Wednesday, October 25, 2017

kinetics - Are there any reactions with no activation energies?


Are there any reactions with no activation energies? Our professor just told us there are no chemical cliffs. Is this true?


I read something about nuclear decay as being a reaction with no activation energies. Why is this so, and how does this work?



Answer



Marcus-Hush theory describes electron/charge transfer rates using conventional transition state theory, e.g.:


$\ce{A+ + B -> A + B+}$


Now one might naively assume that there's no activation barrier in a simple electron transfer, but that's not true. Since the geometry of $\ce{A+}$ and $\ce{A}$ as well as $\ce{B}$ and $\ce{B+}$ are not identical, there is a reorganization energy of both the molecular species and the solvent or environment. This reorganization energy ($\lambda$) serves as an activation barrier for the charge transfer.


Now the convenient thing about charge transfer reactions is that you can tune the $\Delta G$ of the reactants.



Marcus predicted that the maximum rate would occur when the $\Delta G^0 = -\lambda$ and thus the $\Delta G^\ddagger = 0$. If the reaction becomes more thermodynamically favorable, you enter the inverted region and the activation barrier goes up and the reaction rate goes down.


This is a long-winded answer to say there are some chemical reactions with no activation barrier, in addition to the other cases here. Usually the key phrase describing these in papers is "barrier-less reactions"


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