To learn how to use the Nernst equation. The standard reduction potentials liste

Question

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=E2.303RTnFlog10Q

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/nlog10Q

Consider the reaction

Mg(s)+Fe2+(aq)Mg2+(aq)+Fe(s)

at 73 C , where [Fe2+]= 3.70 M and[Mg2+]= 0.110 M .

What is the value for the reaction quotient, Q, for the cell?

What is the value for the temperature, T, in kelvins?

Explanation / Answer

For the given reaction,

Q = [Mg2+]/[Fe2+]

    = (0.110)/(3.70)

    = 0.030

Temperature T = 73 + 273 = 346 K

E = Eo - 0.0592/n logQ

    = (-0.44 - (-2.37) - 0.0592/2 log(0.030)

    = 1.975 V

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