Chapter 20

Describe in words how you would calculate the standard potential of the \(\mathrm{Fe}^{2+} / \mathrm{Fe}(\mathrm{s})\) couple from those of \(\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}\) and \(\mathrm{Fe}^{3+} / \mathrm{Fe}(\mathrm{s})\).

In your own words, define the following symbols or terms: (a) \(E^{\circ} ;\) (b) \(F ;\) (c) anode; (d) cathode.

Briefly describe each of the following ideas, methods, or devices: (a) salt bridge; (b) standard hydrogen electrode (SHE); (c) cathodic protection; (d) fuel cell.

Explain the important distinctions between each pair of terms: (a) half- reaction and overall cell reaction; (b) voltaic cell and electrolytic cell; (c) primary battery and secondary battery; (d) \(E_{\text {cell }}\) and \(E_{\text {cell }}^{\circ}\).

Of the following statements concerning electrochemical cells, the correct ones are: (a) The cathode is the negative electrode in both voltaic and electrolytic cells. (b) The function of a salt bridge is to permit the migration of electrons between the half-cell compartments of an electrochemical cell. (c) The anode is the negative electrode in a voltaic cell. (d) Electrons leave the cell from either the cathode or the anode, depending on what electrodes are used. (e) Reduction occurs at the cathode in both voltaic and electrolytic cells. (f) If electric current is drawn from a voltaic cell long enough, the cell becomes an electrolytic cell. (g) The cell reaction is an oxidationreduction reaction.

For the half-reaction \(\mathrm{Hg}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Hg}(1)\) \(E^{\circ}=0.854 \mathrm{V} .\) This means that \((\mathrm{a}) \mathrm{Hg}(1)\) is more readily oxidized than \(\mathrm{H}_{2}(\mathrm{g}) ;\) (b) \(\mathrm{Hg}^{2+}(\) aq) is more readily reduced than \(\mathrm{H}^{+}(\mathrm{aq}) ;\) (c) \(\mathrm{Hg}(\) l) will dissolve in 1 M HCl; (d) Hg(l) will displace Zn(s) from an aqueous solution of \(\mathrm{Zn}^{2+}\) ion.

The value of \(E_{\text {cell }}^{\circ}\) for the reaction \(\mathrm{Zn}(\mathrm{s})+\) \(\mathrm{Pb}^{2+}(\mathrm{aq}) \longrightarrow \mathrm{Zn}^{2+}(\mathrm{aq})+\mathrm{Pb}(\mathrm{s})\) is \(0.66 \mathrm{V} .\) This means that for the reaction \(\mathrm{Zn}(\mathrm{s})+\mathrm{Pb}^{2+}(0.01 \mathrm{M})\) \(\rightarrow \mathrm{Zn}^{2+}(0.10 \mathrm{M})+\mathrm{Pb}(\mathrm{s}), E_{\text {cell }}\) equals \((\mathrm{a}) 0.72 \mathrm{V}\) (b) \(0.69 \mathrm{V} ;\) (c) \(0.66 \mathrm{V} ;\) (d) \(0.63 \mathrm{V}\)

For the reaction \(\operatorname{Co}(\mathrm{s})+\mathrm{Ni}^{2+}(\mathrm{aq}) \longrightarrow \mathrm{Co}^{2+}(\mathrm{aq})+\) \(\mathrm{Ni}(\mathrm{s}), E_{\mathrm{cell}}^{\circ}=0.03 \mathrm{V} .\) If cobalt metal is added to an aqueous solution in which \(\left[\mathrm{Ni}^{2+}\right]=1.0 \mathrm{M},\) (a) the reaction will not proceed in the forward direction at all; (b) the displacement of \(\mathrm{Ni}(\mathrm{s})\) from the \(\mathrm{Ni}^{2+}(\mathrm{aq})\) will go to completion; (c) the displacement of \(\mathrm{Ni}(\mathrm{s})\) from the solution will proceed to a considerable extent, but the reaction will not go to completion; (d) there is no way to predict how far the reaction will proceed.

The gas evolved at the anode when \(\mathrm{K}_{2} \mathrm{SO}_{4}(\mathrm{aq})\) is electrolyzed between Pt electrodes is most likely to be (a) \(\mathrm{O}_{2} ;\) (b) \(\mathrm{H}_{2} ;\) (c) \(\mathrm{SO}_{2} ;\) (d) \(\mathrm{SO}_{3} ;\) (e) a mixture of sulfur oxides.

The quantity of electric charge that will deposit \(4.5 \mathrm{g}\) Al at a cathode will also produce the following volume at STP of \(\mathrm{H}_{2}(\mathrm{g})\) from \(\mathrm{H}^{+}(\) aq) at a cathode: (a) \(44.8 \mathrm{L} ;\) (b) \(22.4 \mathrm{L} ;\) (c) \(11.2 \mathrm{L} ;\) (d) \(5.6 \mathrm{L}\).

If a chemical reaction is carried out in a fuel cell, the maximum amount of useful work that can be obtained is (a) \(\Delta G ;\) (b) \(\Delta H ;\) (c) \(\Delta G / \Delta H ;\) (d) \(T \Delta S\).

For the reaction \(\mathrm{Zn}(\mathrm{s})+\mathrm{H}^{+}(\mathrm{aq})+\mathrm{NO}_{3}^{-}(\mathrm{aq}) \longrightarrow\) \(\mathrm{Zn}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(1)+\mathrm{NO}(\mathrm{g}),\) describe the voltaic cell in which it occurs, label the anode and cathode,use a table of standard electrode potentials to evaluate \(E_{\text {cell }}^{\circ},\) and balance the equation for the cell reaction.

The following voltaic cell registers an \(E_{\text {cell }}=0.108 \mathrm{V}\) What is the pH of the unknown solution? $$\operatorname{Pt}\left|\mathrm{H}_{2}(\mathrm{g}, 1 \mathrm{atm})\right| \mathrm{H}^{+}(x \mathrm{M}) \| \mathrm{H}^{+}(1.00 \mathrm{M}) |$$ $$\mathrm{H}_{2}(\mathrm{g}, 1 \mathrm{atm}) | \mathrm{Pt}$$

\(E_{\mathrm{cell}}^{\circ}=-0.0050 \mathrm{V}\) for the reaction, \(2 \mathrm{Cu}^{+}(\mathrm{aq})+\) \(\operatorname{sn}^{4+}(\mathrm{aq}) \longrightarrow 2 \mathrm{Cu}^{2+}(\mathrm{aq})+\mathrm{Sn}^{2+}(\mathrm{aq})\) (a) Can a solution be prepared that is \(0.500 \mathrm{M}\) in each of the four ions at \(298 \mathrm{K} ?\) (b) If not, in what direction must a net reaction Occur?

Using the method presented in Appendix \(\mathrm{E}\), construct a concept map showing the relationship between electrochemical cells and thermodynamic properties.

Construct a concept map illustrating the relationship between batteries and electrochemical ideas.

Construct a concept map illustrating the principles of electrolysis and its industrial applications.