Aktiveringsentropi

Aktiveringsentropi, often denoted as ΔS‡, is a thermodynamic parameter crucial in understanding reaction kinetics, particularly within the framework of transition state theory. It represents the change in entropy that occurs when reactants transform into the activated complex, also known as the transition state. This activated complex is a fleeting, high-energy molecular arrangement that exists at the peak of the reaction's energy barrier.

A negative value for activation entropy (ΔS‡ < 0) indicates that the formation of the activated complex involves a decrease in the system's disorder. This often happens when reactants, which are relatively free to move and orient themselves, become more constrained and ordered in the transition state. For instance, if a reaction involves two or more molecules coming together to form a single activated complex, the translational and rotational freedom of the individual molecules is reduced, leading to a more ordered state and thus a negative entropy change.

Conversely, a positive activation entropy (ΔS‡ > 0) suggests an increase in disorder upon forming the activated complex. This can occur in reactions where a larger molecule breaks apart or becomes less structured in the transition state compared to the reactants. For example, if a single reactant molecule undergoes bond breaking to form a more fragmented or less ordered activated complex, the entropy might increase.

The activation entropy, along with activation enthalpy (ΔH‡), helps to determine the activation free energy (ΔG‡), which directly dictates the rate of a chemical reaction. The relationship is given by the Eyring equation, a fundamental equation in chemical kinetics that links the rate constant of a reaction to these thermodynamic parameters. Therefore, studying activation entropy provides valuable insights into the molecular mechanisms and the nature of the transition state for a given reaction.