What an Activated Complex Is and How It Works

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An activated complex is an intermediate state that is formed during the conversion of reactants into products. An activated complex is the structure that results in the maximum energy point along the reaction path. The activation energy of a chemical reaction is the difference between the energy of the activated complex and the energy of the reactants.

How an Activated Complex Works

Consider a chemical reaction between reactants A and B to form products C and D. The reactants must collide with each other and interact in order to form the products. Several factors improve the chances that A and B will encounter each other, including increased temperature, increased concentration of reactants, or adding a catalyst. In a reaction with an activated complex, A and B form the complex A-B. The complex only forms if sufficient energy (the activation energy) is present. The energy of the activated complex is higher than that of either the reactants or products, which makes the activated complex unstable and temporary. If there isn't enough energy for the activated complex to form the products, it eventually breaks apart into the reactants. If enough energy is available, the products form.

Activated Complex Versus Transition State

Some textbooks use the terms transition state and activated complex interchangeably, but they mean different things. The transition state refers only to the highest potential energy of the atoms participating in a chemical reaction. The activated complex covers a range of atom configurations that atoms form on their way from reactant to products. In other words, the transition state is the one molecular configuration that occurs at the peak of the energy diagram of the reaction. The activated complex may be present at any point near the transition state.