Stoichiometric coefficients, convention definition

Note that reactions 2.14, 2.15, and 2.23 involve fractional stoichiometric coefficients on the left-hand sides. This is because we wanted to define conventional enthalpies of formation (etc.) of one mole of each of the respective products. However, if we are not concerned about the conventional thermodynamic quantities of formation, we can get rid of fractional coefficients by multiplying throughout by the appropriate factor. For example, reaction 2.14 could be doubled, whereupon AG° becomes 2AG, AH° = 2AH , and AS° = 2ASf, and the right-hand sides of Eqs. 2.21 and 2.22 must be squared so that the new equilibrium constant K = K2 = 1.23 x 1083 bar-3. Thus, whenever we give a numerical value for an equilibrium constant or an associated thermodynamic quantity, we must make clear how we chose to define the equilibrium. The concentrations we calculate from an equilibrium constant will, of course, be the same, no matter how it was defined. Sometimes, as in Eq. 2.22, the units given for K will imply the definition, but in certain cases such as reaction 2.23 K is dimensionless. 

The overall reaction of the electrolysis cell (Eq. 7) provides the required stoichiometric coefficients for the products and reactant used in Eq. 8. The sign convention is positive for products and negative for reactants with analogous definitions for AB, AC and AD. Data for the constants A, B, C and D are thermodynamic properties and are reproduced in Table 1 from,19... 

In Chapter 2 it was explained why it is absolutely necessary to have a clear definition of plant and process at the beginning of a safety technical assessment. Consequently, one of the first steps has to be the formulation of the stoichiometric equation of the reaction. All descriptions used in this book assume the validity of the common physicochemical convention that the stoichiometric coefficients Vj of all educts are negative and those of products are positive. 

This is the situation exploited by the so-called isolation method to detennine the order of the reaction with respect to each species (see chapter B2.1). It should be stressed that the rate coefficient k in (A3,4,10) depends upon the definition of the in the stoichiometric equation. It is a conventionally defined quantity to within multiplication of the stoichiometric equation by an arbitrary factor (similar to reaction enthalpy). 

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