Hydrogen iodide ions, decomposition

The iodide ion-catalyzed decomposition of hydrogen peroxide to oxygen gas and water is believed to occur via the following mechanism. 

Catalysts are commonly classified as either homogeneous or heterogeneous. A homogeneous catalyst is one that exists in the same phase as the reactants. For example, iodide ion is a homogeneous catalyst for the decomposition of aqueous hydrogen peroxide because both 1 and H202 are present in the same aqueous solution phase. 

Typical among the examples of second-order reactions of type I are the gas-phase thermal decomposition of hydrogen iodide, 2HI —> H2 + I2 the gas-phase thermal decomposition of NO2, 2NO2 2NO + O2 the liquid-phase decomposition of C10 ion, 2C10 " —> 2C1 + 62 and the dimerization of cyclopentadiene in either gas or liquid phase, 2C5H6 —> C10H12. Actually, type I reactions are relatively rare in comparison with type II reactions. 

Adding a drop of potassium iodide solution speeds up the reaction. On the other hand, adding a few crystals of insoluble manganese dioxide, Mn02(x), causes a violent decomposition to occur. The iodide ion, I (a ), and manganese dioxide are two of many catalysts for the decomposition of hydrogen peroxide. 

FIGURE 13.19 The decomposition of hydrogen peroxide is catalyzed by the iodide ion. A few drops of liquid soap have been added to the solution to dramatize the evolution of oxygen gas. (Some of the iodide ions are oxidized to molecular iodine, which then reacts with iodide ions to form the brown triiodide ion, l. j... 

The decomposition of hydrogen peroxide is facilitated by iodide ions (Figure 13.19). The overall reaction is... 

For the decomposition of hydrogen peroxide we saw that the reaction rate depends on the concentration of iodide ions even though 1 does not appear in the overall equation. We noted that 1 acts as a catalyst for that reaction. A cate/js/ is a substance that... 

The decomposition of hydrogen peroxide in the presence of iodide ion is believed to occur via this mechanism. 

Excellent examples are found in the hydrolytic reactions of esters with alkalis, the interaction of alkyl halides with hydroxyl ions, the benzoylation of amines, and the union of tertiary amines with alkyl halides to give quaternary ammonium salts. Among gas-phase reactions the decomposition of hydrogen iodide, on the one hand, and the union of hydrogen with iodine on the other provide the classical examples. The absolute rates in this last case are rather closely given by collision number... 

In this experiment you will study the rate of decomposition of hydrogen peroxide catalyzed by iodide ion, I", or really by 13 produced at the very beginning of the reaction by the oxidation of I by H2O2. The following equation summarizes the reaction... 

The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism ... 

The rate of the decomposition of hydrogen peroxide, H2O2, depends on the concentration of iodide ion present. The rate of decomposition was measured at constant temperature and pressure for various concentrations of H2O2 and of KI. The data appear below. Determine the order of reaction for each substance, write the rate law, and evaluate the rate constant. 

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. 

The stability of the hydrogen dihalide ion is especially interesting. As would be e)q)ected the [HX ] species decompose to form HX and X. The difluoride ion, having a large hydrogen bond energy of about 58 kcals pet mole (10) is the most stable and the di-iodide ion is the least. At low temperatures it appears that more than one solvent molecule is coordinated to the halide ion by means of a hydrogen bond, and there is evidence for the ions [Cl(HCl)j] ,[Bt(HCl) ] , [l(HCl)j] and [I(HBr) ] . The mixed species, MX(HY) are not very stable and there are two possibilities for decomposition. 

We saw in Section 14.5 that the reaction rate for the decomposition of hydrogen per-oxideJepends on thq onc traton erf iodide ions even though I does not appear in the ov ( aQ)n. insyaH I the reaction. A catalyst is a sub-... 

Baxendale (1952), Liebhafsky (1932), and Liebhafsky and Mohammad (1933) have discussed and studied the kinetics of the reduction, in acid solution, of hydrogen peroxide by iodide ion. This decomposition is of the general form... 

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