Application of the Theory of Absolute Reaction Rates to Heterogeneous Processes II. Chemical Reactions on Surfaces

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Abstract
The method developed in the preceding paper is used to derive equations for the rates of unimolecular and bimolecular heterogeneous gas reactions under various conditions of surface coverage by either reactants or poisons. The resulting equations are of the same form as those previously deduced by Langmuir and others, but they are more explicit; provided the activation energy is known they can be utilized to calculate rates of surface reactions which are in good agreement with the observed values. In the case of the dissociation of hydrogen on a tungstensurface, the simple assumption that the activated state consists of hydrogen atoms permits the absolute rate of the reaction to be calculated with an accuracy which is at least as good as that attained by direct experimental measurement. The factors responsible for the difference in rates of the same reaction taking place homogeneously or heterogeneously are considered; it is shown that adsorption of the activated complex lowers the over‐all activation energy, and this has the most important influence in favoring the surface reaction. If one of the products, or any other substance acting as a poison, is strongly adsorbed, the effective activation energy is increased, but there is some compensation resulting from the increase of entropy accompanying the desorption of the poison.

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