composite reaction mechanism

A composite reaction mechanism describes a chemical reaction that occurs in multiple steps rather than a single, direct transformation. Evidence for such mechanisms includes when the reaction's kinetic equation doesn't match its balanced chemical equation, or when reaction intermediates—substances formed and consumed during the reaction—can be detected.123

For example, the reaction between hydrogen and iodine chloride, H₂ + 2ICl → I₂ + 2HCl, has a rate dependent on both [H₂] and [ICl]. This suggests a two-step process: a slow reaction between H₂ and ICl, followed by a rapid reaction of the intermediate HI with another ICl molecule. Other complex mechanisms can lead to oscillatory behavior or even chaos, requiring feedback loops within the reaction. Chain reactions, like those involving hydrogen and bromine, are also composite mechanisms where intermediates are regenerated.1