Are you an EPFL student looking for a semester project?
Work with us on data science and visualisation projects, and deploy your project as an app on top of Graph Search.
Concept
Entropy of activation
Summary
In chemical kinetics, 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 ΔS‡ and equals the change in entropy when the reactants change from their initial state to the activated complex or transition state (Δ = change, S = entropy, ‡ = activation).
Entropy of activation determines the preexponential factor A 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
A = (ekBT/h) exp(ΔS‡/R),
while for bimolecular gas reactions
A = (e2kBT/h) (RT/p) exp(ΔS‡/R).
In these equations e is the base of natural logarithms, h is the Planck constant, kB is the Boltzmann constant and T the absolute temperature. R' 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. R' = 8.3145 × 10−2 (bar·L)/(mol·K).
The value of ΔS‡ 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 ΔS‡ 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.
It is possible to obtain entropy of activation using Eyring equation. This equation is of the form where:
= reaction rate constant
= absolute temperature
= enthalpy of activation
= gas constant
= transmission coefficient
= Boltzmann constant = R/NA, NA = Avogadro constant
= Planck's constant
= entropy of activation
This equation can be turned into the formThe plot of versus gives a straight line with slope from which the enthalpy of activation can be derived and with intercept from which the entropy of activation is derived.
Official source
About this result
This page is automatically generated and may contain information that is not correct, complete, up-to-date, or relevant to your search query. The same applies to every other page on this website. Please make sure to verify the information with EPFL's official sources.
Ontological neighbourhood
Related courses (6)
CH-160(b): General chemistry
Cet enseignement vise l'acquisition des notions essentielles relatives à la structure de la matière, aux équilibres et à la réactivité chimiques. Le cours et les exercices fournissent la méthodologie
CH-160(f): Advanced general chemistry
Cet enseignement vise l'acquisition des notions essentielles relatives à la structure de la matière, aux équilibres et à la réactivité chimiques. Le cours et les exercices fournissent la méthodologie
CH-160(g): Advanced general chemistry
Cet enseignement vise l'acquisition des notions essentielles relatives à la structure de la matière, aux équilibres et à la réactivité chimiques. Le cours et les exercices fournissent la méthodologie
Related lectures (9)
Phosphine Dissociation Mechanisms
Delves into olefin coordination mechanisms, phosphine dissociation, ligand effects, and catalyst reactivity.
Pericyclic Reactions: Mechanisms and Tools
Explores pericyclic reactions, focusing on mechanisms, controls, correlation diagrams, and tools to determine reaction mechanisms.
Chemical Engineering of Heterogeneous Reactions
Explores observed rates in heterogeneous reactions, internal mass transfer impact, reaction kinetics, Arrhenius equation, and heat transfer effects.
Related publications (45)
Bending and Stretching in a Narrow Ribbon of Nematic Polymer Networks
We study the spontaneous out-of-plane bending of a planar untwisted ribbon composed of nematic polymer networks activated by a change in temperature. Our theory accounts for both stretching and bending energies, which compete to establish equilibrium. We s ...
SPRINGER2023Reduction of PdO/Al2O3 in Liquid Cyclohexane Followed In Situ by ATR-IR, High-Energy XRD, and XAS
Davide Ferri, Maarten Nachtegaal
Supported Pd-based catalysts used in heterogeneously catalyzed liquid phase processes often undergo an activation step through reduction of the oxide phase or of the oxidic passivation layer prior to reaction. An oxidized Pd/Al2O3 catalyst was reduced by u ...
AMER CHEMICAL SOC2021Related people (5)
Related units (3)
Related concepts (6)
Pre-exponential 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. It is usually designated by A when determined from experiment, while Z is usually left for collision frequency. The pre-exponential factor can be thought of as a measure of the frequency of properly oriented collisions. It is typically determined experimentally by measuring the rate constant at a particular temperature and fitting the data to the Arrhenius equation.
Eyring equation
The Eyring equation (occasionally also known as Eyring–Polanyi equation) is an equation used in chemical kinetics to describe changes in the rate of a chemical reaction against temperature. It was developed almost simultaneously in 1935 by Henry Eyring, Meredith Gwynne Evans and Michael Polanyi. The equation follows from the transition state theory, also known as activated-complex theory.
Reaction rate constant
In chemical kinetics, a reaction rate constant or reaction rate coefficient (k) is a proportionality constant which quantifies the rate and direction of a chemical reaction by relating it with the concentration of reactants. For a reaction between reactants A and B to form a product C, where A and B are reactants C is a product a, b, and c are stoichiometric coefficients, the reaction rate is often found to have the form: Here k is the reaction rate constant that depends on temperature, and [A] and [B] are the molar concentrations of substances A and B in moles per unit volume of solution, assuming the reaction is taking place throughout the volume of the solution.