Hydrogen-iodide reaction

Example 4.2. The hydrogen-iodide reaction. The reversible formation of hydrogen iodide from hydrogen and iodine in the gas phase, with stoichiometry... 

Examples include nitration of aromatics, the hydrogen-iodide reaction, and radioactive decay. 

Reactions in which a molecule dissociates into two different or equal fragments are very common, although many are so fast that they are practically at equilibrium (e.g., see dissociation of I2 in the hydrogen-iodide reaction in Section 4.2). 

On the other hand, a power-law rate equation with integer reaction orders cannot be taken as evidence against fast pre-dissociation. An example is the hydrogen-iodide reaction, which involves fast pre-dissociation although the reaction orders are integers (see Example 4.2 in Section 4.2). Here, conversion 21 + H2 — 2HI of the dissociation products is second order in these, so that the square-root factor is squared, making the reaction first order in I2. 

Unfortunately, this procedure is not conclusive because an apparently plausible mechanism that avoids highly endothermic steps may well be blocked for other reasons. The hydrogen-iodide reaction... 

Extensive studies have been made of the oxidations of all the halides by hydrogen peroxide. Mellor in 1904 was already able to cite fourteen investigations of kinetics of the hydrogen peroxide-hydrogen iodide reaction including studies of the temperature dependence of the rates and the kinetic form of catalysis by salts of molybdenum and iron. Since the processes (1) and (2)... 

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. 

Figure 19.5 Arrhenius plots of the hydrogen iodide reactions (o)...

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