Reduction Potentials and the Network

Before we discuss the reduction potentials of the alkali metals, we should recall some definitions related to oxidation—reduction, or redox, reactions. (This is but a sparse review. You may need to consult your introductory textbook or lecture notes for further clarification.) Recall that when a substance is reduced, its oxidation state (or the oxidation state of some constituent element) is decreased. When a substance is oxidized, the oxidation state increases. Reduction is associated with a gain of electrons, whereas oxidation corresponds to a loss. (Some students keep these definitions straight by using the mnemonic, or memory device, LEO goes GER, which stands for Loses Llectrons Oxidized, Gains Electrons Reduced.) 

To analyze and tabulate the results of such an experiment, the overall oxidation-reduction equation is often separated into what are known as half-equations or half-reactions, one representing the reduction part and the other the oxidation part. For the Zn—Cu reaction, Equation (12.8a) shows the oxidation half-equation and Equation (12.8b) the reduction half-equation. Notice that the zinc metal releases or loses two electrons (and so is oxidized) and that these electrons are in turn transferred to the Cu ion, which is reduced. Note also that these two half-equations can be added together again to yield Equation (12.7), the overaU equation for the reaction. 

With the preceding as a brief review of redox reactions, we can now turn to a discussion of the standard reduction potentials of the alkali metals, which are listed in Table 12.1. Specifically, we want to know what information they can provide and how such information can be put to use to understand better the characteristics of not only the alkali metals but also other groups of the periodic table. Take lithium as an example. The half-equation for the reduction of aqueous lithium ions to lithium metal is shown in Equation (12.9)  

Now we would like to compare the tendencies of the aqueous lithium cation and the other aqueous alkali metal cations to be reduced. To do this systematically, the reduction potentials must be measured under certain standard-state conditions. We need not concern ourselves with the details of standard states it is enough to note that as a first approximation the standard state for an aqueous solution specifies that all solutes are at a concentration of 1 molar (M) and all gases are at 1 atm of pressure. In addition, these conditions most always specify a temperature of 25°C or 298 K. Under these conditions we can refer to the standard reduction potential as the measure of the tendency of a substance to be reduced under standard conditions. The symbol for this is E°, where the degree sign specifies the standard conditions. 

Now it would be very convenient if we could independently measure the voltage associated with Equation (12.9) or any other individual half-reaction, but it is important to realize that it is not possible to measure such absolute voltages. Why not Because Equation (12.9) is only a / reaction and cannot occur on its own. Free electrons cannot be dumped into a beaker and subsequently combine with the lithium ions. The electrons must come from a substance that has lost its electrons—that is, has been oxidized. 

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