03 XO o2
In those regions of the stratosphere where the atomic oxygen concentration is appreciable, the XO molecules react subsequently with oxygen atoms to produce 02 and to re-form X:
The overall reaction corresponding to this reaction mechanism is obtained by algebraically summing the successive steps that occur in air over and over 323.11*1 an equal number of times. In the case of the additional steps of the mechanism, the reactants in the two steps are added together and become the reactants of the overall reaction, and similarly for the products:
Molecules that are common to both sides of the reaction equation, in this case X and XO, are then canceled, and common terms collected, yielding the balanced overall reaction:
Thus the species X are catalysts for ozone destruction in the stratosphere since they speed up a reaction (here, between 03 and O), but they are eventually re-formed intact and are able to begin the cycle again—with, in this case, the destruction of further ozone molecules.
As previously discussed (Chapman cycle), the above overall reaction can occur as a simple collision between an ozone molecule and an oxygen atom even in the absence of a catalyst, but almost all such direct collisions are ineffective in producing a reaction. The X catalysts greatly increase the efficiency of this reaction, i.e., they effectively increase the value of its rate constant and thereby decrease the steady-state concentration of ozone. All the environmental concerns about ozone depletion arise from the fact that we are inadvertently increasing the stratospheric concentrations of several X catalysts by the release at ground levels of certain gases, especially those containing chlorine and bromine. Such an increase in the catalyst concentration leads to a reduction in the concentration of ozone in the stratosphere by the mechanism shown above and by one discussed later.
Most ozone destruction by the catalytic mechanism (i.e., the combination of sequential steps) described above, hereafter designated Mechanism I, occurs in the middle and upper stratosphere, where the ozone concentration is low to start with. Chemically, all the catalysts X are free radicals, which are atoms or molecules containing an odd number of electrons. As a consequence of the odd number, one electron is not paired with one of opposite spin character (as occurs for all the electrons in almost all stable molecules). Free radicals are usually very reactive, since there is a driving force for their unpaired electron to pair with one of the opposite spin, even if it is located in a different molecule. The determination of the appropriate bonding structure for simple free radicals is described in Chapter 5.
An analysis of which free-radical reactions are and are not feasible in air is given in Box 1-2.
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