Diffusion-controlled reaction
![Figure 1. An illustration to show (a) Alberty-Hammes-Eigen model, and (b) Chou’s model, where E denotes the enzyme whose active site is colored in red, while the substrate S in blue.](/Images/godic/202501/09/Enzyme-substrate_difusion-controlled_reaction5954.jpg")
Diffusion-controlled (or diffusion-limited) reactions are reactions that occur so quickly that the reaction rate is the rate of transport of the reactants through the reaction medium (usually a solution). As quickly as the reactants encounter each other, they react. The process of chemical reaction can be considered as involving the diffusion of reactants until 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 collision frequency.