Stable Reactants and Products

Stable intermediates are those where concentration and lifespan are comparable to those of stable reactants and products. An example is the reaction between methane and oxygen in the gas phase at 700 K and 1 atmosphere. The overall reaction is ... 

If lines are drawn on the surface which pass from reactant to product valleys (or between stable reactant and product configurations) then for each chemical process there are one or more points which represent maxima on minimum activation energy paths. These maxima are transition states for the reaction. However, as we shall see, it is more difficult (at least in many dimensions) to establish that a point represents a transition state than that it represents the configuration of a stable species. 

The reason many global reactions between stable reactants and products have complex mechanisms is that these unstable intermediates have to be produced in order for the reaction to proceed at reasonable rates. Often simplifying assumptions lead to closed-form kinetic rate expressions even for very complex global reactions, but care must be taken when using these since the simplifying assumptions are valid over limited ranges of compositions, temperature, and pressure. These assumptions can fail completely—in that case the full elementary reaction network has to be considered, and no closed-form kinetics can be derived to represent the complex system as a global reaction. 

Evaluation of Formal Potentials 12.3.3.1. Involving Stable Reactants and Products. 

Figure 14.3 is a contour plot that shows two such minima, representing the stable reactant and product configurations, and the transition surface (hne in the figure) that separate their domains of attraction. The minimum of V ) on this surface is the saddle point This dividing TV-dimensional surface is defined in the (TV -+ 1 (-dimensional space by the relation... 

Next use the QSSA to relate C2H5 to stable reactants and products... 

As the reacting system evolves from reactants to products, a number of intermediates appear, reach a certain concentration and ultimately vanish. Corresponding to the distinction between networks, reactions and steps, three different kinds of intermediates can be recognized. First there are intermediates of reactivity, concentration and lifetime comparable to those of stable reactants and products. These intermediates are the ones that appear in the reactions of the network. A typical intermediate of this first kind is formaldehyde CH2O in the oxidation of methane (see below). 

There are about 100 odd elements known to us in the periodic table. Accordingly, nearly 5000 pair-wise combinations of single electrode reactions involving stable reactants and products can be theoretically envisaged leading to a like number of electrochemical cells. Ironically, however, more than a century of effort has resulted in only a few systems of practical importance and efforts are being expended to enhance their specific energy in conjunction with optimum power-density, minimum internal-resistance, maximum... 

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