Entropy of activation

Entropy of activation is the entropy difference between reactants and a reaction’s transition state. In transition-state theory, it helps describe the activation barrier and influences the reaction rate.

Transition-state theory: A framework that relates reaction rates to the properties of transition states and activation free energies. It uses activation entropy alongside activation enthalpy to estimate how quickly a reaction proceeds.
Entropy: A thermodynamic state function associated with energy dispersal and the number of accessible microscopic arrangements. Activation entropy compares this quantity across reactants and the transition state.
Enzyme catalysis: The acceleration of chemical reactions by enzymes through stabilization of reaction pathways and transition states. Enzymes can alter activation entropy by constraining substrates or organizing reacting groups.
Entropy of reaction: The entropy difference between products and reactants in a chemical reaction. It compares stable endpoints, whereas activation entropy compares reactants with the transition state.
Activation free energy: The Gibbs free-energy difference between reactants and a transition state. Its enthalpic and entropic contributions jointly determine the barrier associated with this entropy.
Transition state: A fleeting, highest-free-energy configuration along a reaction pathway, located at a potential-energy saddle point. Its entropy relative to the reactants defines the activation entropy.
Kinetic isotope effect: A change in reaction rate caused by replacing an atom with one of its isotopes. Temperature-dependent isotope effects can help distinguish entropic from enthalpic contributions to a barrier.
Entropy of formation: The entropy change associated with forming a substance from specified reference states. Its reference process differs from the reactant-to-transition-state comparison defining activation entropy.
Activation enthalpy: The enthalpy difference between reactants and a transition state. It is the energetic counterpart to activation entropy in the activation free-energy relation.
Gibbs free energy: A thermodynamic potential combining enthalpy and entropy at constant temperature and pressure. Activation free energy combines the corresponding enthalpic and entropic barrier contributions.

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