Info

Dacey et al., 1984

Source: Adapted from Schwartz (1984a). " Physical solubility; reacts with liquid water. 6 Physical solubility exclusive of acid-base equilibria. ' At 20°C.

'' Temperature dependence also reported as H = exp[7.92 : 10*/T (K) - 15.44],

' See Table 8.1 for references and additional data.

Time

Source: Adapted from Schwartz (1984a). " Physical solubility; reacts with liquid water. 6 Physical solubility exclusive of acid-base equilibria. ' At 20°C.

'' Temperature dependence also reported as H = exp[7.92 : 10*/T (K) - 15.44],

' See Table 8.1 for references and additional data.

assumes that there are no irreversible chemical reactions that are so fast that the equilibrium cannot be established. It also assumes that the surface of the droplet is an unimpeded air-water interface; as discussed in more detail in Chapter 9, some atmospheric aerosols may have an organic surface film (e.g., Husar and Shu, 1975; Chang and Hill, 1980; Graedel and Weschler, 1981; Gill et al., 1983; Pankow et al., 1994a,b, 1997; Goss and Schwarzenbach, 1998), which could alter the establishment of the equilibrium anticipated by Henry's law.

In addition, the high concentrations of ions in solutions of high ionic strength such as sea salt particles (especially near their deliquescence point) can alter gas solubility. In this case, the Henry's law constants must be modified using Setchenow coefficients to take this effect into account (e.g., Kolb et al., 1997).

Ionized and/or hydrolyzed species may be formed from the dissolved gas in some cases. An important example is S02, which dissolves to set up equilibria involving HSO/ and SO2- in a manner similar to C02 (see Chapter 8).

One major difference between reactions in the gas phase and in solution is the presence of solvent molecules in the latter case. In the liquid phase,

Solution

Time

FIGURE 5.13 Patterns of A-B collisions expected in the gas phase and in solution (adapted from Adamson, 1973).

molecules are in close contact, with the space between molecules being ~ 10% of the distance between their centers. Reactants thus have a number of nearest neighbors, in the range of ~4-12, with which they can collide. The reactants can then be thought of as existing in a solvent "cage," in which they undergo many collisions before breaking out of that particular environment. If two participants required for a chemical reaction diffuse into such a solvent cage, they will then be held together for a period of time and undergo a number of collisions with each other; such a series of collisons is known as an encounter. Because of this cage effect, highly reactive species such as atoms and free radicals that are formed in the cage, for example, by photolysis, have a much higher efficiency of recombination than if they were in the gas phase.

Compared to the gas phase, then, reactants take longer to diffuse together, but once they find themselves as nearest neighbors, they undergo a series of collisions rather than separating after one collision; this difference is illustrated in Fig. 5.13 (Adamson, 1973). As a result, for neutral nonpolar reactants (as opposed to ions; see later), the rate constants in solution are expected to be approximately equal to those in the gas phase.

Treatment of systems in which gas-phase diffusion, mass accommodation, liquid phase diffusion, and reaction both in the bulk and at the interface must be taken into account is discussed in Section E.f.

2. Diffusion-Controlled Reactions of Uncharged Nonpolar Species in Solution

Let us first consider a very fast reaction between uncharged nonpolar reactants in solution. In this case, the rate is controlled by the number of encounters. Once A and B diffuse into the same solvent cage, they will react; hence the rate of these diffusion-controlled reactions is determined by how fast A and B diffuse together in solution.

Fick's first law describes the rate of diffusion of a species A in solution across an area E in the direction

Was this article helpful?

0 0

Post a comment