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Question 20.8: Calculating the emf from Standard Potentials Calculate the s...

Calculating the emf from Standard Potentials

Calculate the standard emf of the following voltaic cell at 25^{\circ} \mathrm{C} using standard electrode potentials.

\operatorname{Al}(s)\left|\mathrm{Al}^{3+}(a q) \| \mathrm{Fe}^{2+}(a q)\right| \mathrm{Fe}(s)

What is the cell reaction?


From a table of electrode potentials, write the two reduction half-reactions and standard electrode potentials for the cell. The cell notation assumes that the anode (the oxidation half-cell) is on the left. Change the direction of this half-reaction and the sign of its electrode potential. (Assuming that the cell notation was written correctly, you change the direction of the half-reaction corresponding to the smaller, or more negative, electrode potential.) Multiply the half-reactions (but not the electrode potentials) by factors so that when the half-reactions are added, the electrons cancel. The sum of the half-reactions is the cell reaction. Add the electrode potentials to get the cell emf.

An alternative strategy is to apply the equation

E_{\text {cell }}^{\circ}=E_{\text {cathode }}^{\circ}-E_{\text {anode }}^{\circ}

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