The Chlorine-Hydrogen Reaction

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. 

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. 

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. 

The above results have been discussed by Chapman and Chapman in terms of excited CI2 molecules produced by absorption coupled with an energy exchange between them and oxygen molecules. Since the absorption is now well characterized as dissociative, their discussion is incorrect. 

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His keen analysis of the hydrogen chlorine reaction has also had support in the recent work of Rodebush and Klingelhoefer on the collision efficiency of the reaction Cl + H2. It is found that this reaction goes in one every 10 collisions, whereas the value suggested by Bodenstein was 10. ... 

In the case of the hydrogen-chlorine reaction, the intermediate compound M, by J. W. Draper s hypothesis, is allotropic or active chlorine by E. Pringsheim s hypothesis,4 chlorine monoxide or some analogous compound. Having shown that the additive chlorine monoxide does not accelerate the reaction or abbreviate the period of induction, an imaginary intermediate compound was postulated of a more indefinite and vague form One naturally shirks vague hypotheses... 

The first who used this principle was Chapman (6), and half a year later Bodenstein in his paper on the hydrogen-chlorine reaction (5) also used it. Since the latter defended its use so ardently, it is not unjustly often connected with his name. 

The hydrogen-chlorine reaction H2 -h CI2 2HC1 is known to involve the steps... 

Outside the chemically resistant layers comes the structural portion, which is designed primarily for strength. The exterior surface of the pipe, again, is rich in resin and usually is opaque. Ultraviolet (UV) screeners should be used in outdoor piping to improve its weather resistance. When chlorine has a relatively high hydrogen content, the barriers to UV and visible radiation can prevent photochemical initiation of the hydrogen-chlorine reaction. 

Now, a simplified mechanism of the hydrogen-chlorine reaction is as follows ... 

Unlike the hydrogen-chlorine reaction, that of hydrogen with oxygen follows the chain branching mechanism. For this reason, the summation of the elementary processes occurring in the detonation wave... 

Photochemistry — Grotthuss — Draper — Chemical Photometry — The Hydrogen-Chlorine Reaction — Bunsen and Roscoe — Photosynthesis. 

If two redox electrodes both use an inert electrode material such as platinum, tlie cell EMF can be written down iimnediately. Thus, for the hydrogen/chlorine fiiel cell, which we represent by the cell Fl2(g) Pt FICl(m) Pt Cl2(g) and for which it is clear that the cathodic reaction is the reduction of CI2 as considered in section... 

The reaction with fluorine occurs spontaneously and explosively, even in the dark at low temperatures. This hydrogen—fluorine reaction is of interest in rocket propellant systems (99—102) (see Explosives and propellants, propellants). The reactions with chlorine and bromine are radical-chain reactions initiated by heat or radiation (103—105). The hydrogen-iodine reaction can be carried out thermally or catalyticaHy (106). 

The process involving aHyl alcohol has not been iadustriaHy adopted because of the high production cost of this alcohol However, if the aHyl alcohol production cost can be markedly reduced, and also if the evaluated cost of hydrogen chloride, which is obtained as a by-product from the substitutive chlorination reaction, is cheap, then this process would have commercial potential. The high temperature propylene—chlorination process was started by SheH Chemical Corporation ia 1945 as an iadustrial process (1). The reaction conditions are a temperature of 500°C, residence time 2—3 s, pressure 1.5 MPa (218 psi), and an excess of propylene to chlorine. The yield of aHyl chloride is 75—80% and the main by-product is dichloropropane, which is obtained as a result of addition of chlorine. Other by-products iaclude monochioropropenes, dichloropropenes, 1,5-hexadiene. At low temperatures, the amount of... 

The hydrogen-chlorine chain reaction has proved to be one of the most controversial systems yet studied. After thirty years of investigation Bodenstein43 was able to say in 1931 that every worker on the photochemical synthesis of HC1 had produced his own mechanism even as late as 1940 little positive information had been obtained. However, the accumulated techniques and experience had firmly established the importance of atom chain reactions. The mechanism of photo-initiation and propagation is the same as for the hydrogen bromide photosynthesis, a non-branching chain reaction... 

THE REACTION OF HYDROGEN ATOMS WITH OXYGEN AND THE HYDROGEN CHLORINE REA CTION... 

In this representation of the reaction the hydrogen chlorine chains are cut off short by the disappearance of a hydrogen atom accompanied by the formation of one molecule of water and one half molecule of hydrogen peroxide. However, Salley and Bates have demonstrated that. 

The chief objection to such a mechanism is the shortness of the chains involved in the reaction, as determined experimentally. (1-2.) This, however, does not make this process impossible, but shows that, if it be the correct one, the chain breaking reactions must take place with greater rapidity than the chain propagating reactions. Be that as it may, these results give room for considerable conjecture on how the hydrogen chlorine chains are actually broken by oxygen. We must realize that several reactions may take place, for example... 

The Hydrogen-Chlorine System The formation of hydrochloric acid from hydrogen and chlorine is a well-known gas-phase reaction. This simple reaction system serves to illustrate the important steps that are characteristic of a chain reaction. The overall reaction can be written as... 

Ab initio molecular orbital theory is utilized to study the hydrogen abstraction reaction of n-bromopropane with hydroxyl radical and chlorine atom. The stability of the trans and gauche isomers of n-bromopropane is explored. The potential energy surface of both reactions is characterized by pre- and post-reactive complexes, as well as transition state structures in both trans and gauche isomeric forms. The importance of these two reactions relies on the ultimate product distribution from both reactions. Differences in the reactivity of 1-bromopropane toward OH and Cl are observed. The reaction of n-bromopropane with OH radical favors the abstraction of hydrogen atoms while the reaction with Cl atoms favors the abstraction of hydrogen atoms at the a and p carbon sites. 

This reaction occurs with explosive vigor when fluorine and hydrogen are mixed. On the other hand, hydrogen and chlorine can coexist with little apparent reaction for relatively long periods in the dark. However, ultraviolet light causes an explosively fast reaction, and this is the basis of a popular lecture demonstration, the hydrogen-chlorine cannon. Bromine and iodine also react with hydrogen, but more slowly. 

Corneil and Pimentel have studied, near room temperature, the hydrogen-chlorine explosion laser finding an isotope effect in the CI2-HD system for the reactions... 

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