0.0 0.5 1.0 1.5 2.0 a[|-]"1/2 (units of M"1'2)

FIGURE 5.19 Plot of measured (7ncl)-1 for the uptake of O, into iodide-containing aqueous solutions against the (activity of iodide ion)-1/2 at 277 K (adapted from Hu et al., 1995).

reaction of 03 with I- and a{- is the activity of iodide ion in solution. A plot of l/ync,, where yncl is the measured uptake coefficient, against k~x/1 = (k'a]-)~l/2, i.e., as a function of («,-)"l/2, should give a straight line from whose slope the second-order rate constant k' can be obtained if the Henry's Law constant (H) and liquid-phase diffusion coefficient (D,) are known. The intercept corresponds to 1 /a. As expected the plot of l/yne, against (a,-)-1/2 is linear and the value of k' derived from the slope, 4 X 109 L mol-1 s"\ is in reasonable agreement with literature values (see Problem 9).

Another means of comparing the relative speeds of the various steps represented in Eq. (PPP) involves using the "characteristic time" for each process. Table 5.7 summarizes the parameters involved in each of these characteristic times. These allow an initial assessment of the relative speeds so that those steps which are rate-determining under a particular set of conditions can be readily identified.

6. Reactions at the interface. There is increasing evidence that reactions occur not only in the bulk but also at the interface itself. For example, Hanson and Ravi-shankara (1994) reported evidence that the stratospher-ically important reaction of C10N02 with HC1 (see Chapter f2) occurs both at the surface and in the bulk. Similarly, Hu et al. (1995) investigated the uptake and reaction of Cl2 into aqueous droplets containing bromide ion. Figure 5.20 shows a plot of 1 /yncl, where yncl is the net measured uptake coefficient, against (aUr-)~l/2 and, for comparison, the line expected if Cl2 reacted with Br~ in the bulk phase with a rate constant of 7.7 X 109 L mol-1 s~' as reported earlier in the literature, assuming a mass accommodation coefficient of a = 1. Clearly, the measured rates of uptake exceed those expected, which is all the more surprising since

TABLE 5.7 Characteristic Times Associated with the Uptake of Gases into Liquid Particles and Their Reaction in the Bulk Phase"

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