X O39: 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 O2 and to re-form X:
XO O 9: X O2
The overall reaction corresponding to this reaction mechanism is obtained by algebraically summing the successive steps that occur in air over and over again an equal number of times. In the case of the additional steps of the
Perhaps the alternative name ozone screen is more appropriate than ozone layer.
Review Questions 5–11 are based upon material in the preceding section.
This is the first catalytic destruction mechanism for ozone.
Catalytic Processes of Ozone Destruction 21
mechanism, the reactants in the two steps are added together and become the reactants of the overall reaction, and similarly for the products:
X O3 XO O 9: XO O2 X O2
Molecules that are common to both sides of the reaction equation, in this case X and XO, are then cancelled, and common terms collected, yielding the balanced overall reaction
O3 O 9: 2 O2 overall reaction
Thus the species X are catalysts for ozone destruction in the stratosphere, since they speed up a reaction (here, between O3 and O), but are eventu- ally re-formed intact and are able to begin the cycle again —with, in this case, the destruction of further ozone molecules.
O3 X O XO O2 O2
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 reaction. The X catalysts greatly increase the efficiency of this reaction and thereby decrease the steady-state concentra- tion of ozone. All the environmental concerns about ozone depletion arise from the fact that we have inadvertently increased the stratospheric con- centrations 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 dis- cussed later.
Most ozone destruction by the catalytic mechanism (i.e., the combina- tion 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 X catalysts are free radicals, which are atoms or molecules containing an odd number of electrons. As a con- sequence 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.
An analysis of which free-radical reactions are feasible in air and which are not is given in Box 1-1.
The Rates of Free-Radical Reactions BOX 1-1
The rate of a given chemical reaction is affected by a number of parameters, most notably the magnitude of the activation ener- gy required before the reaction can occur.
Thus reactions with appreciable activation energies are inherently very slow processes and can often be ignored compared to alterna- tive, faster processes for the chemicals involved. In gas-phase reactions involving simple free radicals as reactants, the activa- tion energy exceeds that imposed by their endothermicity by only a small amount. Thus we can assume, conversely, that all exother- mic free-radical reactions will have only a small activation energy (Figure 1a). There- fore, exothermic free-radical reactions usually are fast (providing, of course, the reactants exist in reasonable concentrations in the atmosphere). An example of an exothermic free-radical reaction with a small energy
barrier is
Cl O39: ClO O2
The activation energy here is only 2 kJ mol1. Reactions involving the combining of two free radicals generally are exothermic, since a new bond is formed, so they too pro- ceed quickly with little activation energy, provided that the radical concentrations are high enough that the reactants do in fact col- lide with each other at a fast rate.
In contrast, endothermic reactions in the atmosphere will be much slower since the acti- vation barrier must of necessity be much larger (see Figure 1b). At atmospheric temperatures, few if any collisions between the molecules would have sufficient energy to overcome this large barrier and allow reaction to occur. An example is the endothermic reaction
OH HF 9: H2O F
FIGURE 1 Potential energy profiles for typical atmospheric free-radical reactions, showing (a) exothermic and (b) endothermic patterns.
Potential energy
Products Extent of reaction
Reactants Products Reactants
ΔH > 0
Ea
ΔH < 0
Ea
(a) (b)
Catalytic Processes of Ozone Destruction 23