Reaction mechanisms

Most chemical reactions occur as a sequence of elementary steps. The sequence as a whole is called the reaction mechanism. The slowest step is called the rate-determining step (RDS). The elementary steps must add up to the overall equation. The products of each elementary step are called reaction intermediates.

In the potential energy diagram, each peak represents a separate activated complex, and the valleys between them represent the reaction intermediates. The RDS always has the greatest activation energy.

reaction progressreactantsproductsP3R1AC1AC2AC3P1+R2P2+R3Ep(kJ)HEa2Ea(overall)Ea1Ea3Ear(overall)
Exothermic potential energy diagram with reaction intermediates