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, Tis 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 Vnlog10Q

***Consider the reaction

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

at 57 C , where [Fe2+]= 3.20 M and [Mg2+]= 0.110 M .

Part A) What is the value for the reaction quotient, Q, for the cell?

Part B) Calculate the standard cell potential for

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

Explanation / Answer

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

The value for the reaction quotient, Q, for the cell use following expression:

Q = [prod] / [reactants]
Q = [0.110] / [3.20]
Q = 0.344

Now change unit of temperature as follows:

57 C + 273 = 330 Kelvin

n = 2 electrons transferred from Mg to Fe+2

Now calculate the standard cell potential for the following reaction: Mg(s)+Fe2+(aq)Mg2+(aq)+Fe(s)

Mg(s) ----> Mg^2+ + 2 e- .......Eo = +2.37 V
Fe^2+(aq) + 2 e- --> Fe(s)......Eo = - 0.440 V

Eo cell = 1.93 V

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