Hydrogen iodide, decomposition formation

Even at the ordinary temperature hydrogen iodide is readily decomposed by thionyl chloride, giving hydrogen chloride, iodine, sulphur and sulphur dioxide, but hydrogen bromide undergoes a double decomposition with formation of thionyl bromide and hydrogen chloride 3... 

Rice, Fryling, and Weselowski (J. Amer. Chem. Soc., 1924, 46, 2405) make all reaction rates proportional to the concentration of what they call residual molecules, which have to be formed endothermically from one of the reactants. The proportion of these increases with temperature and accounts for the increase in reaction rate. Something of this kind may be true in special cases, for example, in the formation of HBr the residual molecule would be the bromine atom. But this resolution into atoms is only the limiting case of ordinary activation, and it is difficult indeed to see what the residual molecule could be, or what tautomeric change could occur in the simple decomposition of hydrogen iodide or nitrous oxide. 

Among the alkyl iodides, the iodine yield rises regularly, upon irradiation, for each increase in the number of substituted jS-hydrogen atoms in the molecule (refs. 121, 249, 303, 548, 549, 614). Typical G-values recorded for iodine formation are Mel, 1.26 Etl, 2.12 Pr"I, 1.60 PrT, 2.68 Bu"I, 1.69 Bu I, 2.52. These trends suggest that decomposition of ethyl and higher alkyl iodides may, in part, occur by unimolecular elimination of hydrogen iodide from excited molecules - 303,395 jodine could then be formed by subsequent reaction of hydrogen iodide with alkyl radicals, as follows... 

The decomposition of hydrogen peroxide and the formation of hydrogen iodide from molecular hydrogen and molecular iodine illustrate the elucidation of reaction mechanisms by experimental studies. 

To calculate the second-order rate constants for the formation and decomposition of hydrogen iodide in the gas phase. 

By assigning the values of 13, 10.5, and 10, respectively, to ( — 3/2), the author finds expressions for k which fit the data of Bodenstein (19) on the decomposition and the formation of hydrogen iodide, and the data of Hin-shelwood and Burk (72) on the decomposition of nitrous oxide somewhat... 

How can we find out whether the proposed mechanism for a particular reaction is correct In the case of hydrogen peroxide decomposition we might try to detect the presence of the IO ions by spectroscopic means. Evidence of their presence would support the reaction scheme. Similarly, for the hydrogen iodide reaction, detection of iodine atoms would lend support to the two-step mechanism. For example, I2 dissociates into atoms when it is irradiated with visible light Thus, we might predict that the formation of HI from H2 and I2 would speed up as the intensity of light is increased because that should increase the concentration of I atoms. Indeed, this is just what is observed. 

A general mechanism of surface catalysis involves (a) diffusion of reactants to the surface of the catalyst, (b) a fast reaction between the molecules of the reactants and the atoms in the surface of the solid catalyst (adsorption), followed by (c) the formation of the transition state (the rate-determining step), which then decomposes rapidly to the catalyst and the products. For example, at about BOOOK the homogeneous decomposition of hydrogen iodide,... 

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