Alkali metals standard reduction potentials

Alkali and alkaline-earth metals have the most negative standard reduction potentials these potentials are (at least in ammonia, amines, and ethers) more negative than that of the solvated-electron electrode. As a result, alkali metals (M) dissolve in these highly purified solvents [9, 12] following reactions (1) and (2) to give the well-known blue solutions of solvated electrons. 

Electropositive metals characterized by low standard reduction potentials (alkali metals. Mg, Zn) have been frequently used for the reduction of transition metal halides in the presence of carbon monoxide. The finely divided reducing metal is previously activated by one of the conventional methods. Ethers are frequently used as reaction media. 

How do the standard reduction potentials of the alkali metal cations vary Why ... 

Althongh the transition metals are less electropositive (or more electronegative) than the alkali and alkaline earth metals, their standard reduction potentials suggest that all of them except copper should react with strong acids such as hydrochloric acid to produce hydrogen gas. Flowever, most transition metals are inert toward acids or react slowly with them becanse of a protective layer of oxide. A case in point is chromium ... 

This element is the alkali metal with the least negative standard reduction potential. Write its symbol in reverse order. 

For most tables of standard reduction potentials that start from large positive values at the top and proceed through 0.0 V to negative values at the bottom, the alkali metals are normaUy at that bottom of the table. Use your chemical understanding... 

Because all alkali metals react with water, it is not possible to measure the standard reduction potentials of these metals directly as in the case of, say, zinc. An indirect method is to consider the following hypothetical reaction... 

The rate constants of electron transfer between naphthalene and its anion are in the range 10 -10 liter/mole sec, depending on solvent and alkali metal used (29, 30). The half-wave potential of the first electron addition step is therefore controlled almost solely by the Nernst equation and can be considered as approximately equal to the standard reduction potential. It will differ from it by (RTIF)lii DJDr), where Z>o and are the diffusion coefficients of the oxidized and reduced species, respectively. This term is normally of the order of a few millivolts. 

The potentials are given with respect to the standard reduction potential of biphenyl. These figures show that except for naphthalene all hydrocarbons investigated react quantitatively with alkali metals. At lower temperatures the reduction is generally more nearly quantitative owing to the increased solvation of the ions. At a temperature of —100° C, for instance, biphenyl can be converted completely to the mononegative ion, whereas at room temperature an equilibrium is reached. 

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. 

The trends in standard reduction potentials of the alkali metals. These are related to ionization energies, ionic radius, polarizing power, and energy released upon interaction with water molecules. (Only four water molecules are shown around each ion for clarity.)... 

The standard reduction potentials, particularly those of the heavier congeners, are similar to those of the heavier alkali metals. These are all good reducing agents. The near-constancy of the E° values of calcium, strontium, barium, and radium reflects a balance of the heats of atomization, ionization, and hydration energies. (See Problem 13.23.) Of course, two electrons must be ionized from the alkaline... 

Standard reduction potential (V)t -3.05 Refers to the cation M , where M denotes an alkali metal atom. -2.71 2.93 2.93 -2.92... 

The Group lA elements with their ns valence-electron configurations are all very active metals (they lose their valence electrons very readily), except for hydrogen, which behaves as a nonmetal. We will discuss the chemistry of hydrogen in the next section. Many of the properties of the alkali metals have been described previously (see Section 7.13). The sources and methods of preparation of pure alkali metals are given in Table 19.3. The ionization energies, standard reduction potentials, ionic radii, and melting points for the alkali metals are listed in Table 19.4. Lepidolite, shown in Fig. 19.4, contains several pure alkali metals. 

We will reconsider this process briefly because it illustrates several important concepts. Based on the ionization energies, we might expect lithium to be the weakest of the alkali metals as a reducing agent in water. However, the standard reduction potentials indicate that it is the strongest. This reversal results mainly from the very large energy of hydration... 

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