The Exchange Reaction between the Hydrogen Halides and the Halogens in the Gaseous State

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Abstract
It has been shown that hydrogen fluoride and fluorine gases do not undergo an exchange reaction at room temperature and that heating to 200°C for a matter of an hour or so in a brass vessel is necessary to accomplish exchange. Evidence is obtained that even in this case the observed exchange is heterogeneous and catalyzed by the metal fluorides on the wall of the vessel. Illumination by the light of a mercury arc did not accomplish the exchange under conditions such that an activation free energy of some 9 Kcal/mole for the reaction between a fluorine atom and the fluorine molecule or fluorine atom and HF molecule would have been necessary to prevent exchange. A rapid exchange reaction in the dark at room temperature has been observed to occur between chlorine and HCl in the gaseous state. This is taken as evidence that the hydrogen halides exchange with the halogens through the formation of an intermediate complex of the general formula HX3, and some discussion of the improbability of the alternative atomic mechanism is given. The non‐existence of the HX3 mechanism in the case of the fluorine system is explained by the absence of electrons in the higher shells which are thought to be the main bonding forces involved in the stabilization of the HX3 complex in the case of chlorine, bromine, and iodine, and also by the polar character of the hydrogen‐fluorine bond in HF, increasing the probability of an alternative HX3 complex formation in which the hydrogen rests between the fluorine on the HF molecule and the fluorine molecule itself, thus keeping it clearly labeled and rendering interchange on dissociation of the complex unlikely.

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