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How does an activated complex form during a chemical reaction?

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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.

In chemical kinetics, an activated complex forms as a system moves along a reaction path between the initial reactants and the final products. According to transition-state theory, this process involves a continuous change in the relative positions and potential energies of the constituent atoms and molecules.12

As a reaction progresses, the system must overcome an energy barrier. The activated complex represents an intermediate configuration at the peak of this potential energy surface, where the potential energy reaches its maximum value. During this transition, existing bonds may begin to break while new bonds simultaneously begin to form.13

The energy required to reach this state is known as the activation energy, which is the difference in energy between the initial reactants and the activated complex. In this theoretical framework, the activated complex is considered to be in a state of equilibrium with the reactants, allowing its thermodynamic and statistical properties to be defined. The overall rate of the reaction is determined by the concentration of these activated complexes and the frequency with which they proceed to form the final products.143