Volt-equivalent

Tlic power of these various concepts in codifying and rationalizing the redox chemistry of the clcineiUs is ilhislraled for Ihe case of nitrogen in tbe present section Standard reduction potentials and plots of volt equivalents against oxidation state fur odicr elements are presented in later chapters... 

It also follows that, when three (or more) oxidation states lie approximately on a straight line in the volt-equivalent diagram, they tend to form an equilibrium mixture rather than a reaction going to completion (provided that the attainment of thermodynamic equilibrium is not hindered kinetically). This is because the slopes joining the several points are almost the same, so that E° for the various couples (and hence AG°) are the same there is consequently approximately zero change in free energy and a balanced... 

Figure 11.9 Plot of volt equivalent against oxidation state for various compounds or ions containing N in acidic aqueous solution. Note that values of - AC refer lo N2 as standard (zero) but are quoted per mol of N aloms and per mol of. Nj they refer to reactions in the direction (ox) ne —> (red). Slopes corresponding to some common oxidizing and reducing agents are included for comparison.

A more complete compilation is summarized in Fig. 11.8. It is instructive to use these data to derive a plot of volt equivalent versus oxidation state in basic solution and to compare this with Fig. 11.9 which refers to aeidic solutions. 

Although the reaction has the overall stoichiometry of a dehydration it is more complex than this and involves a mutual redox reaction between N and N. This is at once explicable in terms of the volt-equivalent diagram in Fig. 11.9 which also interprets why NO and N2 are formed simultaneously as byproducts. It is probable that the mechanism involves dissociation of NH4NO3 into NH3 and HNO3, followed by autoprotolysis of HNO3 to give N02, which is the key intermediate ... 

Many of the sulfur oxoaeids and their salts are eonneeted by oxidation-reduetion equilibria some of the more important standard reduetion potentials are summarized in Table 15.19 and displayed in graphie form as a volt-equivalent diagram (p. 435) in Fig. 15.28. By use of the eouples in Table 15.19 data for many other oxidation-reduetion equilibria ean readily be ealeulated. (Indeed, it is an instruetive exereise to eheek the derivation of the numerieal data... 

Figure 15.28 Volt-equivalent diagram for sulfur-containing species in acid solution.

The data in Fig. 17.18 are presented in graphical form in Fig. 17.19 which shows the volt-equivalent diagrams (p. 436) for acid and alkaline solutions. It is clear from these that CI2 and Br2 are much more stable towards disproportionation in acid solution (concave angle at X2) than in alkaline solutions (convex angle). In terms of... 

Figure 25.2 Plot of volt-equivalent against oxidation state for Fe, Ru and Os in acidic aqueous solution.

Figure 30.4 Volt-equivalent versus oxidation state for lanthanides with more than one oxidation state.

This chapter is not concerned with the thermodynamic stability of ions with respect to their formation. Rather, it is concerned with whether or not a given ion is capable of existing in aqueous solution without reacting with the solvent. Hydrolysis reactions of ions are dealt with in Chapter 3. The only reactions discussed in this section are those in which either water is oxidized to dioxygen or reduced to dihydrogen. The Nernst equation is introduced and used to outline the criteria of ionic stability. The bases of construction and interpretation of Latimer and volt-equivalent (Frost) diagrams are described. 

How to construct and interpret volt-equivalent (Frost) diagrams... 

The E value is asserted to be positive, so the accompanying value of AG is negative the value of G" jF for the oxidized state of the couple is higher by 2 X 1.1 V than that for the reduced state. A volt-equivalent consists of a plot of G"//7 values against the oxidation state for ions of the element under consideration. Ions of less stability are placed higher up the G" jF axis in the volt-equivalent diagram those with greater stability are placed lower down. 

Volt-equivalent diagrams convey the same amount of information as do Latimer diagrams about the relative stabilities of the oxidation states of an element and their oxidation/reduction properties, but do it in a graphical manner. Such diagrams are given in subsequent chapters for selected elements to illustrate further the differences in potentials between successive oxidation states. 

The construction and interpretation of volt-equivalent diagrams were described. 

The subject of this chapter is the periodicity of the aqueous chemistry of the elements of the s-block (Groups 1 and 2) and the p-block (Groups 11-18) of the Periodic Table. Modified Latimer diagrams summarize the chemistry of all the elements, and some volt-equivalent diagrams are given to represent the inter-relations between various oxidation states of the elements. Explanations of some trends in redox chemistry are discussed in detail. 

A volt-equivalent diagram for the water-soluble nitrogen species in acidic solution is shown in Figure 6.4. It shows that the nitrate(V) ion is the least stable species, but also indicates the meta-stability of nitrous acid, which is unstable with respect to disproportionation into oxidation states + 5 and zero ... 

Figure 6.4 A volt-equivalent diagram for the water-soluble stales of nitrogen at pH = 0...

Volt-equivalent diagrams for the oxidation states of V are given in Figure 7.16 for pH values of 0 and 14. The reduction potentials on which the diagrams are based are given in the margin as a vertical Latimer diagram. 

Figure 7.16 Volt-equivalent diagrams for the oxidation states of vanadium at pH = 0 (red line) and pH = 14 (black line)...

The redox chemistry of manganese is dealt with volt-equivalent diagrams and a description of the small amount of aqueous chemistry of Tc and Re follows. A volt-equivalent diagram for the oxidation states of Mn is... 

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