Diffusion control

Diffusion control refers to reactions whose rate is governed by transport of the reactants through the reaction medium (usually a solution). The process of chemical reaction can be considered as involving the diffusion of reactants till they encounter each other in the right stoichiometry and form an activated complex which can form the product species. The observed rate of chemical reactions is generally speaking, the rate of the slowest or "rate determining" step. In diffusion controlled reactions the formation of products from the activated complex is much faster than the diffusion of reactants and thus the rate is governed by diffusion.

Rates of diffusion of molecules are generally very high in the gas phase, so diffusion control is rare for homogeneous reactions. Diffusion control is more likely in solution where diffusion of reactants is slower due to the greater number of collisions with solvent molecules. Reactions where the activated complex forms easily and the products form rapidly are most likely to be limited by diffusion control. Examples are those involving catalysis and enzymatic reactions. Heterogeneous reactions where reactants are in different phases are also candidates for diffusion control.

One classical test for diffusion control is to observe whether the rate of reaction is affected by stirring or agitation; if so then the reaction is almost certainly diffusion controlled under those conditions.

Useful reference: Atkins P "Physical Chemistry" Freeman Press 1978

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