Living Reference Work Entry

Encyclopedia of Geochemistry

Part of the series Encyclopedia of Earth Sciences Series pp 1-3

Date: Latest Version

Activation Parameters: Energy, Enthalpy, Entropy, and Volume

  • William H. CaseyAffiliated withDepartment of Chemistry, Department of Earth and Planetary Sciences, UC Davis Email author 
  • , C. André OhlinAffiliated withDepartment of Chemistry, Umeå University


Activation parameters are used in chemical kinetics to describe the equilibrium between reactants of a chemical reaction and a single, unstable molecule called a transition state.

Transition State Theory and Elementary Versus Overall Reactions

Transition-state theory is the central underpinning of all chemical kinetics. It postulates that for elementary reactions, there is equilibrium between reactants and an unstable form called the transition state, which is a single molecule. The equilibrium is conventional in all ways except that the transition state has a loose vibrational mode that dissociates and leads to products (Glasstone et al. 1941). The thermodynamic terms that describe this activated equilibrium between the reactants and the transition state are called activation parameters and have exact analogues with the standard thermodynamic parameters that describe a conventional equilibrium between the ...

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