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

1.2 Time scales for chemical reactions 2
1.2 Time scales for chemical reactions
A chemical reaction requires some “contact” between the respective reaction partners.
The “time between contacts” thus determines the upper limit for the rate of a reaction.
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Reactions in the gas phase:
• Free mean pathlength À molecular dimension (diameter) .
• Reaction rate is determined by time between collisions.
• Time between two collisions @ :
≈ 500 m s (1.1)
y
y
≈ 10 −7 m (1.2)
∆ ≈ ≈ 2 × 10 −10 s (1.3)
• Gas phase reactions take place on the nanosecond time scale:
I @ ≤ 1 ∆ =5× 109 s −1 (1.4)
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Reactions in the liquid phase:
• Molecules are permanently in contact with each other.
• Typical distance between neighboring molecules (upper limit allowing for solvation
shell):
≈ 2 × 10 −8 10 −7 cm (1.5)
• Maximal reaction rate is determined by the rate of diffusion of the reacting molecules
in and out of the solvent cage.
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Reactions in the solid phase:
• Atoms and molecules localized on fixed lattice positions.
• Reaction rate is determined by rate of diffusion (“hopping”) of the atoms and
molecules via vacancies (unoccupied lattice positions, “Fehlstellen”) or interstitial
sites (“Zwischengitterplätze”).
• Hopping from one lattice position to another usually requires a corresponding high
activation energy.
• Solid state reactions are usually very slow.