Ozone decomposition mechanism

Mechanism II begins with fast reversible ozone decomposition followed by a rate-determining bimolecular collision of an oxygen atom with a molecule of NO. The rate of the slow step is as follows Rate = 2[N0][0 This rate expression contains the concentration of an intermediate, atomic oxygen. To convert the rate expression into a form that can be compared with the experimental rate law, assume that the rate of the first step is equal to the rate of its reverse process. Then solve the equality for the concentration of the intermediate ... 

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... 

C15-0051. Do the following for the ozone decomposition mechanism (a) Express the rate in terms of O2 formation, (b) Relate the rate of O3 consumption to the rate of O2 production, (c) If O2 forms at a rate of 2.7 X 10" M, state how fast ozone disappears. 

A knowledge of the kinetics of the decomposition of ozone is essential for the understanding of the chemistry of some important processes which occur in earth s atmosphere. Yet, in spite of numerous studies and the structural simplicity of ozone, the mechanism of its ultraviolet photolysis is still uncertain. Electronically and vibrationally excited species are involved in ozone decomposition and the current knowledge of the chemical behavior of such intermediates is still in its infancy. 

Alder, M. G., and G. R. Hill. The kinetics and mechanism of hydroxide ion catalyzed ozone decomposition in aqueous solution. J. Amer. Chem. Soc. 72 1884-1886, 1950. 

Mechanism of ozone decomposition in water depends on the presence of chemical species that can initiate, promote and/or inhibit its decomposition. The most accepted ozone decomposition mechanism is given in Figrtre 3. 

Figure 3. Scheme of ozone decomposition mechanism in water. P = promoter (e.g. ozone, methanol), S = scavenger or inhibitor (e.g. /-butanol, carbonate ion), I = initiator (e.g. hydroxyl ion, perhydroxyl ion) (adapted by Beltran [35]). 

Tomiyasu, H Fukutomi, H Gordon, G. Kinetics and mechanism of ozone decomposition in basic aqueous soiution. inorganic Chemistry, 1985 24 (19), 2962-2966. 

Tomiyasu H, Fukutomi H, Gordon G (1985) Kinetics and Mechanisms of Ozone Decomposition in Basic Aqueous Solutions, Inorganic Chemistry 24 2962-2985. 

The second is concerned with the need to have a complete and sensible chemical mechanism, valid over a wide range of temperature. Even a relatively simple combustion system will involve dozens of reactions, so that a well established reaction rate data base is essential. It is equivalently essential that the results be verified by comparison with detailed experimental data--such as that provided by laser probes. For example, in a study of the ozone decomposition flame (20). it was found that certain alternative but wrong choices of key input parameters were not discernible if flame speed were used as the sole predicted result for verification however, these choices did produce considerable differences in the profiles of the transient oxygen atom concentration and the temperature. 

On the other hand, the indirect type of ozonation is due to the reactions of free radical species, especially the hydroxyl radical, with the organic matter present in water. These free radicals come from reaction mechanisms of ozone decomposition in water that can be initiated by the hydroxyl ion or, to be more precise, by the hydroperoxide ion as shown in reactions (4) and (5). Ozone reacts very selectively through direct reactions with compounds with specific functional groups in their molecules. Examples are unsaturated and aromatic hydrocarbons with substituents such as hydroxyl, methyl, amine groups, etc. [45,46],... 

The mechanism of decomposition of ozone in water has been the subject of numerous studies, starting from the work of Weiss [47], Among more recent studies, the mechanisms of Hoigne et al. [48] and Tomiyashu et al. [49] are the most accepted in ozone water chemistry. The main conclusion that can be drawn is that ozone stability in water is highly dependent on the presence of substances that initiate, promote, and/or inhibit its decomposition. The ozone decomposition mechanism usually assumed is given in Fig. 4 [50]. 

Gehringer irradiated TCE contaminated waters in the presence and absence of ozone [52]. The use of ozone dramatically increased the rate of 100 ppb TCE decomposition, as shown in Fig. 6 and in Table 7. It was also found that the Z)10 (and thus dose constant) values were independent of initial TCE concentration over a wide range, indicating that the mechanism of decomposition was constant. Thus, the factors for reduction in Dw shown in Table 7 are useful predictive process parameters. 

The fractional value for n—i.e., the apparent order of reaction—is probably due to the complex nature of the competing reactions and the instability of the intermediates. In other words, the mechanism and the kinetics of ozone decomposition in aqueous solution and the possible intermediate unstable ozonide-ring leading from cyanide to cyanate will affect the apparent order of reaction. 

Ozone is formed by the photolytic decomposition of NO2 yielding oxygen radicals and by the reaction sequence NO2 — HNO3 — NO3 O3. In particular, the reaction of NO2 with hydroxyl radicals to form HNO3 increases ozone concentration because two radicals, NO2- and -OH, which catalyze ozone decomposition, are removed. Other radicals are also important for ozone destruction in the stratosphere, especially chlorine oxides see Chlorine, Bromine, Iodine, Astatine Inorganic Chemistif). The mechanism of ozone destruction is complicated as there many compounds involved. Chlorine nitrate and dinitrogen pentoxide can act as reservoir species for CIO-, NO2-, and NO3- radicals. 

During the late 1960s companies in the United States, Europe, and Russia explored the attractive-sounding idea of supersonic transport aircraft ( SSTs, see the photograph). Since SSTs would fly near the altitude of the ozone layer, there was concern about their impact on this vital protective shield. Initially, the concern was that water vapor from fuel combustion by SSTs would be split into reactive species that could decompose ozone and disrupt the balance. Paul Crutzen (1933- ), a Dutch citizen studying at Oxford, discovered that the major ozone decomposition mechanism would, in fact, involve oxides of nitrogen ( NOj ) rather than water. Specifically, he discovered that species such as nitric oxide (NO) could play a catalytic role ... 

Several papers have appeared on the mechanism of decomposition of ozone in aqueous solution. A chain mechanism is proposed for the oxidation of saturated alcohols by O3 in aqueous solution. Acetic acid and acetate ion are knwn to stabilize ozone in aqueous solution, possibly due to their ability to scavenge the hydroxyl radical. A pulse radiolysis studyof the reaction of O3 with CH2C00 leads to the mechanism involving reactions (40)-(44). Reactions of... 

We encountered the second and third reactions earlier in our discussion of mechanisms. The chlorine atoms serve as a catalyst for ozone decomposition because (a) they are not part of the reaction stoichiometry, (b) they are not... 

---