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

11.2 Diffusion-controlled reactions 233
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Typical values:
y Typical magnitude of :
≈ 2 × 10 −5 cm 2 s −1 (11.29)
≈ 5 × 10 −8 cm (11.30)
≈ 8 × 10 9 lmol −1 s −1 (11.31)
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Table 11.1: Diffusion constants of ions in aqueous solution at =298K.
ion (cm 2 s −1 ) ion (cm 2 s −1 )
H + 91 × 10 −5 OH − 52 × 10 −5
Li + 10 × 10 −5 Cl − 20 × 10 −5
Na + 13 × 10 −5 Br − 21 × 10 −5
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Examples of diffusion controlled reaction in solution:
• radical recombination reactions, e.g.
+ → 2 : =13 × 10 10 lmol −1 s −1 (11.32)
• electronic deactivation (quenching) in solution
• H + transfer reactions
+ + − → 2 : =14 × 10 11 lmol −1 s −1 (11.33)
This reaction is ten times faster than a typical diffusion controlled reaction, because
H + transfer is a concerted process (Grotthus mechanism, see Fig. 11.4)!
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Figure 11.4: Structures involved in fast H + transfer processes in liquid water.