A study of the entropic and electrolyte effects in electron transfer reactions

The entropy of activation ΔS^‡ for electron transfer reactions occurring in concentrated electrolyte solutions is calculated, taking into account the electrostatic and electronic contributions. By applying the mean spherical and exponential approximations to treat a simple model of electrolytes (hard charged spheres in a dielectric continuum), the pair distribution function is calculated. These approximations are also used to calculate the electrolyte reorganization term, since both quantities contribute to the electrostatic part of ΔS^‡. Numerical application is made to the ferrous-ferric exchange reaction in 0.55 M HClO_4. Results of a preliminary electron tunneling calculation for the electron transfer are included. Reasonable agreement between observed and calculated ΔS^‡ is obtained.