Molecular diffusivity in air

Illustrative Example 18.1 Estimating Molar Volumes Illustrative Example 18.2 Estimating Molecular Diffusivity in Air Diffusivities in Water... 

Illustrative Example 18.2 Estimating Molecular Diffusivity in Air Problem Estimate the molecular diffusion coefficient in air, >,a, ofCFC-12 (see Illustrative Example 18.1) at 25°C (a) from the mean molecular velocity and the mean free path, (b) from the molar mass, (c) from the molar volume, (d) from the combined molar mass and molar volume relationship by Fuller (Eq. 18-44), (e) from the molecular diffusivity of methane. 

Explain qualitatively the semiempirical relation of Fuller et al. (1966) for molecular diffusivity in air (Eq. 18-44). How is this expression related to the molecular theory of gases ... 

To this end we use the boundary models derived in Chapter 19. Since each model has its own characteristic dependence on substance-specific properties (primarily molecular diffusivity in air or water), the experimental data from different compounds help us recognize the strengths and limitations of the various theoretical concepts. 

To calculate v/a from vwaler3 we use Eq. 20-27 and the simple molar mass relationship (Eq. 18-45) to estimate molecular diffusivity in air, Di3 ... 

Where does molecular diffusion (in air or water) actually play a role in the environment Give reasons why it is not relevant in systems which you exclude in your answer. 

Natural waters formed of —99.7% of H2 0 are also constituted of other stable isotopic molecules, mainly H2 0 (—2%o), H2 0 ( 0.5%o), and HD 0 (—0.3%c), where H and D (deuterium) correspond to and H, respectively. Owing to slight differences in physical properties of these molecules, essentially their saturation vapor pressure, and their molecular diffusivity in air, fractionation processes occur at each phase change of the water except sublimation and melting of compact ice. As a result, the distribution of these water isotopes varies both spatially and temporally in the atmosphere, in the... 

In addition to the theoretical and semiempirical methods for estimating molecular diffusivities in air, several empirical methods, based on regressions of measured diffusivities and other parameters, like molecular weight and molar volume, are available. These methods provide easily calculated estimates when the property data required to apply the more rigorous semiempirical methods are not available. Molecular weight is an excellent parameter for empirical correlations because it is easily determined from the chemical formula and is an important parameter in the theoretical model. Diffusion coefficients in air can be expected to have power-law dependence on molecular... 

Molecular diffusivities in air can also be estimated for compounds when measured values are not readily available using a referencing method in which the diffusivity of a known (preferably a chemical similar to the unknown compound) is used to infer the diffusivity of the unknown species. This method is based on the theoretical... 

FIGURE 5.2 Correlation of molecular diffusivities in air at 25°C with molecular weight. Hg and Br shown as outliers (not included in correlation). (Values compiled by Thibodeaux, L.J. 1996. Environmental Chemodynamics Movement of Chemicals in Air, Water, and Soil, Wiley, New York and from measurements by Gustafson, K.E. and Dickhut, R.M. 1994b. Journal of Chemical and Engineering Data 39,286-289.)... 

The preceding discussion provides the user with an assortment of estimation methods for obtaining reasonable values of molecular diffusivities in air, water, organic liquids, and other environmentally and geochemically relevant media. Methods presented for estimating effective diffusivities in porous media represent extensions of the parameters determined for fluids to situations where diffusion occurs through a fluid imbibed within solid material. The presentation of theoretical, semiempiri-cal, and purely empirical formulae gives the user the ability to choose an estimation method based on the available information about the chemical(s) of interest and the conditions within the medium. 

Equation 13.16 above is a relationship for the overall MTC, K, in terms of the three most active transport processes in the interface region. The is the air-side MTC Drn,Jh is that resulting from molecular diffusion in air-filled soil pore spaces and D Jh is the particle biodiffusion MTC due to earthworm... 

Initially, most studies of air-snow exchange assumed that transport within the snow pack occurs by molecular diffusion through the pore space only. The parameter fesa is then calculated from the molecular diffusivity in air Da, the diffusion path length (dpi), and a factor pt accounting for the porosity of the snow pack and the tortuosity of the diffusion pathway ... 

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