H2—Br2 reaction

Britton and Cole32, in a shock-tube investigation of the H2-Br2 reaction evaluated kl4lk13)x = 8.3 0.7 and 10.1 + 1.7, withno temperature coefficient over the temperature range 1000-1400 °C. They also concluded that El3 Ei 2 kcal.mole-1. 

This reaction and the synthesis of HBr have also received much less attention than the corresponding reactions of the HI system. The problem of the mechanism of the H2+Br2 reaction which Bodenstein and Lind33 found to be complex was later solved independently by Christiansen34, Herzfeld35 and Polanyi36. The well-known mechanism and the kinetic equation resulting from the stationary-state solution are given below... 

Bodenstein explained this result by suggesting that the H2-Br2 reaction was chain in character and initiated by a radical (Ih ) formed by the thermal dissociation of Br2. He proposed the following steps ... 

From the five chain steps written for the H2-Br2 reaction, one can write an expression for the HBr formation rate ... 

Consequently, it is seen, from the measurement of the overall reaction rate and the steady-state approximation, that values of the rate constants of the intermediate radical reactions can be determined without any measurement of radical concentrations. Values k exp and xp evolve from the experimental measurements and the form of Eq. (2.31). Since (ki/k5) is the inverse of the equilibrium constant for Br2 dissociation and this value is known from thermodynamics, k2 can be found from xp. The value of k4 is found from k2 and the equilibrium constant that represents reactions (2.2) and (2.4), as written in the H2 Br2 reaction scheme. From the experimental value of k CX(l and the calculated value of k4, the value k3 can be determined. 

Considering that for a steady system, the termination and initiation steps must be in balance, the definition of chain length could also be defined as the rate of product formation divided by the rate of termination. Such a chain length expression would not necessarily hold for the arbitrary system of reactions (3.1)—(3.6), but would hold for such systems as that written for the H2—Br2 reaction. When chains are long, the types of products formed are determined by the propagating reactions alone, and one can ignore the initiation and termination steps. 

As mentioned, by using the thermal and photochemical reactions all the rate constants in the H2/Br2 reactions can be found. The following problem relates to a reaction where this is not the case. 

It is important to realize that a steady state treatment on a mechanism does not necessarily generate a rate expression in which all the individual rate constants appear. If, as in the H2/Br2 reaction above, all the rate constants do appear in the rate expression, then it may be possible to determine the magnitudes of all the rate constants from a steady state analysis. But if they do not all appear, then the steady state treatment can only allow determination of those rate constants which do appear in the rate expression, and alternative ways will have to be found to give an independent determination of the remaining rate constants. 

The long chains approximation must never be used when one or other or both of the chain carriers are removed or formed in steps other than propagation, initiation and termination. For example, when inhibition is present and a product removes or forms one or other of the chain carriers, the long chains approximation is invalid see the H2/Br2 reaction, Section 6.10. 

CH3CHO decomposition - determination of the mechanism, 211-213 the steady state analysis, 233-238 setting up of the steady state expression for the overall activation energy in terms of the activation energies for the individual steps, 238-239 H2/Br2 reaction - a steady state analysis on the reaction with inhibition, 213-216 without inhibition 216-217 determination of the individual rate constants, 217-218 Stylised Rice-Herzfeld mechanisms, 221-224, with surface termination, 240-243 RH/Br2 reaction - a steady state analysis, 225-227... 

The H2 + Br2 reaction provides a classic example of a much more general type of chain reaction, the two-center chain. In the case of H2 + Br2, the two centers are the H and Br atoms, both of which are required to complete the chain cycle and both of which may be involved in chain termination. A more general type of two-center chain system is the one exemplified by the over-all reaction. ... 

By using the methods employed in the case of the H2 + Br2 reaction (Sec. XIII.3) we can compare these reactions to the competing reaction 1. By using the available thermodynamic data and making reasonable assumptions about frequency factors it is possible to show that all of these steps are negligibly small, even in the cases of reactions such as 14, 15, and 16, which have appreciably lower activation energies than has reaction 1. 

Using a steady-state flow system, Levy has investigated the H2-Br2 reaction over the temperature range 600-1470 °K and has shown the Christiansen, Herzfeld and Polanyi free radical mechanism to be obeyed. Pressures of Br2 and HBr were measured after collection of products. Initial Br2 concentrations were obtained by material balance. Reactor residence times were calculated from measurements of total flow rate and reactor volume. 

A shock-wave study of the H2-Br2 reaction has been performed by Plooster and Garvin over the temperature range 850-1140 °K for the purpose of extending thermal and photochemical work done at temperatures below 600 The reaction was followed by spectrophotometry of the Br2 concentration in the 4100-4200 A region. The shock tube was of the open-ended variety, the driver gas being the laboratory atmosphere. 

In the case of the H2 Br2 reaction, the concept of reaction order is clearly without meaning, except under limiting conditions where one or the other of the terms in the denominator of the rate law [Eq. (1-7)] is dominant. 

The process is unimolecular in both H and Br2. Since elementary reactions such as (4.15) represent molecular collision processes the exponents in the predicted rate expressions must always be positive integers. Thus experimental rate laws like those for the H2 + Br2 reaction (4.5) or the H2 + D2 reaction (4.14) indicate mechanisms which involve more than one elementary step. 

In subsequent years, a succession of brilliant physical chemists interested in the fundamental laws of chemical kinetics began to interpret their results in terms of radical reactions. In 1918, J. A. Christiansen, K. F. Herzfeld, and M. Polanyi independently suggested a radical chain process for the H2-Br2 reaction. In 1925, H. S. Taylor postulated the occurrence of the ethyl radical to rationalize a gas-phase photolysis. In 1931, Norrish suggested that radicals occur in the photolysis of carbonyl compounds. And then in 1939, in a very influential paper, F. Paneth showed that small alkyl radicals could be produced in a flowing gas... 

The concentration of chlorine atoms [Cl] in this mechanism is much lower than that of [Br] in the H2/Br2 reaction. This is a direct consequence of the lower activation energy of C1+H2 HC1+H when compared with of Br+H2 HBr+H, as will be discussed in Section 12.4. 

---