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Chapter 20: Problem 4
$$E_{\text {cathode }}^{\circ}=(2.71-2.310) V=+0.40 V$$VVV
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A concentration cell is constructed of two hydrogen electrodes: one immersed in a solution with \(\left[\mathrm{H}^{+}\right]=1.0 \mathrm{M}\) and the other in \(0.65 \mathrm{M} \mathrm{KOH}\) (a) Determine \(E_{\text {cell for the reaction that occurs. }}\) (b) Compare this value of \(E_{\text {cell }}\) with \(E^{\circ}\) for the reduction of \(\mathrm{H}_{2} \mathrm{O}\) to \(\mathrm{H}_{2}(\mathrm{g})\) in basic solution, and explain the relationship between them.
A test for completeness of electrodeposition of \(\mathrm{Cu}\) from a solution of \(\mathrm{Cu}^{2+}(\mathrm{aq})\) is to add \(\mathrm{NH}_{3}(\mathrm{aq}) .\) A blue color signifies the formation of the complex ion \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\left(K_{\mathrm{f}}=1.1 \times 10^{13}\right) .\) Let \(250.0 \mathrm{mL}\) of \(0.1000 \mathrm{M} \mathrm{CuSO}_{4}(\text { aq })\) be electrolyzed with a \(3.512 \mathrm{A}\) current for 1368 s. At this time, add a sufficient quantity of \(\mathrm{NH}_{3}(\text { aq })\) to complex any remaining \(\mathrm{Cu}^{2+}\) and to maintain a free \(\left[\mathrm{NH}_{3}\right]=0.10 \mathrm{M} .\) If \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\) is detectable at concentrations as low as \(1 \times 10^{-5} \mathrm{M}\) should the blue color appear?
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.
Consider the reaction \(\operatorname{Co}(\mathrm{s})+\mathrm{Ni}^{2+}(\mathrm{aq}) \longrightarrow\) \(\mathrm{Co}^{2+}(\mathrm{aq})+\mathrm{Ni}(\mathrm{s}), \quad\) with \(\quad E_{\mathrm{cell}}^{\circ}=0.02 \mathrm{V} . \quad\) If \(\quad \mathrm{Co}(\mathrm{s}) \quad\) is added to a solution with \(\left[\mathrm{Ni}^{2+}\right]=1 \mathrm{M},\) should the reaction go to completion? Explain.
In the construction of the Statue of Liberty, a framework of iron ribs was covered with thin sheets of copper less than \(2.5 \mathrm{mm}\) thick. A layer of asbestos separated the copper skin and iron framework. Over time, the asbestos wore away and the iron ribs corroded. Some of the ribs lost more than half their mass in the 100 years before the statue was restored. At the same time, the copper skin lost only about \(4 \%\) of its thickness. Use electrochemical principles to explain these observations.
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