Hydrogen bromide reactions

Another case in point is chain reactions (see Chapter 9). Asa rule, these are non-simple because their termination step involves two intermediates as reactants. Nevertheless, explicit rate equations can often be derived with the Bodenstein and long-chain approximations. The classical example is that of the hydrogen-bromide reaction (see Section 9.3). 

Some chain reactions can be initiated photochemically [3]. In fact, most of the early work on kinetics of chain reactions was done with photochemical initiation. For example, in the hydrogen-bromide reaction (see next section), initiation Br2 — 2 Br can be achieved with ultraviolet light. Such initiation allows the reaction to be conducted at a lower temperature at which thermal initiation is ineffective. This may be an advantage in an industrial process, and also offers opportunities for elucidation of reaction mechanisms. 

Reactions with two chain carriers the hydrogen-bromide reaction... 

The rates of product formation (and reactant consumption) are seen to be of order one half in the initiator or, if the reaction is initiated by a reactant converted in the propagation cycle, the rate equation involves exponents of one half or integer multiples of one half. For an example, see the hydrogen-bromide reaction below. This is one of the exceptions to the rule that reasonably simple mechanisms do not yield rate equations with fractional exponents. [The other exceptions are reactions with fast pre-dissociation (see Section 5.6) and of heterogeneous catalysis with a reactant that dissociates upon adsorption.]... 

Example 9.1. The hydrogen-bromide reaction. The elucidation of the mechanism of the gas-phase reaction... 

Because of its relative simplicity, the hydrogen-bromide reaction (Example 9.1 in the preceding section) serves well to illustrate the application of the estimates to the identification of relevant steps. 

Table 9.1. Approximate standard enthalpies of steps in hydrogen-bromide reaction.

Taking once more the hydrogen-bromide reaction as an example The one-plus form of the rate equation 9.24 contains two phenomenological coefficients, k3 and kb. The first is given by... 

Examples include the hydrogen-bromide reaction, thermal cracking of ethane and n-butane, oxidation of cyclohexane, and the hydrogen-oxygen reaction. 

The cause is the increasing retardation by the reverse step with progressing conversion as the product builds up [43]. This retardation can become effective even if the other propagation step and therefore the overall reaction are irreversible, or at a conversion so low that the reverse overall reaction is still insignificant. The hydrogen-bromide reaction is a simple example (see Example 10.1 in Section 10.3). 

As seen in the example of ethane cracking and the hydrogen-bromide reaction, such a "creeping up" of the reaction order with progressing conversion is a typical symptom of a reverse step in the propagation cycle [43]. 

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