Quantitative Character of a Redox Reaction

The initial state of the system is defined by its temperature (which is equal to the temperature of the surroundings), its pressure (equal to that of the surroundings), and also the activities (Ox,), (Red, ) of the different species (Fig. 14.2). These are the activities before the reaction started. We can remark that these initial conditions are quite general because of the fact that the final products Redi and 0x2 already exist in the initial state. From a practical standpoint, this may mean that the reaction has been begun before and is now continuing with the addition of the species Oxi or Reda. This is exactly the case with a redox titration, except at the first point. A particular case of the initial state is that in which the activities of the final species Redi and 0x2 are null. As a matter of fact, it corresponds to the first point of a titration. 

The final state is defined by the same temperamre and pressure as the initial ones, but the activities have changed. These are the activities at equilibrium (see Chap. 2). 

We approached the concept of quantitative character of a chemical reaction in Part II, which is devoted to acids and bases. It can also be applied to redox reactions. Additionally, it will be thoroughly discussed later in another chapter. Now, we limit ourselves to saying that the quantitative character of a reaction may be defined by the number of moles of one of the reactants reacting when the reaction reached its state of equilibrium, that is, when it stopped. The reactant, whose modified number of moles is studied, can be chosen arbitrarily. 

Determining the quantitative character necessitates evaluating the concentrations of the different species in the initial and final states. Calculating the concentrations of the different species at equilibrium is identical to that developed to determine the equilibrium potentials of solutions (see Chap. 16). It is based on handling the different mathematical relations that are obligatorily satisfied and that govern the equilibria in solution. Let s consider the redox reaction 

Cl and C2 are the mass balances on couples 1 and couples 2. The last relation expresses the equilibrium in electrons. It is called the electron balance relation. The equilibrium constant K° is easily calculated by starting from the standard potentials of couples (see Chaps. 2 and 13). Indeed, Nernst s law permits us to write 

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