Page 47 - Modeling of Chemical Kinetics and Reactor Design

Reaction Mechanisms and Rate Expressions 17

cycle is formed. The intermediate species are intermittently formed by
the reaction, and the final products are the result of a cyclic repetition
of the intervening processes. Chain reactions play active roles in
industrial processes.
Bodenstein [4] first suggested the idea of a chain reaction by
+
postulating that ions such as chlorine, Cl , are chain carriers. Later,
2
Nernst [5] proposed his mechanism for the hydrogen in chlorine
reaction that gave rise to the idea that organic free radicals are
important in reaction mechanisms. Taylor [6] investigated the reactions
of hydrogen atoms with various substances. He proposed a reaction
between a hydrogen atom and ethane. The resulting ethyl radical plays
an important role in hydrocarbon reactions. Rice and Herzfeld [7]
postulated reaction schemes involving the participation of free radicals
in the pyrolysis of organic compounds. Techniques such as spectro-
scopy, electron-spin resonance spectroscopy, and mass spectrometry
have confirmed the validity of these types of reaction mechanism.
They confirmed that free radicals are important in many reactions, and
thus act as chain carriers. Chain reactions are involved in such pro-
cesses as combustion, polymerization, and photochemical processes.
A chain reaction consists of three main steps:

1. Initiation (or activation)
2. Propagation (closed sequence steps)
3. Termination

The reaction between hydrogen and bromine is the first step in the
initiation reaction in which the chain carriers are formed. The thermal
hydrogen bromine reaction begins with the initiation reaction:

Br → Br + Br * (1-66)
*
2
This is followed by chain propagation reactions:

Br + H → HBr + H * (1-67)
*
2
H + Br → HBr Br * (1-68)
+
*
2
H + HBr→ H + Br * (1-69)
*
2
The termination reaction is the removal of the carrier from the system: