Chemical reactions are usually very difficult to occur in ice at low temperature, where atoms and molecules are frozen in position with minimal thermal energy and entropy. Contrary to this general behavior, certain weak acids including fluoroacetic acids dissociate spontaneously in ice at temperature as low as 10 K. The acid dissociation yields in cryogenic ice are higher than that in aqueous solution at room temperaure. The enhanced reactivity of these weak acids is an unexpected consequence of proton transfer equilibrium in ice. The configurational entropy of highly mobile protons in ice shifts the acid dissociation equilibrium forward. Although this entropy is generated in the solid phase of water, it is comparatively large in magnitude with the entropies of water vaporization and acid dissociation reactions in aqueous phase. Therefore, this themodynamic driving force may be quite generally effective for driving proton transfer reactions in the ice environments.

Enhanced dissociation of weak acids in cryogenic ice: Configurational entropy of mobile proton is the driving force

2019 Fall National Meeting

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