Reaction diameter

The diffusion coefficient corresponding to the measured values of /ch (D = kn/4nRn, is the reaction diameter, supposed to be equal to 2 A) equals 2.7 x 10 cm s at 4.2K and 1.9K. The self-diffusion in H2 crystals at 11-14 K is thermally activated with = 0.4 kcal/mol [Weinhaus and Meyer 1972]. At T < 11 K self-diffusion in the H2 crystal involves tunneling of a molecule from the lattice node to the vacancy, formation of the latter requiring 0.22 kcal/mol [Silvera 1980], so that the Arrhenius behavior is preserved. Were the mechanism of diffusion of the H atom the same, the diffusion coefficient at 1.9 K would be ten orders smaller than that at 4.2 K, while the measured values coincide. The diffusion coefficient of the D atoms in the D2 crystal is also the same for 1.9 and 4.2 K. It is 4 orders of magnitude smaller (3 x 10 cm /s) than the diffusion coefficient for H in H2 [Lee et al. 1987]. 

Glass tube with cooling mantle, equipped at the bottom with a ceramic filter plate to allow easy separation of the catalyst upon reaction, diameter 4 cm... 

The usual way to check the simple collision theory was to choose some ad hoc values of the energy of activation E and of the reaction diameter D, and to fit the experimental data by the Arrhenius equation, or similar equations which can be deduced by taking into account the internal degrees of freedom of the reacting molecules. However, it is doubtful if these values of E and D are real physical quantities or whether they are only adjusted parameters. To answer this point, a critique of these quantities has been done by this author and is referred to hereafter.2-4... 

As far as the collision diameter D is concerned, the general use is to take the reaction diameter D as the average kinetic diameter between the two reacting molecules, as deduced from transport phenomena determinations.17 This choice has no sound basis, and some improvements can be made if one considers the influence of the intermolecular field. Such an analysis shows that, if the reaction needs an activation energy E, then one must use an impact diameter D,... 

On the other hand, if for the reaction to take place, it necessary that the two molecules get close together withoi extra energy (for example, the deactivation process of th molecular reactions, the reactions between alkaline metals halogen molecules, as Na-f-Cl2, etc.), then the reaction diameter can be defined from inspection of the trajectories of the... 

Enzyme and source Detailed reaction diameter, swelling centration for free and... 

Figure 9. Schematic representation of the expected and actual distribution of the three trimethylbenzenes in the intracrystalline space and in the gaseous products of the reaction. Diameters of the circles are proportional to the intracrystalline content of each compound the effective molecular size increases from top to bottom of the Figure.

Calculate the rate constant at 298 K. Assume that the reaction diameter is 4.0 x 10 m. The diffusion coefficient of iodine atoms in CCI4 at this temperature is equal to... 

Since the reaction diameter d occurs both in the expression for k in Eq. (12.2-2) and in the expression for y in Eq. (12.2-5), we solve by successive approximation. We guess a value for d and calculate the value of k for that value, then guess another value of d and calculate k again, and so forth, until an adequate approximation for k is obtained. By this procedure the correct value of k is obtained with a value of d equal to 8.64 x 10 m = 0.864 nm = 8.64 A. 

The reaction diameter for the reaction between hydrogen ions and hydroxide ions in aqueous solution is larger than the sum of the radii of these ions. The explanation for this fact is that water molecules can exchange hydrogen and hydroxide ions as described in Chapter 10. 

Calculate the values of the electrostatic factor / for 2223 equal to 2,1,0, — Land —2 at 298.15 K, assuming a dielectric constant equal to 78.4 and a reaction diameter equal to 0.50 nm. Comment on your results. 

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