PROPERTIES OF REACTION NETWORKS

In this section we discuss some simple properties of reaction networks and give a few examples to show the consequences of various postulated network features. Section 2.6 and Chapters 6 and 7 will introduce the more difficult problem of reasoning from observations to realistic reaction models. 

A thermodynamically consistent reaction model will have equilibrium states (b, T ) at which the reaction rates TZj all vanish and the generalized concentrations 5, are positive in each phase. Then a Taylor expansion of TZj gives 

Multireaction systems often have some quasi-equilibrium steps whose forward and reverse rates greatly exceed the net rate TZj at all conditions of interest. For such a reaction, the approximation 

the rate expression is replaced by an algebraic constraint on the concentrations. This treatment is customary for physical changes of state such as vaporization and is often applied to fast chemical steps as well. Note that the true equilibrium condition of vanishing TZj is replaced here by the less stringent condition of Eq. (2.5-2a), which permits step j to proceed at a nonzero rate in response to concentration changes caused by other reactions. 

Highly reactive intermediates, such as atoms and free radicals, occur at small concentrations in many reaction systems. Their net rates of production, Rj, are normally small relative to their total rates of formation and consumption, except for small regions near the start and end of the reaction interval of space or time. This behavior prompted the assumption 

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