Decomposition of ozone

Aqueous Phase. In pure water, the decomposition of ozone at 20°C iavolves a complex radical chain mechanism, initiated by OH and propagated by O2 radical ions and HO radicals (25). O3 is a radical ion. 

Hydrogen peroxide greatly accelerates the decomposition of ozone in alkaline solutions because of formation of HOg, which reacts rapidly with ozone to form the radical ion (25). When the concentration of H2O2 exceeds 10 Af, the decomposition of ozone is initiated faster by HOg than by... 

Inorganic Reactions. Ozone reacts rapidly with various free radicals and radical ions such as O, 0 , H, HO, N, NO, Cl, and Br. Some of these radicals (HO, NO, Cl, and Br) can initiate the catalytic decomposition of ozone. 

Above pH 9, decomposition of ozone to the reactive intermediate, HO, determines the kinetics of ammonia oxidation. Catalysts, such as WO, Pt, Pd, Ir, and Rh, promote the oxidation of dilute aqueous solutions of ammonia at 25°C, only two of the three oxygen atoms of ozone can react, whereas at 75°C, all three atoms react (42). The oxidation of ammonia by ozone depends not only on the pH of the system but also on the presence of other oxidizable species (39,43,44). Because the ozonation rate of organic materials in wastewater is much faster than that of ammonia, oxidation of ammonia does not occur in the presence of ozone-reactive organics. 

Unpiotonated hydioxylamine is oxidized rapidly by ozone, / = 2.1 X 10 (39). The reaction of ozone with the lower oxides of nitrogen (NO and NO2) is also rapid and quantitative the end product is nitrogen pentoxide, which is also a catalyst for the decomposition of ozone (45). Nitrous oxide, however, reacts slowly (k < 10 ) (39). Nitrogen-containing anions, eg, nitrite and cyanide, also ate oxidized by ozone (39). Nitrite is oxidized to nitrate (fc = 3.7 X 10 and cyanide is oxidized rapidly to cyanate (fc = 2.6 X 10 (46) and 10 -10 (39)). Cyanate, however, is oxidized slowly. 

Relative rates of eompetitive reaetions ehemieal oxidation, lysing baeteria, decomposition of ozone in aqueous solutions, and so on. 

P. Crutzen (Max Planck Institute for Chemistry, Mainz), M. Molina (Massachusetts Institute of Technology) andF. S. Rowland (Irvine, California) work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone. 

The decomposition of ozone, 03, to diatomic oxygen, 02, is believed to occur by a two-step mechanism ... 

The chlorine atoms that catalyze the decomposition of ozone come from chlorofluorocarbons (CFCs) used in many refrigerators and air conditioners. A major culprit is CF2CI2, Freon, which forms Cl atoms when exposed to ultraviolet radiation at 200 nm ... 

The two scientists who first suggested (in 1974) that CFCs could deplete the ozone layer, F. Sherwood Rowland (1927-) and Mario Molina (1943-), won the 1995 Nobel Prize in chemistry, along with Paul Crutzen (1933—), who first suggested that oxides of nitrogen in the atmosphere could catalyze the decomposition of ozone. 

The catalyzed decomposition of ozone is known to be responsible for the ozone hole (Figure A) that develops in Antarctica each year in September and October, at the end of winter in the Southern Hemisphere. No ozone is generated during the long, dark Antarctic winter. Meanwhile, a heterogeneous reaction occurring on clouds of ice... 

For tiie first-order thermal decomposition of ozone... 

As a final example of numerical simulations, consider the base-catalyzed decomposition of ozone in aqueous solution. This multistep reaction is controversial in that contradictory mechanisms have been suggested.33 34 The set of reactions that appears to be the most consistent with the experimental data is shown in Table 5-1, with a set of rate constants. Most of these values were reported in the literature, but several were refined to give agreement with experiments that measured the decline in concentration O3. 

Reactions" and rate constants"6 for the decomposition of ozone in alkaline solutions... 

Chain reaction. A possible mechanism for the chlorine-catalyzed decomposition of ozone is... 

Orders can be negative, as in (concentration) corresponding to order — l. Because [AJ 1 = 1/fA], a negative order implies that the concentration appears in the denominator of the rate law. Increasing the concentration of that species, usually a product, slows down the reaction because the species participates in a reverse reaction. An example is the decomposition of ozone, 03, in the upper atmosphere ... 

FIGURE 13.16 A representation of a proposed one-step mechanism for the decomposition of ozone in the atmosphere. This reaction takes place in a single bimolecular collision. 

FIGURE 13.17 An alternative, two-step mechanism for the decomposition of ozone. In the first step, an energized ozone molecule shakes off an oxygen atom. In the second step, that oxygen atom attacks another ozone molecule. 

When we substitute this expression into the rate law for net rate of decomposition of ozone, we get... 

The oxygen atom produced in this step can react with oxygen molecules to produce more ozone, and so the ozone concentration in the stratosphere normally remains constant, with seasonal variations. Because the decomposition of ozone... 

Photolytic reactions such as the decomposition of ozone by light are also unimolecular processes ... 

What is needed now is some means for calculating e. To do this, it is useful to consider some component, H, which is formed only by Reaction I, which does not appear in the feed, and which has a stoichiometric coefficient of v/// = 1 for Reaction I and stoichiometric coefficients of zero for all other reactions. It is always possible to write the chemical equation for Reaction I so that a real product has a stoichiometric coefficient of +1. For example, the decomposition of ozone, 2O3 3O2, can be rewritten as 2/3O3 —> O2. However, you may prefer to maintain integer coefficients. Also, it is necessary that H not occur in the feed, that there is a unique H for each reaction, and that H participates only in the reaction that forms it. Think of H as a kind of chemical neutrino formed by the particular reaction. Since H participates only in Reaction I and does not occur in the feed, Equation (2.40) gives... 

C15-0078. A student proposes the following mechanism for the atmospheric decomposition of ozone to... 

The following rate expression has been determined for the low temperature chlorine catalyzed decomposition of ozone. 

The gas phase decomposition of ozone is catalyzed by nitrogen pent oxide. Two mechanisms that purport to explain this process are given below. Can relatively simple kinetic measurements be used to eliminate one of the proposed mechanisms from further consideration Explain. 

Experimental studies of the catalytic decomposition of ozone in the presence of nitrogen pentoxide follow a rate expression of the form... 

Bauer et al. (1981) measured the influence of bed diameter on the catalytic decomposition of ozone. Figure 6 shows the decrease of the conversion with bed diameter for Bauer s data. This figure also shows the influence of distributor design on conversion. In many small scale experiments, a porous plate is used which will give better performance than the distributors used in large shallow bed commercial designs. 

Figure 6. Conversion catalytic decomposition of ozone for different bed diameters and distributors. (From Werther, 1992.)...

As the reaction temperature is increased, chemiluminescence is observed in the reactions of ozone with aromatic hydrocarbons and even alkanes. Variation of temperature has been used to control the selectivity in a gas chromatography (GC) detector [35], At room temperature, only olefins are detected at a temperature of 150°C, aromatic compounds begin to exhibit a chemiluminescent response and at 250°C alkanes respond, giving the detector a nearly universal response similar to a flame ionization detector (FID). The mechanisms of these reactions are complex and unknown. However, it seems likely that oxygen atoms produced in the thermal decomposition of ozone may play a significant role, as may surface reactions with 03 and O atoms. 

Chain decomposition of ozone was also observed in the oxidation of cumene by an O3-O2 mixture [151]. The rate of ozone consumption was found to be... 

The second term characterizes the chain decomposition of ozone. The mechanism of chain reaction was found to include the following reactions ... 

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