Entropy of Activation
Entropy of activation (ΔS*) is a thermodynamic parameter that describes the change in entropy when a reactant moves to the activated complex or transition state during a chemical reaction. In other words we can say that the entropy of activation is a measure of the randomness or disorderness associated with the transition from reactants to the transition state in a chemical reaction.
Entropy of activation may be positive or negative. It is positive when a reaction is initiated or catalyzed by heat because, the molecules gain thermal energy leads to increase molecular motion and disorder. Entropy of activation for a bimolecular process is usually negative, as the formation of the activated complex leads to a decrease in entropy of the system due to the reactants becomes more ordered in the transition state.
The standard free energy change (ΔFo) is related with equilibrium constant (K) of a reaction as-
ΔFo = −RTlnK
or, K = e−ΔFo/RT
or, K = e−ΔHo/RT. eΔSo/R
Where, ΔFo = standard free energy change
ΔSo = standard entropy change
ΔHo = standard enthalpy change
K = euqilibrium constant
For equilibrium constant K*-
or, K* = e−ΔH*/RT. eΔS*/R
Where ΔS* and ΔH* are the difference in the value of standard entropy chnage and standard enthalpy change respectively for the activated complex and reactant-
Reactants → Activated Complex → Products
