Qualitative Prediction of Redox Reactions After Standard Potentials

3 Qualitative Prediction of Redox Reactions After Standard Potentials 

Considering standard potentials or, if necessary, apparent standard potentials is probably the simplest way to predict the direction of a redox reaction from the qualitative standpoint. The rule is that the oxidized form of the couple that has the highest standard potential spontaneously reacts with the reduced form of the couple that has the 

The redox couples are often located on a scale of increasing potentials according to their standard potential values (as acids are located on the acidity scale according to their pKa values). The origin of the scale is fixed at the value E = 0.00 V, which is the standard potential of the couple H+/H2(g) (Fig. 14.1). The strongest oxidants are located farthest on the right. The strongest reductants are located farthest on the left. For example, if the Zn +/Zn(s) and Cu +/Cu(s) couples are placed face to face, we can, with the rule above, predict the spontaneous reaction 

When the antagonist redox couples exchange protons or some other particules (of course, in addition to that of electrons), the comparison must be accomplished by examining the apparent standard potentials, adapted, of course, to the experimental conditions. Basing ourselves on apparent standard potentials indeed also permits us to make predictions in any aqueous solution. The following example illustrates these assertions. Let s consider the reaction 

The first equilibrium involves a proton exchange, while the second equilibrium does not involve such an exchange. As a result, it does not depend on the pH value until about pH 9 (see Chap. 18 for a further explanation). The apparent standard potential of the first couple is 

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