Frost Ebsworth diagrams

The following potential diagram summarizes some of the redox chemistry of iron in aqueous solution. Calculate the value of for the reduction of Fe (aq) to iron metal. 

Although there are short cuts to this problem, the most rigorous method is to determine AG°(298K) for each step. 

Fe to Fe is a three-electron reduction the standard reduction potential for the process is found from the corresponding value of AG°  

Although the method given here is probably tbe safest way to perform tbe calculation, substitution of a value for tbe Faraday constant may in fact be excluded. Wby  

Construct a potential diagram (at pH 0) for the reduction of aqueous HNO2 to NO and then to N2O given that E for the HNO2/NO and NO/N2O couples are +0.98 and +1.59 V respectively. Calculate F for the following half-reaction  

This indicates that reaction 8.48 is spontaneous. Similarly, at 

Construct a potential diagram for the reduction of aqueous Cr to Cr, followed by reduction to Cr. Values 

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Standard reduction potentials are often tabulated as in Appendix 11, but it is also useful to present data in the form of a potential diagram (also known as Latimer diagrams) or Frost-Ebsworth diagram (see Section 7.6). 

Frost-Ebsworth diagrams and their relationship to potential diagrams... 

Fig. 7.3 The Frost-Ebsworth diagram for manganese in aqueous solution at pH 0, i.e. [H+] = 1 moldm ".

Worked example 7.8 Using Frost-Ebsworth diagrams... 

Construct Frost-Ebsworth diagrams for Ga, In and T1 at pH = 0. Use the diagrams to comment on (a) the relative abilities of Ga +, In + and Tl to act as oxidizing agents under these conditions, and (b) the relative stabilities of the +1 oxidation state of each element. 

Fig. 14.6. Potential diagram for nitrogen at pH = 0. A Frost-Ebsworth diagram for nitrogen is given in Figure 7.4c.

Explain how the Frost-Ebsworth diagram for nitrogen (Figure 7.4c) illustrates that NH30H (at pHO) is unstable with respect to disproportionation. 

Fig. 21.9 Potential diagram for chromium at pH 0. A Frost-Ebsworth diagram for Cr is shown in Figure 7.4a.

When negative oxidation states are involved, care must be taken in plotting appropriate values of — AG"/F. All points on a Frost Ebsworth diagram refer to stability with respect to — AG°/F = 0 for the zero oxidation state of the element. Thus, for example, starting from " = +1.09 V for the jBr2/Br couple, a value of —AG"/F = +1.09 V is calculated for the reduction of jBi2 to Br+ For a Frost-Ebsworth diagram, we require a value of —ACr IF that corresponds to the process Br — jBtj + e and therefore the appropriate value of —ACr IF is —1.09V. This concept is further explored in problem 8.24 at the end of the chapter. 

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