Chapter 20

$$ \mathrm{Pt}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Pt}(\mathrm{s}) \text { and } \mathrm{Sn}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Sn}(\mathrm{s}) $$

$$ \mathrm{Hg}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Hg}(\mathrm{l}) \text { and } \mathrm{Cr}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Cr}(\mathrm{s}) $$

Challenge Write the balanced equation for the cell reaction and calculate the standard cell potential for the reaction that occurs when these half-cells are connected. Describe the reaction using cell notation. $$ \begin{array}{l}{\mathrm{NO}_{3}^{-}+4 \mathrm{H}^{+}+3 \mathrm{e}^{-} \rightarrow \mathrm{NO}+2 \mathrm{H}_{2} \mathrm{O}} \\ {\mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O}+4 \mathrm{e}^{-} \rightarrow 4 \mathrm{OH}^{-}}\end{array} $$

$$ \mathrm{Sn}(\mathrm{s})+\mathrm{Cu}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Sn}^{2+}(\mathrm{aq})+\mathrm{Cu}(\mathrm{s}) $$

$$ \mathrm{Mg}(\mathrm{s})+\mathrm{Pb}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Pb}(\mathrm{s})+\mathrm{Mg}^{2+}(\mathrm{aq}) $$

$$ \begin{array}{l}{2 \mathrm{Mn}^{2+}(\mathrm{aq})+8 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+10 \mathrm{Hg}^{2+}(\mathrm{aq}) \rightarrow} \\ {2 \mathrm{MnO}_{4}-(\mathrm{aq})+16 \mathrm{H}^{+}(\mathrm{aq})+5 \mathrm{Hg}_{2}^{2+}(\mathrm{aq})}\end{array} $$

Describe the conditions under which a redox reaction causes an electric current to flow through a wire.

Identify the components of a voltaic cell. Explain the role of each component in the operation of the cell.

Write the balanced equation for the spontaneous cell reaction that occurs in a cell with these reduction half-reactions. $$ \begin{array}{l}{\text { a. } A g^{+}(a q)+e^{-} \rightarrow A g(s) \text { and } N i^{2+}(a q)+2 e^{-} \rightarrow \text { Nils } )} \\ {\text { b. } M g^{2+}(a q)+2 e^{-} \rightarrow M g(s) \text { and } 2 H^{+}(a q)+2 e^{-} \rightarrow H_{2}(g)}\end{array} $$ $$ \begin{array}{l}{\text { c. } 5 n^{2+}(a q)+2 e^{-} \rightarrow S n(s) \text { and } F e^{3+}(a q)+3 e^{-} \rightarrow F e(s)} \\ {\text { d. } P b l_{2}(s)+2 e^{-} \rightarrow P b(s)+2 l^{-}(a q) \text { and } P t^{2+}(a q)+2 e^{-} \rightarrow P t(s)}\end{array} $$

Determine the standard potential for electrochemical cells in which each equation represents the overall cell reaction. Identify the reactions as spontaneous or nonspontaneous as written. $$ \begin{array}{l}{\text { a. } 2 \mathrm{Al}^{3+}(\mathrm{aq})+3 \mathrm{Cu}(\mathrm{s}) \rightarrow 3 \mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{Al}(\mathrm{s})} \\ {\text { b. } \mathrm{Hg}^{2+}(\mathrm{aq})+2 \mathrm{Cu}^{+}(\mathrm{aq}) \rightarrow 2 \mathrm{Cu}^{2+}(\mathrm{aq})+\mathrm{Hg}(\mathrm{l})}\end{array} $$ $$ \mathrm{c} \cdot \mathrm{Cd}(\mathrm{s})+2 \mathrm{NO}_{3}-(\mathrm{aq})+4 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{Cd}^{2+}(\mathrm{aq})+2 \mathrm{NO}_{2}(\mathrm{g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) $$

Identify what is reduced and what is oxidized in the zinc-carbon dry-cell battery. What features make the alkaline dry cell an improvement over the earlier type of dry-cell battery?

Explain what happens when a battery is recharged.

Describe the half-reactions that occur in a hydrogen fuel cell, and write the equation for the overall reaction.

Describe the function of a sacrificial anode. How is the function of a sacrificial anode similar to galvanization?

Explain why lithium is a good choice for the anode of a battery

Design an Experiment Use your knowledge of acids from Chapter 18 to devise a method for determining whether a lead-acid battery can deliver full charge or is beginning to run down

Define electrolysis and relate the definition to the spontaneity of redox reactions.

Explain why the products of the electrolysis of brine and the electrolysis of molten sodium chloride are different.

Describe how impure copper obtained from the smelting of ore is purified by electrolysis

Explain, by referring to the Hall-Héroult process, why recycling aluminum is very important.

Describe the anode and cathode of an electrolytic cell in which gold is to be plated on an object.

Explain why producing a kilogram of silver from its ions by electrolysis requires much less electric energy than producing a kilogram of aluminum from its ions.

What feature of an oxidation-reduction reaction allows it to be used to generate an electric current?

Describe the process that releases electrons in a zinccopper voltaic cel

What is the function of a salt bridge in a voltaic cell?

What information do you need in order to determine the standard voltage of a voltaic cell?

In a voltaic cell represented by All \(\mathrm{Al}^{3+} \| \mathrm{Cu}^{2+} | \mathrm{Cu},\) what is oxidized and what is reduced as the cell delivers current?

Under what conditions are standard reduction potentials measured?

A salt bridge is filled with KNO \(_{3}\) . Explain why it is necessary that the potassium ions move through the salt bridge to the cathode.

Write the balanced chemical equation for the standard cell notations listed below. $$ \begin{array}{l}{\text { a. } \mathrm{I}^{-}\left|\mathrm{I}_{2} \| \mathrm{Fe}^{3+}\right| \mathrm{Fe}^{2+}} \\ {\text { b. } \mathrm{sn}\left|\mathrm{Sn}^{2+} \| \mathrm{Ag}^{+}\right| \mathrm{Ag}} \\\ {\text { c. } \mathrm{Zn}\left|\mathrm{Zn}^{2+} \| \mathrm{Cd}^{2+}\right| \mathrm{Cd}}\end{array} $$

Calculate the cell potential for the following voltaic cells. $$ \begin{array}{l}{\text { a. } 2 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{Pb}(\mathrm{s}) \rightarrow \mathrm{Pb}^{2+}(\mathrm{aq})+2 \mathrm{Ag}(\mathrm{s})} \\ {\text { b. } \mathrm{Mn}(\mathrm{s})+\mathrm{Ni}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Mn}^{2+}(\mathrm{aq})+\mathrm{Ni}} \\ {\text { c. } \mathrm{I}_{2}(\mathrm{aq})+\mathrm{Sn}(\mathrm{s}) \rightarrow 2 \mathrm{I}^{-}(\mathrm{aq})+\mathrm{Sn}^{2+}(\mathrm{aq})}\end{array} $$

What part of a zinc-carbon dry cell is the anode? Describe the reaction that takes place there

How do primary and secondary batteries differ?

Lead-Acid Battery What substance is reduced in a lead-acid storage battery? What substance is oxidized? What substances are produced in each reaction?

Biofuel Cell At the cathode of a biofuel cell, Fe \(^{3+}\) in potassium hexacyanoiron(III) \(\left(\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\right)\) is reduced to \(\mathrm{Fe}^{2+}\) in potassium hexacyanoiron (II) \(\left(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\right)\) At the anode, reduced nicotinamide-adenine-dinucleo-tide (NADH) is oxidized to NAD+. Use the following standard reduction potential to determine the potential of the cell. $$ \begin{array}{ll}{\mathrm{NAD}^{+}+\mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{NADH}} & {E^{0}=-0.320 \mathrm{V}} \\\ {\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}+1 \mathrm{e}^{-} \rightarrow\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}} & {E^{0}=+0.36 \mathrm{V}}\end{array} $$

Fuel Cells List two ways in which a fuel cell differs from an ordinary battery.

Galvanization What is galvanization? How does galvanizing iron protect it from corrosion?

Batteries Explain why a lead storage battery does not produce a current when the level of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is low.

Steel Wool is a bundle of filaments made of steel, an alloy of iron and carbon. Which would be the best way to store steel wool? a. Store it in water. b. Store it in open air. c. Store it with a desiccant.

Corrosion Protection List three ways metals can be protected from corrosion.

Half-reactions for a lead-acid storage battery are below. $$ \begin{array}{l}{\mathrm{PbO}_{2}(\mathrm{s})+\mathrm{SO}_{4}^{2-}(\mathrm{aq})+4 \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow} \\\ {\mathrm{PbSO}_{4}(\mathrm{s})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \quad E^{0}=+1.685 \mathrm{V}} \\ {\mathrm{PbSO}_{4}(\mathrm{s})+2 \mathrm{e}^{-} \rightarrow \mathrm{Pb}(\mathrm{s})+\mathrm{SO}_{4}^{2-}(\mathrm{aq}) \quad E^{0}=-0.356 \mathrm{V}}\end{array} $$ What is the standard cell potential for one cell in a car battery?

You design a battery that uses a half-cell containing Sn and \(\mathrm{Sn}^{2+}\) and another half-cell containing Cu and \(\mathrm{Cu}^{2+} .\) The copper electrode is the cathode, and the tin electrode is the anode. Draw the battery and write the half-reactions that occur in each half-cell. What is the maximum voltage this battery can produce?

How can the spontaneous redox reaction of a voltaic cell be reversed?

Where does oxidation take place in an electrolytic cell?

Down’s Cell What reaction takes place at the cathode when molten sodium chloride is electrolyzed?

Industry Explain why the electrolysis of brine is done on a large scale at many sites around the world.

Recycling Explain how recycling aluminum conserves energy

Describe what happens at the anode and the cathode in the electrolysis of KI (aq).

Answer the following questions based on Figure \(20.28 .\) a. Which electrode grows? Write the reaction that occurs at this electrode. b. Which electrode disappears? Write the reaction that occurs at this electrode.

Why do electrons flow from one electrode to the other in a voltaic cell?