Notice that a chlorine atom is used up in the first reaction but a chlorineatom is formed again by the second reaction. Therefore, the amount of chlo-rine does not change as the overall process occurs. This means that thechlorine atom is a true catalyst: it participates in the process but is not con-sumed. Estimates show that one chlorine atom can catalyze the destruction ofabout one million ozone molecules per second.
The chlorine atoms that promote this damage to the ozone layer are pre-sent because of pollution. Specifically, they come from the decompositionof compounds called Freons, such as CF2Cl2, which have been widely usedin refrigerators and air conditioners. The Freons have leaked into the at-mosphere, where they are decomposed by light to produce chlorine atomsand other substances. As a result, the manufacture of Freons was banned by
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C H E M I S T R Y I N F O C U S
Protecting the Ozone
Chlorofluorocarbons (CFCs) are ideal compoundsfor refrigerators and air conditioners because theyare nontoxic and noncorrosive. However, the chem-ical inertness of these substances, once thought tobe their major virtue, turns out to be their fatalflaw. When these compounds leak into the atmos-phere, as they inevitably do, they are so unreactivethey persist there for decades. Eventually theseCFCs reach altitudes where ultraviolet light causesthem to decompose, producing chlorine atoms thatpromote the destruction of the ozone in thestratosphere (see discussion in Section 16.2).Because of this problem, theworld’s industrialized nations havesigned an agreement (called theMontreal Protocol) that bannedCFCs in 1996 (with a 10-year graceperiod for developing nations). Sowe must find substitutes for theCFCs—and fast.
In fact, the search for substi-tutes is now well under way.Worldwide production of CFCs hasalready decreased to half of the1986 level of 1.13 million metrictons. One strategy for replacingthe CFCs has been to switch tosimilar compounds that containcarbon and hydrogen atoms sub-stituted for chlorine atoms. Forexample, the U. S. appliance industry has switched from Freon-12 (CF2Cl2) to the compoundCH2FCH3 (called HFC-134a) forhome refrigerators, and most ofthe new cars and trucks sold in the
United States have air conditioners that employHFC-134a. Converting the 140 million autos cur-rently on the road in the United States that useCF2Cl2 will pose a major headache, but experiencesuggests that replacement of Freon-12 with HFC-134a is less expensive than was originally feared.For example, Volvo Cars of North America esti-mates that a Volvo can be converted from Freon-12to HFC-134a for around $300.
A related environmental issue involves replac-ing the halons for firefighting applications. In par-ticular, scientists are seeking an effective replace-
ment for CF3Br (halon-1301), thenontoxic “magic gas” used toflood enclosed spaces such asoffices, aircraft, race cars, and mil-itary tanks in case of fire. Thecompound CF3I, which appears tohave a lifetime in the atmosphereof only a few days, looks like apromising candidate but muchmore research on the toxicologyand ozone-depleting properties ofCF3I will be required before itreceives government approval asa halon substitute.
The chemical industry hasresponded amazingly fast to theozone depletion emergency. It isencouraging that we can act rapid-ly when an environmental crisisoccurs. Now we need to get betterat keeping the environment at ahigher priority as we plan for thefuture.
An Amana refrigerator, one ofmany appliances that now useHFC-134a. This compound isreplacing CFCs, which lead tothe destruction of theatmospheric ozone.
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