Reactionlimited

Reactionlimited is a term used to describe a regime in which the overall rate of a process is governed by the intrinsic rate of the chemical reaction step rather than by transport of reactants to the reactive site. In a reaction-limited process, increasing the concentration of reactants or improving mixing will have a smaller effect on the rate once the reaction step remains the bottleneck. Conversely, the rate is sensitive to factors that affect the activation energy and the rate constant k of the elementary step, such as temperature, catalysts, and surface properties.

Two common contexts are crystal growth in solution, where adsorption or incorporation at steps is slower than the arrival of solute; and catalysis or electrochemical reactions, where the surface reaction (such as a charge-transfer step) is slower than mass transport. In contrast, diffusion-limited or mass-transport-limited regimes arise when transport to the reactive site is the bottleneck.

Quantitative description often involves a rate expression R ≈ k [A]^m [B]^n, where k is the temperature-dependent rate constant. In transport-limited cases, diffusion coefficients and concentration gradients govern R rather than the chemical rate constant. The Damköhler number Da is used to diagnose the regime, with Da << 1 indicating reaction-limited behavior and Da >> 1 indicating diffusion-limited behavior.

Implications for design include strategies to alter the chemical step, such as improving catalysts or increasing temperature, rather than focusing solely on transport enhancements. Understanding whether a process is reaction-limited guides targeted improvements and efficiency optimization.