Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

11.2 Diffusion-controlled reactions 230
11.2 Diffusion-controlled reactions
11.2.1 Experimental observations
The transition to diffusion control can be observed in the gas phase by studying the
kinetics in highly compressed gases (up to =1000bar) or, even better, in supercritical
media. At ≈ 1000 bar, the gas density reaches the density of the liquid phase.
I Smoluchowsky-Kramers limit: At high densities, the rate constant becomes
∝ ∝ 1
(11.11)
I
Figure 11.2: Diffusion control of a unimolecular reaction: At very high pressures, the
fragments recombine before they become separated by diffusion y the reaction becomes
diffusion controlled.
11.2.2 Derivation of k
In order to derive a theoretical expression for , we consider the distribution of the
molecules around a molecule inthecenterofasphereasshowninFig.11.3.