H2-CI2 reaction

Most of the chain reactions show such chemical inhibition, and marked inhibition by trace impurities is excellent evidence for a chain reaction. Thus 0.01 mole per cent of O2 is capable of reducing the quantum yield of the H2 + CI2 reaction by 1000 fold.- In a similar fashion free radicals are easily adsorbed on vessel surfaces, and it is common experience to find that a change in vessel shape, in surface/volume ratio (e.g., that obtained by packing the vessel with glass), or the addition of inert gases such as... 

It is obvious, then, that only the H2-CI2 reaction can be exploded photochemi-cally, that is, at low temperatures. The H2-Br2 and H2-I2 systems can support only thermal (high-temperature) explosions. A thermal explosion occurs when a chemical system undergoes an exothermic reaction during which insufficient heat is removed from the system so that the reaction process becomes self-heating. Since the rate of reaction, and hence the rate of heat release, increases exponentially with temperature, the reaction rapidly runs away that is, the system explodes. This phenomenon is the same as that involved in ignition processes and is treated in detail in the chapter on thermal ignition (Chapter 7). 

Since the H2-CI2 reaction has been reviewed several times -" we will consider only briefly the early work. Draper appears to have made the earliest studies. Later, Bodenstein et carried out important experiments which served to stimulate the development of the present understanding of the reaction. 

The activation energy for (2) has been calculated to be 5.5 kcal.mole while (3) proceeds with little or no activation energy. Reaction (3), on the other hand, is about 100 times as fast as (4) near room temperature . In treating their results on the H2-CI2 reaction in the presence of NOCl, Ashmore and Chan-mugam have found the rate coefficient of (2) to be 5 x 10 l.mole . sec at 250 °C. Combining this result with earlier work at other temperatures leads to... 

We will illustrate the use of the steady-state approximation for solving the rate expressions in the reaction between hydrogen and bromine, which proceeds according to a scheme that is slightly more complicated than that for the H2 + CI2 reaction... 

This is exactly the way Bodenstein derived his equation for the H2 + CI2 reaction, assuming steady state in the chain reaction. 

In contrast, for the Hj/Brj and H2/CI2 reactions, which have much lower E2, the chain reaction mechanisms have much lower activation energies than the molecular process (see Table 12.2). 

Almost all of the reactions dealt with in this book are overall reactions that consist of a series of elementary steps involving a large number of intermediates. This is true even for such apparently simple reactions as 2O3 3O2 or H2 + CI2 —> 2HC1. 

Historically, the steady state approximation has played an important role in unraveling mechanisms of apparently simple reactions such as H2 + CI2 = 2HC1, which involve radicals and chain mechanisms. We discuss here the formation of NO from N2 and O2, responsible for NO formation in the engines of cars. In Chapter 10 we will describe how NO is removed catalytically from automotive exhausts. 

The reaction also can be carried out reversibly if additional constraints are placed on the system, as in the cell illustrated by Figure 7.2. The H2 and CI2 electrodes are connected to a potentiometer. If the electromotive force of the cell is opposed by the eleetromotive force of the potentiometer, which is maintained at an infinitesimally lower value than that of the H2-CI2 cell, then the conversion to HCl can be carried out reversibly, although it would take an infinitely long time to obtain one mole of reaction. The change in the Gibbs function is the same for either the reversible or the explosively spontaneous path for carrying out the transformation, because the initial and final states are the same in both cases. However, the amount of useful (electrical) work is different, and, for the reversible path... 

The free energy functions are defined by explicit equations in which the variables are functions of the state of the system. The change of a state function depends only on the initial and final states. It follows that the change of the Gibbs free energy (AG) at fixed temperature and pressure gives the limiting value of the electrical work that could be obtained from chemical transformations. AG is the same for either the reversible or the explosively spontaneous path (e.g. H2 -I- CI2 reaction) however, the amount of (electrical) work is different. Under reversible conditions... 

Three conditions must be fulfilled obtain complete conversion of the reactants, H2 and CI2. The first condition is that thermal equilibrium of the system be favorable. This condition is fulfilled at low and intermediate temperatures, where formation of the product HC1 is thermodynamically favored. At very high temperatures, equilibrium favors the reactants, and thereby serves to limit the fractional conversion. The second requirement is that the overall reaction rate be nonnegligible. There are numerous examples of chemical systems where a reaction does not occur within reasonable time scales, even though it is thermodynamically favored. To initiate reaction, the temperature of the H2-CI2 mixture must be above some critical value. The third condition for full conversion is that the chain terminating reaction steps not become dominant. In a chain reaction system, as opposed to a chain-branching system discussed below, the reaction progress is very sensitive to the competition between chain initiation and chain termination. This competition determines the amount of chain carriers (batons) in the system and thereby the rate of conversion of reactants. 

These two reactions constitute a chain reaction mechanism similar to that found for the H2-CI2 system. A third reaction was later added to the NO formation mechanism,... 

Table 7.4 compares with experiment [82] the effect of functionals and of basis set size on the reaction enthalpies of the important H2/CI2 and H2/O2 reactions. 

Exotic reactions like the ignition of a H2-CI2 mixture by optical radiation will not be covered in this book. 

A gas phase stepwise reaction H2 + CI2 2HC1 follows the unbranched chain mechanism that involves atomic H and Cl as active intermediates ... 

Bra + H2, and 50.6 Kcal for I2 + Ha. Since AS1.2 is about the same for all the halogens, we might thus expect that these activation energies would account for a major part of the differences that might exist between CI2, Br2, and I2. This turns out to be the case. For Ha + I2, the activation energy is sufficiently high that at lower temperatures the bimolecular reaction with an activation energy of about 39 Kcal (Table XII.4) provides the principal reaction path. The Ha + CI2 reaction is faster than the... 

In practice one measures explosion limits by permitting a reaction mixture (of fixed composition) to enter a flask at a previously determined higher temperature and observing the minimum pressure at which explosion takes place. If the rate law for the reaction is known (actually this is seldom true) then any of the above equations can be tested. In this way Sagulin showed that the critical explosion pressures for H2 + CI2 mixtures followed an equation of the form... 

If we consider the mechanism of any typical chain reaction, we see that the atoms or free radicals produced are, by virtue of the existence of the chain, autocatalytic agents for the reaction. Thus in the chain reaction H2 + CI2 —> 2HC1 both H and Cl arc chain carriers, the chain mechanism being... 

It is, however, possible to induce explosions in these systems by the use of additives which are frequently referred to as sensitizers. Thus Ashmore has shown that the addition of 0.5 mm Ilg of NO to 50 mm Ilg of an equimolar mixture of H2 + CI2 lowers the critical explosion temperature from 400 to 270°C. The explosion in this case is still, however, a thermal explosion, and it has been shown that the lowering of the explosion temperature was produced by an increase in the concentration of Cl atoms, not by a change in the chain mechanism. This increase in concentration of Cl atoms was produced by the replacement of the slow, high-activation-energy initiation reaction, M + CI2 2C1 + M(E > 57 Real), by the much-lower-activation-energy reaction, NO + CI2 NOCl + C1(jE = 22 Real). 

DOT CLASSIFICATION Forbidden SAFETY PROFILE A severe explosion hazard when shocked, exposed to heat or flame, or by spontaneous chemical reaction. It has no known uses as an explosive because it is far too sensidve in the dry state to store or handle safely. If this material must be worked with, it should be kept wet. A convenient way of keeping it wet is with ether when it is needed in the dry state, it simply has to be taken out into the open and the ether wiU evaporate, leaving it perfecdy dry. When dry, it will explode when given the slightest touch, vibradon, or rise in temperature. Even a puff of air directed into it can cause it to detonate. It is a high explosive and is very violent. Incompadble with O3. H2S, CI2, Br2, acids. See also IODIDES. 

In equation (29) the symbol hv denotes the energy of a quantum of light of frequency v. The notation signifies that when the H2-CI2 mixture is irradiated by light of an appropriate frequency, light quanta are absorbed by chlorine molecules, causing them to dissociate. Any process of this kind, where radiation induces a reaction in an otherwise metastable system, is termed photosensitization. There are also many examples of processes in which reaction intermediaries or products are formed in excited states and deactivate by emitting nonthermal radiation this phenomenon is called... 

For the H2-CI2 system, the photoyield is of the order 10 to 10 . In this case the chain step is much faster because the reaction... 

Values of have been obtained less directly by adding NO2 to H2+O2 mixtures and to H2+CI2 mixtures . In these experiments the ratios of the rate coefficients were obtained for the following competing reactions. 

The work on the H2-CI2 system up to about 1925 was characterized by disagreements over the quantum yield of HCl production and the influence of drying and impurities on the course of the reaction. For example, Bodenstein found a quantum yield as high as 10 while Kornfield and Mueller obtained 2x10 Much of the controversy stems from experimental techniques which were inadequate to cope with such a rapid and sensitive reaction. 

Bodenstein and Dux studied the photochemical H2-CI2 system by freezing out CI2 and HCI periodically and measuring the H2 pressure. They found that the reaction rate was independent of the HCI concentration in disagreement with the results of Ritchie and Norrish. These authors criticize the method of Bodenstein and Dux, suggesting that complete freezing out of CI2 and HCI may not have been achieved. Bodenstein and Dux" also observed the rate of HCI formation to be inversely proportional to the amount of added oxygen and proportional to the square of the chlorine atom concentration. 

The photochemical formation of HCI from H2-CI2 mixtures has been studied by Chapman. The reaction was followed by dissolving the HCI formed in water which was present in the reaction vessel thereby reducing the total pressure in the system as time passed and permitting the rate to be calculated. Only qualitative significance can be attached to this data since the influence of the presence of water in the system is not accounted for. 

Tafel s original work in 1905 was concerned with organic reactions and H2 evolution at electrodes, and Eq. (1) was written as an empirical representation of the behavior he first observed. A particular value o b = RT/2F has come to be associated specifically with Tafel s name for the behavior of the cathodic H2 evolution reaction (h.e.r.) when under kinetic control by the recombination of two (adsorbed) H atoms following their discharge from or H2O in a prior step. Such kinetic behavior of the h.e.r. is observed under certain conditions at active Pt electrodes and in anodic CI2 evolution at Pt. (We note here, in parentheses, that an alternative origin for a Tafel slope of RT/2F for the h.e.r. at Pt has been discussed by Breiter and by Schuldiner in terms of a quasiequilibrium diffusion potential for H2 diffusing away from a very active Pt electrode at which H2 supersaturation arises). 

In many reactions, it is the water which plays the r61e of the catalyst. It is thus that, H2 + CI2, H2 + 2 O2, NO -H NH -f HCl, etc., do not combine under the conditions under which they are ordinarily made, if these substances are kept entirely free from moisture. 

We may sharpen the comparison by removing the approximation Ea, = AH3 4. Table 3-4 lists experimental activation energies for all the steps in the generalized mechanism except the initial dissociation of X2 into atoms, for which is approximated by A/fJ 2 - Thus, for H2 + CI2 2HC1, E is very nearly 34 kcal/mole compared with about 41 kcal/mole for H2 + Br2 2HBr, and this corresponds to a factor of about 2000 at the very low temperature of 180°C. In the photolytic reactions the Aif 2 I m is not present in E, which is essentially just. In such cases the difference in activation energies for the CI2 and Br2 reactions is about 13 kcal/mole. 

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