Transition State Theory
Chemical reaction rates are influenced by a variety of factors, including temperature and the presence of catalysts. For example, an increase in temperature will increase reaction rates. According to collision theory, the reaction rate is equal to the frequency of successful collisions. What is mainly required for a successful collision of reactants (molecules entering a reaction) is a minimal quantity of energy (activation energy) as well as a specific spatial orientation of the reacting molecules. In other words, collisions not satisfying these prerequisite do not lead to a reaction. Collision theory explains why an increase in temperature accelerates a reaction: the kinetic energy of molecules rises with temperature. However, another theory is needed to explain the phenomenon of activation energy: transition state theory.
Transition state theory does not define colliding molecules as compact objects; in fact, the term "collision," although current in chemical vocabulary, does not accurately describe the interaction of molecules during a chemical reaction. According to transition state theory, as molecules approach one another prior to a reaction, their orbitals connect. As a result, the orbitals become deformed, weakening the existing bond between molecules. Weakened, bonds can break, and new bonds can be formed. For example, when hydrogen iodide (HI) molecule interact with chlorine (Cl) molecules, chlorine, which is negatively charged, will attract the electrons shared by the hydrogen and iodine atoms in the HI molecule, weakening the H-I bond. In order for the reaction to proceed, the H-I bond must be broken, which enables the creation of a new bond: H- Cl. Indeed, HCl, along with a molecule of iodine, is the outcome of the reaction. There is a point, however, during the reaction, as the old bond is breaking and the new one is forming, when an intermediary grouping of atoms appears: the activated complex. For example, the activated complex for a HI + Cl reaction would be the unstable I-H-Cl group of atoms.
Transition state theory also describes the changes in energy during a reaction. During the reaction process, as molecules approach each other and connect, their kinetic energy turns into potential energy. As the process proceeds, potential energy rises, reaching a maximum at the point of contact. This energy rise is also known as a reaction profile. Each reaction has its characteristic reaction profile. For example, for the HI + Cl reaction, there needs to minimum of kinetic energy converted into potential energy if the reaction is to succeed. This minimum of energy is called activation energy, the level of energy needed to activate a reaction.
Transition state theory stems from the pioneering research by Michael Polanyi (1891-1976) and Henry Eyring (1901-1981) in the 1920s and 1930s. Assuming that the current theory of chemical change was incomplete, Polanyi and Eyring proceeded to study the changes in potential energy during chemical reactions, eventually formulating a theory which was in accord with quantum mechanics.
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