The Free Energy of Activation as the critical factor in geochemical processes

By studying the Free Energy of Activation, ΔG≠, of various geochemical transformations we have revealed the importance of the contribution of the entropy of activation, ΔS≠, in the energetics of the processes. In studies performed so far including changes of phases, adsorption, desorption, formation or breaking of bonds, ordering of ions, etc. only the activation energy was considered as the main factor determining the rate of the transformation process through the Arrhenius equation. The above changes result in change of degrees of freedom of the systems. If the above changes happen during the “reactants to transition state or activated complex” step, they result in changes of the entropy of activation. Through the use of the Eyring–Polanyi equation and literature data we were able to determine ΔG≠ = ΔH≠ − TΔS≠ where ΔH≠ is related to Eact = RT + ΔΗ≠ and ΔS≠ is related to A of the Arrhenius equation. It was found that the combination of enthalpy and entropy of activation in ΔG≠ gives a more realistic/true value of the energy requirements of the activation step that the processes need in order to take place. Also, an explanation is given of why calculated activation energy values (that are related only to enthalpy of activation values) for certain transformations deviate from the expected and observed energy requirements that characterize the processes when the entropic component is substantial. This analysis shows that similar processes have similar ΔG≠ values and therefore there is a way of foreseeing the ΔG≠ of a process, if a number of similar processes have been studied and their ΔG≠ values have been calculated.

► Free Energy of Activation in geochemical processes. ► Free Energy of Activation rather than Activation Energy. ► Enthalpy of activation related to Activation Energy. ► The Eyring vs. the Arrhenius equation. ► Entropy of activation as a tool for suggesting mechanisms.