How do flow methods allow scientists to measure very fast reactions?

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A graph showing a potential energy curve for a chemical reaction, with the y-axis labeled potential energy and the x-axis labeled progress of reaction. The curve rises from the initial state (reactants) to a peak labeled transition state (activated complex), indicating activation energy, then drops to the final state (products). — chemical kinetics In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy required to transform reactants into products in a chemical reaction. On a potential energy curve, the value of the activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (known as the activated complex, or transition state) and particles of reactants in their initial state. The activation energy thus can be visualized as a barrier that must be overcome by reactants before products can be formed.

Flow methods are designed to overcome the challenge of accurately measuring the initial time of fast reactions, where the duration required for mixing reactants or changing system temperature is significant relative to the reaction's half-life.1

In these methods, two gases or solutions are introduced rapidly into a mixing vessel. The resulting mixture then flows along a tube, allowing scientists to measure the concentrations of reactants or products at various positions along the tube, which correspond to different reaction times.1

A common modification is the stopped-flow technique. In this approach, reactants are forced rapidly into a reaction chamber, and the flow is suddenly stopped. Scientists then measure the amounts of substances using physical methods at various short time intervals.1

While effective for many rapid processes, flow methods are limited by the time required for mixing and are generally not suitable for reactions with a half-life of less than approximately one-hundredth of a second.1