Frost diagrams

Figure 7.11 Enthalpy of formation of binary oxides of the 3d transition metals (a) and (c), Frost diagram for the same 3d metals (b) Enthalpy of formation of binary oxides of the group 6 transition metals.

The Frost diagrams for the first series of the d block elements in acidic solution, pH = 0, given in Figure 7.11(b) show many similarities with the variation of the enthalpy of formation of the oxides. Only the oxidation states observed for solid oxides are included. 

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... 

There are two main methods of summarizing the thermodynamic stabilities of the oxidation states of elements in aqueous solution, known after their inventors. Latimer and Frost diagrams are usually restricted to the two extremes of standard hydrogen ion (pH = 0) or hydroxide ion (pH = 14) solutions. 

Fig. Frost diagram showing the oxidation states of Al, Ga, In, Tl in aqueous solution at pH= 0.

The frost diagram shown in figure above shows much larger negative slope (negative electride potential) of aluminium. The compounds of thallium (Tl) are toxic but they pose no environmental hazard as they are little used. 

Figure below shows the Frost diagram for the elements of 3d-series (i.e., Sc to Zn). It shows the electrode potentials for aqueous species appropriate to acid solution (pH = 0). In this diagram, the... 

Fig Frost diagram for dements of the 3d series in aqueous solution at pH = 0. 

Fig Frost diagram showing the redox states of nitrogen in water at pH = 0 continuous line) and pH = 14 dashed line). 

Except for fluorine the elements have an extensive oxoacid chemistry. Figure shows Frost diagrams with the oxidation states found in acid and alkaline solution. The sharp trend in oxidising power of the elements (X2/X- potential) can be seen. As expected from Pauling s rules the hypohalous acids X(OH)... 

Fig Frost diagrams for the halogens in aqueous solution at pH = 0 a) and pH =14 (h). X represents any halogen, except F for positive oxidation states. 

A Frost diagram for the chromium group in the J-block (Group 6) in acidic solution (pH = 0). 

Frost diagrams for the Group 13/111 9.27 Frost diagrams for the Group 14/IV... 

Figure 2.2. Frost diagram (i.e., AG°/F vs oxidation state) for the oxygen system in water.

Example 2.12 involves the Latimer and Frost diagrams of Mn. Key data are summarized in Table 2.5. 

Example 2.12 In the Latimer diagram for manganese in acidic medium, (a) find the values of E and Eg (see the table below), (b) draw the Frost diagram, and (c) find which is the most stable species of this system in acidic aqueous solution. The standard potential data are summarized in the table given above. 

This means that the equilibrium in the Mn02(S)/Mn2+ system is strongly driven toward Mn2+. This agrees well with the Frost diagram in Example 2.12, that shows the high stability of Mn2+ in aqueous media. 

Martinez de Ilarduya, J. M. Villafane, F. A Warning for Frost Diagrams Users, J. Chem. Educ. 1994, 71, 480-482. 

Note to solve this problem, you may use additional E° data from the Mn Frost diagram (see Example 2.12). 

In any case, such a K value means that the Mn3+ disproportionation into Mn2++ Mn02 is spontaneous, as can also be predicted from its Frost diagram and from the A G°. (The fact that Mn3+ can be produced in oxic waters is probably due to kinetic rather than thermodynamic stability). 

The Frost diagram for chlorine species (see Experiment 19) shows that Cl- is the most stable species of this system in water. Accordingly, Cl2, HCIO, and CIO- each require 2e in order to be converted into Cl and, in this sense, the three will be taken as equivalents in the following equations, assuming that the final product in the three cases is the chloride ion. The free available chlorine (or free chlorine residual) can thus be understood as the sum of the concentrations of Cl2, HCIO, and CIO- ... 

FIGURE 8-4 Frost Diagrams for Hydrogen, (a) Acidic solution, (b) Basic solution. 

FIGURE 8-26 Frost Diagram for Nitrogen Compounds in Acid. 

FIGURE 8-29 Frost Diagram for Chlorine Species. The solid line is for acidic solutions and the dashed line is for basic solutions. 

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