174 Ultimately, AG , values must be based on experimental results; in many cases

Question

174 Ultimately, AG , values must be based on experimental results; in many cases, these experimental results are obtained from E values. Early in the twentieth century, G. N. Lewis conceived of an experimental approach for obtaining standard potentials of the alkalai metals.. This approach involved using a solvent with which the alkalai metals do not react, in this case ethylamine. In the ollowing cell diagram, Na (amalg, 0.206%) represents a solution of 0.26% Na in liquid mercury Na (s) Na+ (in ethylamine) Na (amalg 0.206%) E e -o 8453 v Although Na(s) reacts violently with water to produce H2 (g), at least for a short time, a sodium amalgam electrode does not react with water. This makes it possible to determine Eell for the following voltaic cell Na (amalg 0.206%) I Na' (1 M) I H"(1 M) H, (g, 1 atm) 1 Pt (s) E 1.8673 V a. Write the Nernst equations for the cell reactions that occur in the voltaic b. Use the equation AG--nFE to determine AG for the cell reactions c. Write the overall equation obtained by combining the two equations in d. Use the G o value to calculate Ecell o for the overall reaction and use that to cells (1) and (2) written in part a. part a and determine AG ° for this overall reaction. calculate E Nat/Na Compare this with the accepted value.

Explanation / Answer

From the given data

a. Nernst equation for 1,

Ecell = Eo - 0.0592 log(Na+)

Nernst equation for 2,

Ecell = Eo - 0.0592/2 log(Na+)

b. dG = -nFEo

          = -1 x 96500 x 0.8453/1000

          = -81.57 kJ

c. Overall equation,

2Na + 2H+ --> 2Na+ + H2

dGo for overall reaction = -2 x 96500 (0.8453 - 1.8673)/1000

                                      = 197.25 kJ

d. Eocell = Eocell = 197250/-2 x 96500 = -1.02 V

which is way off from the accepted value of -2.71 V

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