to recover by 2050. Due to its widespread adoption and implementation it has been hailed as an example of exceptional international co-operation. It has been ratified by 196 states.
