CFC molecules

Susan Solomon and James Anderson showed that CFCs produce chlorine atoms and chlorine oxide under the conditions of the ozone layer and identified the CFCs emanating from everyday objects, such as cans of hair spray, refrigerators, and air conditioners, as the primary culprits in the destruction of stratospheric ozone. The CFC molecules are not very polar, and so they do not dissolve in rain or the oceans. Instead, they rise to the stratosphere, where they are exposed to ultraviolet radiation from the Sun. They readily dissociate in the presence of this radiation and form chlorine atoms, which destroy ozone by various mechanisms, one of which is... 

The net reaction for this two-step mechanism is the conversion of an O3 molecule and an oxygen atom into two O2 molecules. In this mechanism, chlorine atoms catalyze ozone decomposition. They participate in the mechanism, but they do not appear in the overall stoichiometry. Although chlorine atoms are consumed in the first step, they are regenerated in the second. The cyclical nature of this process means that each chlorine atom can catalyze the destruction of many O3 molecules. It has been estimated that each chlorine atom produced by a CFC molecule in the upper stratosphere destroys about 100,000 molecules of ozone before it is removed by other reactions such as recombination CF2 Cl -b Cl CF2 CI2... 

CFCs are nearly ideal substances for attacking ozone molecules and damaging the ozone layer. On the one hand, they tend to be very stable, even in the stratosphere. Many CFCs have half-lives of 100 years or more that means that once they have escaped into the upper atmosphere, they are likely to remain there for very long periods. On the other hand, some small number of CFC molecules do dissociate to form chlorine free radicals, with the ability to destroy ozone molecules. Although the number of CFC molecules that do dissociate is relatively small, the actual number is not important since chlorine free radicals that are generated in the process are used over and over again. That is, they are catalysts in the destruction of ozone and are not, themselves, used up in their reactions with ozone molecules. 

This set of reactions shows that ultraviolet radiation strikes a CFC molecule removing a chlorine atom. The chlorine atom collides with an ozone molecule and bonds with one of ozone s oxygen atoms. The result is the formation of chlorine monoxide, CIO, and molecular oxygen. Chlorine monoxide is a... 

Estimates are that CFCs are so stable that they remain in the atmosphere from 80 to 120 years, and they are now spread throughout the atmosphere. Even in the most remote location, there are no fewer than 25 trillion CFC molecules in every liter of air you breathe Molina. Rowland, and Crutzen realized that CFC molecules reaching the stratosphere are Iragmented when exposed to the harsh ultraviolet rays at this altitude, as illustrated in Figure 17.18. One of these fragments is atomic chlorine, which can catalyze the destruction of ozone. One chlorine atom, it is now estimated, can cause the destruction of at least 100,000 ozone molecules in the one or two years before the chlorine forms a hydrogen chloride molecule, HC1, and is carried away by atmospheric moisture. 

How many moles of CFC molecules are there in every liter of air you breathe, and what percentage of air is this ... 

You would need to know the number of air molecules in a liter of air. You would divide the number of CFC molecules by the number of air molecules and multiply by 100 to get the percentage of CFC molecules. 

Now we turn to the origin of the rate laws themselves. How do molecules of ozone change into molecules of oxygen What atomic interactions turn a mixture of fuel and air into carbon dioxide and water when it ignites in an engine What is really going on in terms of atoms high in the atmosphere as CFC molecules punch holes in the ozone layer Chemical kinetics and particularly the rate law for a reaction provide the kind of information we need to build a model of the reaction at a molecular level. 

Look at the overall result of the above reaction sequence. Chlorine atoms are used up in the first step but are regenerated in the third step, so they don t appear in the net equation. Thus, the net sequence is a never-ending chain reaction, in which the generation of just a few chlorine atoms from a few CFC molecules leads to the destruction of a great many ozone molecules. 

CFC-12. CFCs escape into the atmosphere and, because of their inertness, remain without further reaction until they reach the stratosphere and the ozone layer. In the stratosphere the high-energy ultraviolet radiation causes a chlorine atom to split off from the CFC molecule. This chlorine atom, or free radical, then reacts with the ozone. 

CFCs released from a can of hairspray do not just disappear, the researchers found. They float up high into the ozone layer, where they are broken apart by ultraviolet radiation. The wreckage of a CFC molecule contains highly active chlorine atoms that can destroy ozone. 

Step [1] Bond cleavage in a CFC molecule forms two radicals. 

The immediate cause of the depletions of stratospheric ozone is thought to involve atoms of chlorine or simple compounds such as chlorine monoxide (CIO). However, these chemicals are thought to have an indirect origin through human activities, especially the emission of CFCs to the atmosphere. Once formed in the stratosphere by the degradation of a CFC molecule, a single chlorine atom is capable of destroying as many as 100,000 ozone molecules before it is removed from the upper atmosphere. 

When a CFC molecule breaks apart, it forms a single chlorine atom CFG sunlights CFC + Cl... 

The CFC indicates a CFC molecule without one chlorine atom.) The chlorine formed in this reaction can react with an ozone (O3) molecule ... 

On page 845, study the three equations that describe the photodissociation of a CFC molecule, destruction of an ozone molecule, and the regeneration of a free chlorine atom. 

In Section 4.6.4, the role of CFCs in stratospheric ozone destruction was discussed. CFCs also are of concern because they are radiatively active in portions of the infrared spectrum not strongly attenuated by water vapor, C02, CH4, or N20. Currently, a CFC molecule added to the atmosphere absorbs about 10,000 times as much long-wave infrared radiation as does a C02 molecule. C02 has a radiative forcing of 1.8 X 10-5 W/(m2 ppb(v)), whereas CFCs range from 0.22 to 0.32 W/(m2 ppb(v)) (Prather et al., 1996). CFCs also have long atmospheric residence times, ranging from 50 to 1700 years. The locations of some CFC absorbance bands are shown in Fig. 4-42. Unlike the several radiatively active trace gases that have both natural and... 

Explain why the radiant energy found in the troposphere is unable to liberate chlorine atoms from CFC molecules but the radiant energy in the stratosphere is able to do this. 

They either dissolve in the clouds and are rained out, or they react chemically to be converted into other substances. Neither of these mechanisms are important for CFCs. Chlorofluorocarbons are insoluble in water, and they are so stable that they can exist in the lower atmosphere for years. During this time, the CFC molecules wander around in the atmosphere, moving wherever the air currents take them. They can eventually make their way up into the stratosphere. 

When CFC gases are released into the atmosphere, they gradually rise into the lower stratosphere because they are not washed out of the troposphere by rain. In the stratosphere the chlorine atoms are released when the CFC molecules are broken up by energetic UV-C radiation as indicated by the reaction ... 

SCHEME 10.3 The three steps of the chain reaction by which CFCs, such as CF2CI2, destroy ozone. Initially, the CFC molecule absorbs UV radiation and generates two radicals. In step (b), the CT radical destroys an ozone molecule, and this reaction forms a new radical, CIO, which combines with O in step (c) and regenerates the Cl radical. The newly generated CT radical repeats step (b). And the chain process continues, in principle indefinitely. 

The catalytic destruction of ozone in the stratosphere involves reactions between gases there, so it is an example of homogeneous catalysis. The most important catalyst for this process is chlorine. Much of the chlorine present in the stratosphere comes from CFC molecules that were released in the troposphere and slowly migrated to the stratosphere. (Because they are very unreactive at ground level, nearly all CFCs that are released into the atmosphere eventually find their way to the stratosphere.) Upon absorption of UV light, the CFCs initiate a catalyzed reaction mechanism ... 

In recent years ozone in the stratosphere has been depleted at an alarmingly fast rate by chlorofluoro-carbons (CECs). A CFC molecule such as CFCI3 is first decomposed hy UV radiation ... 

So one CFC molecule cem initiate a process that can destroy many molecules of ozone. 

As long as there were no C-H bonds in the CFC molecule, the eluting peak showed good recovery. With C-H bonds present the CFC was decomposed. The selectivity of aluminum oxide for CFC s is very high, as shown in Fig. 7-46. Due to the decomposition of the partly halogenated CFC s the quantitative analysis for these CFC s is very difficult. 

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