Chapter 20

For each of these pairs of half-reactions, write the balanced equation for the overall cell reaction, and calculate the standard cell potential. Describe the reaction using cell notation. Refer to Chapter 19 to review writing and balancing redox equations. $$\mathrm{Co}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Co}(\mathrm{s}) \text { and } \mathrm{Cr}^{3+}(\mathrm{aq})+3 \mathrm{e}^{-} \rightarrow \mathrm{Cr}(\mathrm{s})$$

For each of these pairs of half-reactions, write the balanced equation for the overall cell reaction, and calculate the standard cell potential. Describe the reaction using cell notation. Refer to Chapter 19 to review writing and balancing redox equations. $$\mathrm{Hg}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Hg}(\mathrm{l}) \text { and } \mathrm{Cr}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Cr}(\mathrm{s})$$

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

Write the balanced equation for the spontaneous cell reaction that occurs in a cell with these reduction half-reactions. a. \(A g^{+}(a q)+e^{-} \rightarrow A g(s)\) and \(N i^{2+}(a q)+2 e^{-} \rightarrow\) Ni(s) b. \(\mathrm{Mg}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Mg}(\mathrm{s})\) and \(2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2}(\mathrm{g})\) c. \(\mathrm{Sn}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Sn}(\mathrm{s})\) and \(\mathrm{Fe}^{3+}(\mathrm{aq})+3 \mathrm{e}^{-} \rightarrow \mathrm{Fe}(\mathrm{s})\) d. Pbl \(_{2}(s)+2 e^{-} \rightarrow P b(s)+21-(a q)\) and \(P t^{2+}(a q)+2 e^{-} \rightarrow P t(s)\)

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?

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-Heroult 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 zinc- copper voltaic cell.

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 \(A l^{3+} \| \mathrm{Cu}^{2+} | \mathrm{Cu},\) 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. $$ \begin{array}{l}{\text { a. } \mathrm{I}^{-}\left|\mathrm{I}_{2} \| \mathrm{Fe}^{3+}\right| \mathrm{Fe}^{2+}} \\ {\text { b. } \mathrm{Sn}\left|\mathrm{Sn}^{2+} \| \mathrm{Ag}^{+}\right| \mathrm{Ag}} \\\ {\text { c. } \mathrm{Zn}\left|\mathrm{Zn}^{2+} \| \mathrm{Cd}^{2+}\right| \mathrm{Cd}}\end{array} $$

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, \(\mathrm{Fe}^{3+}\) 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-dinucleotide (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? $$ \begin{array}{l}{\text { a. Store it in water. }} \\ {\text { b. Store it in open air. }} \\ {\text { c. Store it with a desiccant. }}\end{array} $$

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

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

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 \(\mathrm{KI}(\mathrm{aq}) .\)

Electroplating Figure 20.27 shows a key being electroplated with copper in an electrolytic cell. Where does oxidation occur? Explain your answer.

Aluminum Production What substance is electrolyzed in the industrial process to produce aluminum metal?

Corrosion Explain why water is necessary for the corrosion of iron.

Space Travel The space shuttle uses a \(\mathrm{H}_{2} / \mathrm{O}_{2}\) fuel cell to produce electricity. a. What is the reaction at the anode? At the cathode? b. What is the standard cell potential for the fuel cell?

Fuel Cells Explain how the oxidation of hydrogen in a fuel cell differs from the oxidation of hydrogen when it burns in air.

Copper Refining In the electrolytic refining of copper, what factor determines which piece of copper is the anode and which is the cathode?

Storage Batteries Lead-acid batteries and other rechargeable batteries are sometimes called storage batteries. What is being stored in these batteries?

Predict Suppose that scientists had chosen the \(\mathrm{Cu}^{2+} |\) Cu half-cell as a standard instead of the \(\mathrm{H}^{+} | \mathrm{H}_{2}\) half-cell? What would the potential of the hydrogen electrode be if the copper electrode were the standard? How would the relationships among the standard reduction potentials change?

Hypothesize The potential of a half-cell varies with concentration of reactants and products. For this reason, standard potentials are measured at 1\(M\) concentration. Maintaining a pressure of 1 atm is especially important in half-cells that involve gases as reactants or products. Suggest a reason why gas pressure is critical in these cells.

Apply During electrolysis, an electrolytic cell releases bromine vapor and hydrogen gas. After electrolysis, the cell is found to contain a concentrated solution of potassium hydroxide. What was the composition of the cell before electrolysis began?

Hypothesize Suppose in galvanization, copper was plated on iron instead of zinc. Would copper continue to protect the iron from corrosion, as zinc does, if the copper coating became broken or cracked? Explain.