In
chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in ...
, the entropy of activation of a reaction is one of the two parameters (along with the enthalpy of activation) which are typically obtained from the temperature dependence of a reaction
rate constant, when these data are analyzed using the
Eyring equation of the
transition state theory. The standard entropy of activation is symbolized and equals the change in
entropy
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
when the reactants change from their initial state to the
activated complex or transition state ( = change, = entropy, = activation). It determines the
preexponential factor
In chemical kinetics, the pre-exponential factor or A factor is the pre-exponential constant in the Arrhenius equation (equation shown below), an empirical relationship between temperature and rate coefficient In chemical kinetics a reaction r ...
of the
Arrhenius equation for temperature dependence of reaction rates. The relationship depends on the
molecularity of the reaction: for reactions in solution and unimolecular gas reactions , while for bimolecular gas reactions . In these equations is the base of
natural logarithm
The natural logarithm of a number is its logarithm to the base of the mathematical constant , which is an irrational and transcendental number approximately equal to . The natural logarithm of is generally written as , , or sometimes, if ...
s, is the
Planck constant
The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivalen ...
, is the
Boltzmann constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas consta ...
and the
absolute temperature.
' is the ideal gas constant in units of (bar·L)/(mol·K). The factor is needed because of the pressure dependence of the reaction rate. (bar·L)/(mol·K).
The value of provides clues about the
molecularity of the
rate determining step in a reaction, i.e. the number of molecules that enter this step. Positive values suggest that entropy increases upon achieving the transition state, which often indicates a
dissociative mechanism in which the activated complex is loosely bound and about to dissociate. Negative values for indicate that entropy decreases on forming the transition state, which often indicates an
associative mechanism in which two reaction partners form a single activated complex.
[James H. Espenson ''Chemical Kinetics and Reaction Mechanisms'' (2nd ed., McGraw-Hill 2002), p.156-160 {{ISBN, 0-07-288362-6]
References
Chemical kinetics