A toy model solved by statistical mechanics for teaching reaction kinetics beyond ideality

TL;DR

This paper introduces a simple statistical mechanics toy model to help students understand the complex and interaction-dependent reaction rates in non-ideal chemical systems, illustrating the lack of universal rules.

Contribution

It presents an educational toy model formulated via statistical mechanics to explore reaction kinetics beyond ideal solutions, emphasizing the complexity of non-ideal interactions.

Findings

Reaction rates are not universally connected to thermodynamic quantities.

The toy model demonstrates the dependence of reaction rates on microscopic interactions.

Students can solve the model with various interaction types as an educational exercise.

Abstract

Chemical equilibrium is fully characterized by thermodynamics, while the rates of chemical reactions can be calculated for ideal solutions by using mass-action equations. The evaluation of the rates of reactions in a non-ideal system is instead much more complex, even at equilibrium, being dependent on the microscopic features of the interactions: no universal theory exist. Here we propose a toy model to help students understand such complexity. It is formulated by means of statistical mechanics and aims at the evaluation of the exchange reaction rate at equilibrium. The toy model can be solved by the students, with various types of interactions, as an exercise. The results prove that no general rule connects the reaction rates to the thermodynamic quantities, such as the activity coefficients, dramatically proving the complexity and richness of the field.