Decomposition of hydrogen iodide

Many second-order reactions follow Class I rate expressions. Among these are the gas-phase thermal decomposition of hydrogen iodide (2HI - H2 + I2), dimerization of cyclopen-tadiene (2C5H6 -> C10H12), and the gas phase thermal decomposition of nitrogen dioxide (2N02 2NO + 02). 

The decomposition of hydrogen iodide is a reversible second order reaction in both directions ... 

For the decomposition of hydrogen iodide in the range 550-750 Bodenstein represented the temperature dependence of the specific rate by... 

A second-order reaction has an overall reaction order of 2. An example of a second-order reaction is the decomposition of hydrogen iodide. 

Decomposition of Hydrogen Iodide Porous Vycor glass membranes Nonporous Pd/Ag membranes Itoh et al (1984) Yeheskel, Leger and Courvoisier (1979)... 

Decomposition of hydrogen iodide. No catalyst present (gas phase decomposition)... 

According to the authors cited above, equation (16) holds for the decomposition of hydrogen iodide on platinum and of nitrous oxide on gold, and furthermore, according to Schwab and Drikos (12), for the oxidation of carbon monoxide to carbon dioxide on copper oxide. 

Fig. 1 a.—Influence of temperature on the rate of decomposition of hydrogen iodide. 

The Arrhenius Equation applies equally well to homogeneous and heterogeneous reactions. The figure shows Bodenstein s results for the homogeneous decomposition of hydrogen iodide. Other examples are shown also. 

Taylor studied the decomposition of hydrogen iodide by a streaming method, using a tube of Pyrex glass of 600 J. Physical Chem., 1924, 28, 984. 

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. 

Where the rate of reaction depends upon the presence of an accidental catalyst, measurements are characterized by great lack of reproducibility. Many homogeneous reactions, on the other hand, have quite definite and reproducible rates. For example, the measurements of Bodenstein and of Kistiakowsky on the rate of decomposition of hydrogen iodide agree excellently, as do those of numerous investigators of the rate of decomposition of nitrogen pentoxide. 

Many examples of this type of reaction are known the decomposition of arsine the decomposition of phosphine on surfaces of glass, f porcelain, J silica the decomposition of formic acid vapour on a variety of different surfaces— glass, platinum, rhodium, titanium oxide, and others the decomposition of nitrous oxide on the surface of gold Tf the decomposition of sulphuryl chloride on the surface of glass the decomposition of hydrogen iodide on the surface of platinum ff the decomposition of hydrogen selenide on the surface of selenium. J J A general discussion... 

Another example of a reaction nearly independent of pressure in this way is the catalytic decomposition of hydrogen iodide on the surface of a heated gold wire. X The initial pressure of the gas can be varied from 100 mm. to 400 mm. with a resulting change in the absolute rate of reaction which amounts to about 45% only instead of 400%. 

The decomposition of hydrogen iodide on the surface of gold, and the decomposition of ammonia on the surface of tungsten, nearly conform to the condition. 

It is probable that the actual mechanism of the decomposition is unimolecular, but, the observed order of the reaction being zero, we cannot tell whether the molecules decompose singly or by interaction with their neighbours. The catalytic decomposition of hydrogen iodide on the surface of platinum can actually be shown to be unimolecular. The heat of activation in this instance is even lower (14,000 cals.), but is again subject to the same uncertainty as the values for the unimolecular reactions of nitrous oxide. 

Another instance of the same curious phenomenon can be cited. Hydrogen has a most pronounced inhibiting effect on the thermal decomposition of ammonia at the surface of a heated platinum wire. On the other hand, it is found to be almost without influence on the decomposition of hydrogen iodide on the surface of the same wire, Compt. Rend., 1922, 175, 277. 

Phosphoric acid is a much weaker oxidizing agent than sulphuric acid, and it fails to oxidize either hydrogen bromide or hydrogen iodide. The brown color, it is true, indicates a trace of free iodine, but this is accounted for by a direct decomposition of hydrogen iodide by heat. 

Should, e. g. the decomposition of hydrogen iodide solution (anJ =1), proceeding according to the equation... 

Different Ways of Expressing the Rate of Reaction There is usually more than one way to measure the rate of a reaction. We can study the decomposition of hydrogen iodide, for example, by measuring the rate at which either H2 or I2 is formed in the following reaction or the rate at which HI is consumed. 

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