KEY2CHEM

Electrochemical Reactions

Electrochemistry is the study of the relationship between chemical change and electrical work. The type of chemical reaction that occurs via a net movement of electrons is an oxidation-reduction (commonly called redox) reaction. A spontaneous redox reaction is one that occurs with a positive cell (or half-cell) potential, since the relationship between cell potential and free energy change is $$\Delta{G}^\circ = -nFE^{\circ}$$, where $$F =$$ Faraday’s constant ($$96,485\frac{\text{C}}{\text{mol e}^-}$$).

Example 1.

Which half reaction is spontaneous?

A. $$\require{mhchem}\ce{Zn^2+(aq) + 2 e- -> Zn(s) \;\;\;\;E^{\circ}} = -0.76 \;V$$

B. $$\require{mhchem}\ce{Cu^2+(aq) + 2 e- -> Cu(s) \;\;\;\;E^{\circ}} = 0.34 \;V$$

C. $$\require{mhchem}\ce{Ni^2+(aq) + 2 e- -> Ni(s) \;\;\;\;E^{\circ}} = -0.25 \;V$$

Solution

B. $$\require{mhchem}\ce{Cu^2+(aq) + 2 e- -> Cu(s) \;\;\;\;E^{\circ}} = 0.34 \;V$$

A positive value of $$E^\circ$$ corresponds to a negative value of $$\Delta G^\circ$$, and the reaction is spontaneous.

Example 2.

A galvanic electrochemical cell contains the following redox reaction. Which species is the anode in this galvanic cell?

$$\require{mhchem}\ce{2 Ag+(aq) + Fe(s) -> 2 Ag(s) + Fe^2+(aq)}$$

A. $$\require{mhchem}\ce{Ag+(aq)}$$

B. $$\require{mhchem}\ce{Fe(s)}$$

C. $$\require{mhchem}\ce{Ag(s)}$$

Solution

B. $$\require{mhchem}\ce{Fe(s)}$$

Oxidation occurs at the anode in an electrochemical cell. Since $$\require{mhchem}\ce{Fe(s)}$$ is undergoing oxidation, $$\require{mhchem}\ce{Fe(s)}$$ is the anode. $$\require{mhchem}\ce{Ag(s)}$$ is the cathode, since $$\require{mhchem}\ce{Ag+(aq)}$$ is reduced to $$\require{mhchem}\ce{Ag(s)}$$.

Example 3.

How many minutes will it take to electrolyze $$2.99\text{ g}$$ of $$\require{mhchem}\ce{Ni(s)}$$ from an aqueous solution of $$\require{mhchem}\ce{NiSO4}$$ if a constant current of $$15.1\text{ A}$$ is applied?

A. $$10.9\text{ minutes}$$

B. $$5.43\text{ minutes}$$

C. $$21.7\text{ minutes}$$

Solution

A. $$10.9\text{ minutes}$$

This is a stoichiometry problem that can be solved using Faraday’s laws.

$$\require{mhchem}\ce{Ni^2+(aq) + 2 e- -> Ni(s)}$$

$$2.99\text{ g}\times \frac{1\text{ mol Ni}}{58.69\text{ g Ni}} \times \frac{2\text{ mol e}^-}{1\text{ mol Ni}}\times \frac{96,485\text{ C}}{1\text{ mol e}^-}\times \frac{1 \text{ second}}{15.1\text{ C}}\times \frac{1\text{ minute}}{60\text{ seconds}} = 10.9\text{ minutes}$$