Reactions and the Steady-State Assumption

The steady-state approximation enables one to formulate an overall rate equation for a system of coupled reactions in terms of elementary steps. For a system at steady state, the entropy production is at minimum. 

Usually a reaction is the result of a number of successive elementary steps 

In this example there are two elementary steps and one overall reaction equation. Let us generalize and consider a chain reaction that consists of n successive steps 

Of course, further generalizations are conceivable in which each elementary step involves the reaction of more than one compound and with stoichiometric coefficients larger than one, but this does not lead to kinetic expressions that are essentially different from the ones derived here. 

We will see later that the system of coupled reactions can go to a stable state (the steady state), which lies close to equilibrium. This has a non-vanishing rate of entropy production which represents the minimum achievable under the given conditions. In general, the rate of entropy production a = dS/dt is the sum of the contributions from the elementary steps and becomes [see (2.30) and (2.19)]  

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