Ozone equilibrium with oxygen

Stratospheric ozone is in a dynamic equilibrium with a balance between the chemical processes of formation and destruchon. The primary components in this balance are ultraviolet (UV) solar radiation, oxygen molecules (O2), and oxygen atoms (O) and may be represented by the following reactions ... 

A convention used in most literature on ozone mass transfer and in the rest of this book is to define the mass transfer coefficient as the one that describes the mass transfer rate without reaction, and to use the enhancement factor E to describe the increase due to the chemical reaction. Furthermore, the simplification that the major resistance lies in the liquid phase is used throughout the rest of the book. This is also based on the assumption that the mass transfer rate describes physical absorption of ozone or oxygen, since the presence of a chemical reaction can change this. This means that KLa - kLa and the concentration gradient can be described by the difference between the concentration in equilibrium with the bulk gas phase cL and the bulk liquid concentration cL. So the mass transfer rate is defined as ... 

At 30 km above the surface of the earth, the temperature is about 230. K and the partial pressure of oxygen is about 1.0 X 10-3 atm. Estimate the partial pressure of ozone in equilibrium with oxygen at 30 km above the earth s surface. Is it reasonable to assume that the equilibrium between oxygen and ozone is maintained under these conditions Explain. 

Reactive species such as NOx molecules, chlorine radicals, and hydrogen radicals present in the stratosphere can also cause the catalytic conversion of ozone to oxygen, in competition with the last reaction shown above. Reactions such as the following, therefore interfere with the equilibrium photochemical processes, reducing the concentration of ozone in the stratosphere. The net reaction is one where ozone is converted to oxygen and catalytic Cl species survives for further reaction. Particularly effective in this regard are halogen monoxides (CIO and BrO). 

For dissociation at 226 and 230 nm, the determined j3 values for the fast and slow oxygen atoms are 1.3/1.5 and 0.7/0.8, respectively. The former value corresponds to a bond angle of 120°, close to the ozone ground state equilibrium bond angle of 117°. The reduced anisotropy parameter of 0.8 implies a more strongly bent geometry with a bond angle of 100°. 

At T= 20 °C the aqueous solubility of ozone is about one third of the gas-phase concentration seven times higher than that of oxygen However, we must consider that the gas does not contain pure ozone, but rather approximately 20 % wt O-, in 02 (corresponding to cG = 320.1 g nrf3 at STP, cf. Table B 1-5), which can be achieved with modern electrical discharge ozone generators at standard pressure. Thus, the equilibrium concentration of ozone in the liquid is usually less than cL = 108 g m ... 

Thus, when studying atmospheric chemistry, it is necessary always to take into account the vertical and horizontal movements in the atmosphere, as well as the conditions controlling those chemical reactions that do not spontaneously lead to photochemical equilibrium. These conditions are applicable not only to ozone in the lower stratosphere, but also to atomic oxygen in the upper mesosphere above 75 km. In fact, equation (4) shows that, with increasing height, the formation of O3 becomes less and less important because of the decrease in the concentration of 02 and N2. Above 60 km the concentration of atomic oxygen exceeds that of ozone, but it is still in photochemical equilibrium up to 70 km. However, at the mesospause (85 km), it is subject to atmospheric movements, and its local concentration depends more on transport than on the rate of production. 

Another interesting feature of gas phase photolysis includes the possible generation of ozone by VUV irradiation of oxygen. Dohan and Masschelein previously presented the mechanistic and preparative aspects of this method in 1987. Unfortunately, the use of LP Hg lamps with a Suprasil quartz envelope for the irradiation of a gas stream containing oxygen inevitably leads to a photo stationary equilibrium between ozone formation at X of 184.9 nm and ozone destruction at X of... 

But if, in these various cases, we can put beyond doubt the existence of the same gas in two distinct polymeric forms, we are indebted to the phenomena of false equilibrium in the conditions of temperature and pressure for which the states of false equilibrium would not be produced, oxygen, taken in definite conditions, would always enclose a determined amount of ozone at a given pressure and temperature its properties would.be perfectly determined but its density taken with respect to a perfect gas would vary with the pressure and with the temperature oxygen would behave, in terms of the variation of density produced by a rise in temperature, just as do sulphur vapor, iodine vapor, acetic acid vapor one may not, therefore, argue from this fact that at a given pressure and temperature each of these gases exists in a perfectly determined state in order to deny, for each of them, the coexistence of two polymers one may nierely conclude there are not produced phenomena of false equilibrium in the conditions of temperature and pressure for which the experiments have been performed. 

The absorption of ozone by cyanide solutions in stirred reactors is complicated by mass transfer considerations. The presence of ozone gas in the exhaust from such a reactor does not indicate that equilibrium has been obtained between ozone gas bubbles and ozone in solution, but rather that the mass transfer through the individual bubbles is not complete, because of the resistance on the gas side. In other words, mass transfer controls the reaction, as the ozone will react almost instantaneously with the cyanide ion in solution. The presence of some metals, particularly copper, appears to speed up the absorption by acting as oxygen carriers. A solution of ozone in dilute acid decomposes somewhat more quickly when a trace of cupric ion is added. The presence of these metal catalysts, if this be their function, does not appear to be a necessary condition to ozone oxidation. What is important is that adequate mass transfer time and surface be available, as would be found in a countercurrent packed tower. 

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