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

10.2 Kinetics of heterogeneous reactions (surface reactions) 227
11. Reactions in solution
“Everyone has problems - chemists have solutions.”
I
Comparison of gas and liquid phase reactions (@ T = 298 K and p =1bar):
• Molecules in the gas phase occupy ≈ 02 % of the total volume, while molecules
in the liquid phase occupy ≈ 50 % of the total volume.
• Molecules in liquid are in permanent contact with each other, and undergo permanent
collisions with each other.
• The crossing of a potential barrier in a reaction is not continuous as in gas phase
but affected by multiple collisions during the crossing.
• As a result of the permanent collisions, we can expect the reactant molecules to
be in thermodynamic equilibrium even above 0 . Thus TST should be applicable.
• However, it is very difficult to evaluate the partition functions in liquids. Hence,
one usually employs the thermodynamic version TST.
• Solvent molecules act as an efficient heat bath.
I Observations on liquid phase reactions: Experience has shown that reactions in
liquids have similar rates as in the gas phase, except if
• the reaction occurs with the solvent,
• there are strong interactions of the reactant molecules with the solvent (e.g., ionic
reactions; ⇒ solvent shells),
• the reactions are diffusion controlled.
11.1 Qualitative model of liquid phase reactions
In contrast to gas phase reactions for which we have advanced theories (⇒ transition
state theory, unimolecular rate theory), the theory for liquid phase reactions is much
less well developed. However, we can understand the required basic reaction steps as
sketched in Fig. 11.1 and a number of limiting cases resulting from these steps.