H2-I2 reaction

For many years, it was assumed that the H2-I2 reaction occurs in a single step, a collision between an H2 molecule and an I2 molecule. That would, of course, be compatible with the rate expression above. However, there is now evidence to indicate that a quite different and more complex mechanism is involved (see Problem 77). 

There are many conceivable analogues of this reaction. We mention only the famous H2 + I2 reaction, long supposed to proceed via a rectangular transition state, but shown by Sullivan7 to involve iodine atoms. The rectangular transition state is, in the light of the Woodward-Hoffmann rules, obviously forbidden. 

Inorganic chain mechanisms are of much lower prevalence than the typical organic reactions that make up the vast bulk of chain reactions. Of the inorganic reactions, the classic examples are the H2/Br2 and H2/C12 reactions, with the first being the most straightforward. As mentioned earlier, the H2/I2 reaction was always used as an example of an elementary bimolecular reaction, but was finally shown to have a complex, non-chain mechanism. In contrast, the H2/Br2 and H2/C12 reactions are chain reactions. 

While this work is requiring the revision of many textbooks which have used the hydrogen-iodine reaction as a classic example of a bimolecular reaction, it has also aroused interest in its implications for absolute reaction rate theory. Noyes has suggested that the results present a paradox in kinetics. In his discussion, he suggests that absolute rate theory as normally formulated fails to account for momentum effects which in some cases, namely the H2-I2 reaction, place severe restrictions on the path leading from reactants to products. In the... 

Bodenstein and Lind have studied the thermal reaction between H2 and Br2 to form HBr. Their careful study showed this reaction to be of apparently much greater complexity than the H2-I2 reaction. As was pointed out later in the interpretations of this data by Christiansen, Herzfeld, and Polanyi the reaction proceeds by a free radical chain mechanism involving bromine and hydrogen atoms. Assuming the bromine atom concentration to be governed by the Br2 = 2Br thermal equilibrium, the mechanism of HBr formation is... 

It is possible to use K to calculate the extent to which reaction occurs when an equilibrium is disturbed by adding or removing a product or reactant To show how this is done, consider the effect of adding hydrogen iodide to the HI-H2-I2 system (Example 12.7). 

Colorimetric analysis based on visual estimation is not very exact. Some more accurate data on the H2, I2, HI system at equilibrium are shown in Table 9-1. The reaction is... 

Discussion. The iodimetric method utilises the reversible reaction H2S + I2 2H+ +21- +S... 

Use reactant molecules to write appropriate elementary reactions that satisfy the following criteria (a) a unimolecular decomposition that generates I (b) a bimolecular collision that forms a square H2 I2 complex and (c) a bimolecular collision in which a hydrogen atom is transferred between... 

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). 

Class II second-order rate expressions are one of the most common forms one encounters in the laboratory. They include the gas phase reaction of molecular hydrogen and iodine (H2 + I2 -> 2HI), the reactions of free radicals with molecules (e.g., H -f Br2 -> HBr -f Br), and the hydrolysis of organic esters in nonaqueous media. 

If one has data on the reaction rate constant at several temperatures, this equation provides the basis for the most commonly used method for determining the activation energy of a reaction. If E is temperature invariant, a plot of in k versus the reciprocal of the absolute temperature should be linear with slope —(E/R). A typical plot is shown in Figure 3.9 for the reaction H2 + I2 - 2HI. The slope corresponds to an activation energy of 44.3 kcal/mole. 

Since initially, [H2] = [I2] = 0, the forward reaction will predominate. 

Bimolecular reaction.—In the bimolecular reaction A+B M+N, let C A and CB respectively denote the concentrations of the substances A and B, expressed in mol. per litre. Similarly, let CM and CN respectively denote the concentrations of M and N. It has previously been shown that the speed of the reaction is equal to the product of the affinity or the driving force of the reaction, k, and the concentrations of the reacting substances, that is, the velocity of the reaction A+B is equal to kC CB. If A and B are the same, so that 2Av M+N, the speed of the -> reaction at any instant will be represented by kCA2. When hydrogen iodide dissociates 2HI H2+I2. The speed of the - reaction at any instant will be represented by kCBI2 and the speed of the <- reaction by k CiCB. When equilibrium occurs, the speeds of these two reactions are the same, and therefore the condition o equilibrium is kCrr —k C Ci, or K—kjV—C HCi/C Hi2. At 440°, when the system is in equilibrium, nearly 20 per cent, of the hydrogen iodide will have dissociated. Hence, at 440° (80 per cent.) 2HIt H2- -I2(20 per cent.). This means that if 100... 

Develop a reaction mechanism for iodine (I2-O2-H2 system) from the information in the NIST Chemical Kinetics Database [256], Start with the H2-O2 reaction subset hydrogen.mec. Using the database, identify the relevant reactions with I2. Add these reactions to the starting mechanism, including product channels and rate constants. List the additional I-containing species formed in reactions of I2. Extend the reaction mechanism with reactions of these species. Continue this procedure until reactions of all relevant iodine species in the I2-O2-H2 system is included in the mechanism. 

Let us first consider kinetic energy of translation for the sake of simplicity. At the moment of collision most of this kinetic energy may be transferred to the interior of the molecules, and the result may be a sufficient loosening of the two molecules of HI to permit their rearrangement to H2 + I2. Activated reaction products are thus produced, which in due season lose their excess energy by collision or otherwise. 

The equilibrium composition of mixtures of H2 and h is studied at various temperatures in order to get the following values of Kp. Use these data, preferably with the method of least squares, to determine the enthalpy of the reaction H2 + I2 2HI... 

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