A Physical Basis for Kinetic Compensation.

Area of Science:

• Physical Chemistry
• Chemical Kinetics
• Thermodynamics

Background:

• The kinetic compensation effect (KCE) describes a positive correlation between Arrhenius activation energy (E) and frequency factor (A).
• Despite over 100 years of research and thousands of publications, the cause of KCE remains debated.
• KCE is often visualized using Constable plots, showing a linear relationship between ln[A] and E/R.

Purpose of the Study:

• To propose a physical basis for the kinetic compensation effect (KCE) and isokinetic relationship (IKR).
• To suggest that KCE arises from the path dependence of reaction history.
• To reconcile KCE with IKR through a proposed model.

Main Methods:

• Analysis of the single-step rate law approximation for reversible reactions.
• Introduction of a dynamic thermal equilibrium temperature (T0 = H°/ΔS°).
• Relating the slope of Constable/KCE plots to T0 and reaction path dependence constant (k0).

Main Results:

• A linear relationship between ln[A] and E is attributed to path dependence of reaction history.
• The proposed model provides a physical basis for KCE and IKR, defining a crossover temperature (1/T0).
• Qualitative agreement was found between calculated ΔH° and ΔS° values and literature data for thermal decomposition reactions.

Conclusions:

• The study suggests reaction path dependence as the underlying cause of the kinetic compensation effect.
• The proposed physical model successfully reconciles KCE with isokinetic relationships.
• The findings are supported by experimental data from thermal decomposition of organic peroxides, calcium carbonate, and poly(methyl methacrylate).