TRANSITION-STATE THEORY

In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy also called activated-complex theory, or theory of absolute reaction rates, a conception of chemical reactions or other processes that involve rearrangement of matter as proceeding through a continuous change in the relative positions and potential energies of the constituent atoms and molecules. Between the initial and final arrangements of atoms or molecules there exists an intermediate configuration for which the energy arising from interatomic and intermolecular forces (potential energy) reaches a maximum; this configuration corresponds to the transition state. The difference between the energies of the transition and the initial states is called the activation energy; it represents the minimum energy that a reacting or flowing system must acquire for the transformation to take place. The variation of energy during a typical transformation from an initial to a final state is shown in the Figure. The reaction coordinate represents the extent of the transformation from the initial state to the final state rather than distances between atoms or molecules in the system undergoing change. The activated complex is a hypothetical, transient molecule considered to be in a state of equilibrium with the atoms or molecules in the initial state and therefore amenable (to some extent) to specification of thermodynamic properties. The rate by which the final state is attained is determined by the number of activated complexes formed and the frequency with which they go over to the final state. These quantities may be calculated for simple systems using statistical-mechanical principles. In this way the rate constant k of a chemical reaction or a transport process may be expressed in terms of measurable properties, such as atomic or molecular dimensions, atomic masses, and interatomic or intermolecular forces and differences in thermodynamic properties between the initial and transition statese.g., free energy, entropy, and enthalpy.