Learning Goal: To learn how to use the Nernst equation. The standard reduction p

Question

Learning Goal: To learn how to use the Nernst equation. The standard reduction potentials listed in any reference table are only valid at standard-state conditions of 25 C and 1 M. To calculate the cell potential at non-standard-state conditions, one uses the Nernst equation, E=E2.303RT/nF [log10Q] where E is the potential in volts, E is the standard potential in volts, R=8.314J/(Kmol) is the gas constant, T is the temperature in kelvins, n is the number of moles of electrons transferred, F=96,500C/(mol e) is the Faraday constant, and Q is the reaction quotient. Substituting each constant into the equation the result is E=E0.0592 V/n [log10Q].

What is the cell potential for the reaction

Mg(s)+Fe2+(aq)Mg2+(aq)+Fe(s) at 79 C when [Fe2+]= 3.80 M and [Mg2+]= 0.210 M .

Express your answer to three significant figures and include the appropriate units.

Explanation / Answer

Mg2+ + 2 e– --> Mg(s)          E= -2.3568 V     Anode

Fe2+ + 2 e– --> Fe(s)           E= -0.440 V     Cathode

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Mg(s) + Fe2+(aq) Mg2+(aq) + Fe(s)     E = +1.93 V

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E = E2.303RT/nF [log10Q]

= +1.93 V - (2.303 X 8.314J/(Kmol) X 352) / 2 X 96,500C/(mol e) [log10(0.21/3.8)]

E = +1.974 V your required answer

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