Standard reduction potentials transition metals

Transition metals tend to have higher melting points than representative metals. Because they are metals, transition elements have relatively low ionization energies. Ions of transition metals often are colored in aqueous solution. Because they are metals and thus readily form cations, they have negative standard reduction potentials. Their compounds often have unpaired electrons because of the diversity of -electron configurations, and thus, they often are paramagnetic. Consequently, the correct answers are (c) and (e). 

Table 20.2 lists standard potentials E° for oxidation of first-series transition metals. Note that these potentials are the negative of the corresponding standard reduction potentials (Table 18.1, page 775). Except for copper, all the E° values are positive, which means that the solid metal is oxidized to its aqueous cation more readily than H2 gas is oxidized to H+(aq). 

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. 

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 ... 

Many of the transition metals can take more than one oxidation state. Whenever this occurs, it is useful to show the relationship between the standard reduction potentials graphically using a Latimer diagram. Consider the stepwise reduction potentials for Cu and Cu + shown by Equations (5.27) and (5.28). In order to determine the skip potential for the two-electron reduction of Cu " shown by Equation (5.29), we cannot simply add the two stepwise potentials together to get the final result. 

A Compare the extents to which the properties of successive elements across the periodic table differ for representative elements and d-transition metals. Explain. Compare the metals and nonmetals with respect to (a) number of outer-shell electrons, (b) electronegativities, (c) standard reduction potentials, and (d) ionization energies. 

Most of the transition elements do not react with strong acids, such as HCl and H2SO4. Some do have negative standard reduction potentials for the reaction M"+ -I- ne - M, and liberate hydrogen from hydrochloric acid. These include Mn, Cr, and the iron triad. Silver, gold, the palladium triad, and the platinum triad, the so-called noble metals, are especially inert to acids, both to the nonoxidizing species, such as hydrochloric and hydrofluoric acids, and to the oxidizing acids, such as nitric acid. 

More recently it has been found15 that a correlation exists between spectroscopic parameters of the divalent aqua ions of the metals Cr to Ni, and the polarographic y2. A linear relationship was found between A0 and crystal field splitting parameter, ot the transfer coefficient, n the number of electrons transferred in the reduction, EVl the polarographic half-wave potential and E° the standard electrode potential. The use of the crystal field splitting parameter would seem to be a more sensible parameter to use than the position of Amax for the main absorption band as the measured Amax may not be a true estimate of the relevant electronic transition. This arises because the symmetry of the complex is less than octahedral so that the main absorption band in octahedral symmetry is split into at least two components with the result that... 

The standard potentials of metals are usually given for the reaction of cathodic reduction from simple hydrated metal ions (see Section 1). It is of interest to analyze what changes in their values can be expected if (a) metal ions are deposited from some other solvent, and (b) metal ions form complexes with some components of the solution. Solvation itself is quite often a form of complexation, e.g., in the case of transition metal ions. 

The abihty of transition metals to exhibit a wide range of oxidation states is marked with metals such as vanadium, where the standard potentials can be rather small, making a switch between states relatively easy. Thus successive reduction potentials of vanadium (values for acidic solution) are -VOj -I- e — VO +( = l.OV) VO + + e V +( = 0.337V) V + - - e — V +( = -0.255V). In contrast, for lanthanides, the reduction potentials for Ln + + e — Ln + are greater than -2.3 V, except for Eu (-0.34 V) Sm (-1.55 V) and Yb (-1.05 V) similarly all the potentials for Ln" + + e — Ln + are greater than - - 3.3 V, except for cerium (+1.7 V). These are the only lanthanides with any tendency to adopt other oxidation states in their aqueous chemistry. 

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