Electrochemistry: Chemical Change and Electrical Work

What does a negative  ${\mathbf{\text{E}}}^{\mathbf{\text{o}}}$cell indicate about a redox reaction? What does it indicate about the reverse reaction?

A voltaic cell is constructed with a ${\text{Cu/Cu}}^{\text{2 + }}$ half-cell and an${\text{Ni/Ni}}^{\text{2 + }}$ half-cell. The nickel electrode is negative. 

(a) Write balanced half-reactions and the overall reaction. 

(b) Diagram the cell, labeling electrodes with their charges and showing the directions of electron flow in the circuit and of cation and anion flow in the salt bridge

How is a standard reference electrode used to determine unknown ${\text{E}}^{\text{o}}$  half-cell values?  

When metal A is placed in a solution of a salt of metal B, the surface of metal A changes colour. When metal B is placed in acid solution, gas bubbles form on the surface of the metal. When metal A is placed in a solution of a salt of metal C, no change is observed in the solution or on the metal A surface. Will metal C cause formation of $H_2$ when placed in acid solution? Rank metals A, B, and C in order of decreasing reducing strength

Write the cell notation for the voltaic cell that incorporates each of the following redox reactions:

 (a)  ${\mathbf{\text{Al(s) + Cr}}}^{\mathbf{\text{3 + }}}{\mathbf{\text{(aq) n Al}}}^{\mathbf{\text{3 + }}}\mathbf{\text{(aq) + Cr(s)}}$

 (b)  ${\mathbf{\text{Cu}}}^{\mathbf{\text{2 + }}}{\mathbf{\text{(aq) + SO}}}_{\mathbf{\text{2}}}{\mathbf{\text{(g) + 2H}}}_{\mathbf{\text{2}}}\mathbf{\text{O(l) n}}$$\mathbf{\text{Cu(s) + S}}{{\mathbf{\text{O}}}_{\mathbf{\text{4}}}}^{\mathbf{\text{2 - }}}{\mathbf{\text{(aq) + 4H}}}^{\mathbf{\text{ + }}}\mathbf{\text{(aq)}}$

The standard cell potential is a thermodynamic state function. How are $\mathbf{E}\mathbf{°}$ values treated similarly to $\mathbf{∆}\mathbf{H}\mathbf{°}\mathbf{,}\mathbf{∆}\mathbf{G}\mathbf{°}\mathbf{,}\mathbf{ }\mathbf{and}\mathbf{ }\mathbf{∆}\mathbf{S}\mathbf{°}$ values? How are they treated differently?

In basic solution${\mathbf{\text{Se}}}^{\mathbf{2}\mathbf{-}}$,  and ${\mathbf{\text{SO}}}_{\mathbf{\text{3}}}^{\mathbf{\text{2 - }}}$ ions react spontaneously

   $$\begin{gathered} {\mathbf{\text{2Se}}}^{\mathbf{\text{2 - }}}{\mathbf{\text{(aq) + 2SO}}}_{\mathbf{\text{3}}}^{\mathbf{\text{2 - }}}{\mathbf{\text{(aq) + 3H}}}_{\mathbf{\text{2}}}\mathbf{\text{O(l)n}} \\ {\mathbf{\text{2Se(s) + 6OH}}}^{\mathbf{\text{ - }}}{\mathbf{\text{(aq) + S}}}_{\mathbf{\text{2}}}{{\mathbf{\text{O}}}_{\mathbf{\text{3}}}}^{\mathbf{\text{2 - }}}\mathbf{\text{(aq)}}\mathbf{ }\mathbf{ }\mathbf{ }{\mathbf{\text{E}}}_{\mathbf{\text{cell}}}^{\mathbf{\text{o}}}\mathbf{\text{ = 0.35V}} \end{gathered}$$

(a) Write balanced half-reactions for the process. 

(b) If   ${\text{E}}_{\text{sulfite}}^{\text{o}}$is $\text{ - 0.57V}$ , calculate  ${\text{E}}_{\text{selenium.}}^{\text{o}}$

Use the following half-reactions to write three spontaneous reactions, calculate E°cell for each reaction, and rank the strengths of the oxidizing and reducing agents 

$$\begin{gathered} {\mathbf{\text{(1)I}}}_{\mathbf{\text{2}}}{\mathbf{\text{(s) + 2e}}}^{\mathbf{\text{ - }}}{\mathbf{\text{n2I}}}^{\mathbf{\text{ - }}}{\mathbf{\text{(aq) E}}}^{\mathbf{\text{0}}}\mathbf{\text{=0.53V}} \\ {\mathbf{\text{(2)S}}}_{\mathbf{\text{2}}}{{\mathbf{\text{O}}}_{\mathbf{\text{8}}}}^{\mathbf{\text{2 - }}}{\mathbf{\text{(aq) + 2e}}}^{\mathbf{\text{ - }}}\mathbf{\text{n2S}}{{\mathbf{\text{O}}}_{\mathbf{\text{4}}}}^{\mathbf{\text{2 - }}}\mathbf{\text{(aq)}}{\mathit{E}}^{\mathbf{\text{0}}}\mathbf{=}\mathbf{2}\mathbf{.}\mathbf{01}\mathit{V} \\ {\mathbf{\text{(3)Cr}}}_{\mathbf{\text{2}}}{\mathbf{\text{O}}}_{\mathbf{\text{7}}}^{\mathbf{\text{2 - }}}{\mathbf{\text{(aq) + 14H}}}^{\mathbf{\text{ + }}}{\mathbf{\text{(aq) + 6e}}}^{\mathbf{\text{ - }}}{\mathbf{\text{n2Cr}}}^{\mathbf{\text{3 + }}}{\mathbf{\text{(aq) + 7H}}}_{\mathbf{\text{2}}}{\mathbf{\text{O(l) E}}}^{\mathbf{\text{0}}}\mathbf{\text{=1.33V}} \end{gathered}$$

In acidic solution${\mathbf{\text{O}}}_{\mathbf{\text{3}}}$, and ${\mathbf{\text{Mn}}}^{\mathbf{\text{2+}}}$ion react spontaneously:

${\mathbf{\text{O}}}_{\mathbf{\text{3}}}{\mathbf{\text{(g)+Mn}}}^{\mathbf{\text{2+}}}{\mathbf{\text{ (aq)+H}}}_{\mathbf{\text{2}}}{\mathbf{\text{O(l)O}}}_{\mathbf{\text{2}}}{\mathbf{\text{(g)+MnO}}}_{\mathbf{\text{2}}}{\mathbf{\text{(s)+2H}}}^{\mathbf{\text{+}}}{\mathbf{\text{(aq)\hspace{1em}E}}}_{\mathbf{\text{cell}}}^{\mathbf{\text{°}}}\mathbf{\text{=0.84 V}}$

 (a) Write the balanced half-reactions.

 (b) Using Appendix D to find  ${\mathbf{\text{E}}}_{\mathbf{\text{casne }}}^{\mathbf{\text{°}}}{\mathbf{\text{,calculateE}}}_{\mathbf{\text{manganese }}}^{\mathbf{\text{°}}}\mathbf{\text{.}}$

Use the emf series (Appendix D) to arrange the species.

(a) In order of decreasing strength as oxidizing agents:  ${\mathbf{\text{Fe}}}^{\mathbf{\text{3+}}}{\mathbf{\text{,Br}}}_{\mathbf{\text{2}}}{\mathbf{\text{,Cu}}}^{\mathbf{\text{2+}}}$

(b) In order of increasing strength as oxidizing agents : ${\mathbf{\text{Ca}}}^{\mathbf{\text{2+}}}\mathbf{\text{,}}$${\mathbf{\text{Cr}}}_{\mathbf{\text{2}}}{\mathbf{\text{O}}}_{\mathbf{\text{7}}}^{\mathbf{\text{2-}}}$,${\mathbf{\text{Ag}}}^{\mathbf{\text{+}}}$

Use the following half-reactions to write three spontaneous reactions  , calculate for each reaction, and rank the strengths of the oxidizing and reducing agents:

(1)  

(2)   

(3)  

(a) How do the relative magnitudes of Q and K relate to the signs of  $\mathbf{\text{ΔG}}$ and  ${\mathbf{\text{E}}}_{\mathbf{\text{cell}}}$ ? Explain. 

(b) Can a cell do work when Q/K> $\mathbf{1}$ or Q/K< $\mathbf{1}$? Explain.

A Voltaic cell consists of a metal $\mathbf{A}\mathbf{/}{\mathbf{A}}^{+}$   electrode and a metal  $\mathbf{B}\mathbf{/}{\mathbf{B}}^{+}$ electrode, with the $\mathbf{A}\mathbf{/}{\mathbf{A}}^{+}$  electrode negative. The initial  $\mathbf{\left[}{\mathbf{\text{A}}}^{\mathbf{\text{+}}}\mathbf{\right]}\mathbf{\text{/}}\mathbf{\left[}{\mathbf{\text{B}}}^{\mathbf{\text{+}}}\mathbf{\right]}$  is such that   ${\mathbf{\text{E}}}_{\mathbf{\text{cell }}}$ >   ${\mathbf{\text{E}}}_{\mathbf{\text{cell }}}^{\mathbf{\text{o}}}$

(a) How do  $\mathbf{\left[}{\mathbf{\text{A}}}^{\mathbf{\text{+}}}\mathbf{\right]}$ and  $\mathbf{\left[}{\mathbf{\text{B}}}^{\mathbf{\text{+}}}\mathbf{\right]}$ change as the cell operates? 

(b) How does  ${\mathbf{\text{E}}}_{\mathbf{\text{cell }}}$  change as the cell operates? 

(c) What is   $\mathbf{\left[}{\mathbf{\text{A}}}^{\mathbf{\text{+}}}\mathbf{\right]}\mathbf{\text{/}}\mathbf{\left[}{\mathbf{\text{B}}}^{\mathbf{\text{+}}}\mathbf{\right]}$  when ${\mathbf{\text{E}}}_{\mathbf{\text{cell }}}$ = ${\mathbf{\text{E}}}_{\mathbf{\text{cell }}}^{\mathbf{\text{o}}}$  ? Explain. 

(d) Is it possible for  ${\mathbf{\text{E}}}_{\mathbf{\text{cell }}}^{}$  to be less than  ${\mathbf{\text{E}}}_{\mathbf{\text{cell }}}^{\mathbf{\text{o}}}$? Explain

In a concentration cell, is the more concentrated electrolyte in the cathode or the anode compartment? Explain.

What is the value of the equilibrium constant for the reaction between each pair at 25°C?

 $\begin{array}{l}{\mathbf{\text{(a) Ni(s) and Ag}}}^{\mathbf{\text{+}}}\mathbf{\text{(aq)}}\\ {\mathbf{\text{(b) Fe(s) and Cr}}}^{\mathbf{\text{3+}}}\mathbf{\text{(aq)}}\end{array}$

What is the value of the equilibrium constant for the reaction between each pair at $\mathbf{25}\mathbf{°}\mathbf{C}$?

$\begin{array}{l}{\mathbf{\text{(a) Al(s) and Cd}}}^{\mathbf{\text{2+}}}\mathbf{\text{(aq)}}\\ {\mathbf{\text{(b) I}}}_{\mathbf{\text{2}}}{\mathbf{\text{(s) and Br}}}^{\mathbf{\text{-}}}\mathbf{\text{(aq)}}\end{array}$

What is the value of the equilibrium constant for the reaction between each pair at $\mathbf{25}\mathbf{°}\mathbf{C}$?

$\begin{array}{l}{\mathbf{\text{(a) Ag(s) and Mn}}}^{\mathbf{\text{2+}}}\mathbf{\text{(aq)}}\\ {\mathbf{\text{(b) Cl}}}_{\mathbf{\text{2}}}{\mathbf{\text{(g) and Br}}}^{\mathbf{\text{-}}}\mathbf{\text{(aq)}}\end{array}$

What is the value of the equilibrium constant for the reaction between each pair at $\mathbf{25}\mathbf{°}\mathbf{C}$?

$\begin{array}{l}{\mathbf{\text{(a) Cr(s) and Cu}}}^{\mathbf{\text{2+}}}\mathbf{\text{(aq)}}\\ {\mathbf{\text{(b) Sn(s) and Pb}}}^{\mathbf{\text{2+}}}\mathbf{\text{(aq)}}\end{array}$

A voltaic cell consists of a standard hydrogen electrode in one half-cell and a ${\mathbf{\text{Cu/Cu}}}^{\mathbf{\text{2+}}}$ half-cell. Calculate  $\mathbf{\left[}{\mathbf{\text{Cu}}}^{\mathbf{\text{2+}}}\mathbf{\right]}$ when ${\mathbf{\text{E}}}_{\mathbf{\text{cell }}}^{\mathbf{\text{°}}}$  is  $\mathbf{0}\mathbf{.}\mathbf{22}$V

During reconstruction of the Statue of Liberty, Teflon spacers were placed between the iron skeleton and the copper plates that cover the statue. What purpose do these spacers serve? 

Why do steel bridge-supports rust at the waterline but not above or below it?

After the $\mathbf{1930}\mathit{s}$, chromium replaced nickel for corrosion resistance and appearance on car bumpers and trim. How does chromium protect steel from corrosion? 

Which of the following metals are suitable for use as sacrificial anodes to protect against corrosion of underground iron pipes? If any are not suitable, explain why: 

(a) Aluminium 

(b) Magnesium

 (c) Sodium 

(d) Lead 

(e) Nickel 

(f) Zinc 

(g) Chromium

In the everyday batteries used for flash lights, toys, etc., no salt bridge is evident. What is used in these cells to separate the anode and cathode compartments?

Subterranean brines in parts of the United States are rich in iodides and bromides and serve as an industrial source of these elements. In one recovery method, the brines are evaporated to dryness and then melted and electrolyzed. Which halogen is more likely to form from this treatment? Why?

Zinc plating (galvanizing) is an important means of corrosion protection. Although the process is done customarily by dipping the object into molten zinc, the metal can also be electroplated from aqueous solutions. How many grams of zinc can be deposited on a steel tank from a  ${\mathbf{\text{ZnSO}}}_{\mathbf{\text{4}}}$ solution when a $\mathbf{\text{0.855 - A}}$  current flows for $\mathbf{\text{2.50}}$  days?

Consider the following general electrolytic cell:

(a) At which electrode does oxidation occur? 

(b) At which electrode does elemental M form? 

(c) At which electrode are electrons being released by ions?

 (d) At which electrode are electrons entering the cell?

A voltaic cell consists of Cr/Cr3 and Cd/Cd2 half-cells with all components in their standard states. After  minutes of operation, a thin coating of cadmium metal has plated out on the cathode. Describe what will happen if you attach the negative terminal of a dry cell (V) to the cell cathode and the positive terminal to the cell anode.

Why are E half-cell values for the oxidation and reduction of water different from E° half-cell values for the same processes? In an aqueous electrolytic cell, nitrate ions never react at the anode, but nitrite ions do. Explain.

In an aqueous electrolytic cell, nitrate ions never react at the anode, but nitrite ions do. Explain.

How does over voltage influence the products in the electrolysis of aqueous salts?

In the electrolysis of molten NaBr,

 (a) What product forms at the anode?

 (b) What product forms at the cathode?

In the electrolysis of molten BaI₂, 

(a) What product forms at the negative electrode?

 (b) What product forms at the positive electrode?

A professor adds  to water to facilitate its electrolysis in a lecture demonstration. 

(a) What is the purpose of the ${\mathbf{Na}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$ ? 

(b)Why is the water electrolyzed instead of the salt?

How many seconds does it take to deposit $\mathbf{1}\mathbf{.}\mathbf{63}\mathbf{ }\mathbf{g}$ of on a decorative drawer handle when $\mathbf{13}\mathbf{.}\mathbf{7}\mathbf{ }\mathbf{A}$ is passed through a $\mathbf{Ni}{\left({\mathrm{NO}}_3\right)}_{\mathbf{2}}$ solution?

How many seconds does it take to deposit $\mathbf{65}\mathbf{.}\mathbf{5}\mathbf{ }\mathbf{g}$  of $\mathbf{Zn}$  on a steel gate when $\mathbf{21}\mathbf{.}\mathbf{0}\mathbf{ }\mathbf{A}$  is passed through a  ${\mathbf{ZnSO}}_{\mathbf{4}}$ solution?

How many grams of aluminum can form by passing  $\mathbf{305}\mathbf{C}$ through an electrolytic cell containing a molten aluminum salt?

How many grams of radium can form by passing $\mathbf{235}\mathbf{C}$ through an electrolytic cell containing a molten radium salt?

Electrolysis of molten $\mathbf{NaCl}$ in a Downs cell is the major isolation step in the production of sodium metal. Assuming that $\mathbf{215}\mathbf{ }\mathbf{g}$ of Na metal forms,

(a) How many moles of electrons are required?

(b) How many coulombs are required?

(c) How many amps will produce this amount in $\mathbf{9}\mathbf{.}\mathbf{50}\mathbf{ }\mathbf{h}$?

Electrolysis of molten ${\mathbf{MgCl}}_{\mathbf{2}}$ is the final production step in the isolation of magnesium from seawater by the Dow process. Assuming that $\mathbf{45}\mathbf{.}\mathbf{6}\mathbf{ }\mathbf{g}$ of $\mathbf{Mg}$ metal forms,

(a) How many moles of electrons are required?

(b) How many coulombs are required?

(c) How many amps will produce this amount in$\mathbf{3}\mathbf{.}\mathbf{50}\mathbf{ }\mathbf{h}$?

What product forms at each electrode in the aqueous electrolysis of the following salts: (a) ${\mathbf{FeI}}_{\mathbf{2}}$ ; (b) ${\mathbf{K}}_{\mathbf{3}}{\mathbf{PO}}_{\mathbf{4}}$?

What product forms at each electrode in the aqueous electrolysis of the following salts: (a)$\mathbf{Cr}{\left({\mathrm{NO}}_3\right)}_{\mathbf{3}}\mathbf{;}$ (b)${\mathbf{MnCl}}_{\mathbf{2}}$?

Question: To examine the effect of ion removal on cell voltage, a chemist constructs two voltaic cells, each with a standard hydrogen electrode in one compartment. One cell also contains a  ${\text{Pb/Pb}}^{\text{2 + }}$half-cell; the other contains a ${\text{Cu/Cu}}^{\text{2 + }}$half-cell. 

(a) What is ${\text{E}}^{\text{o}}$ of each cell 298 K at ? 

(b) Which electrode in each cell is negative? 

(c) When ${\text{Na}}_{\text{2}}\text{S}$ solution is added to the ${\text{Pb}}^{\text{2 + }}$electrolyte, solid $\text{PbS}$ forms. What happens to the cell voltage? 

(d) When sufficient ${\text{Na}}_{\text{2}}\text{S}$ is added to the ${\text{Cu}}^{\text{2 + }}$electrolyte,  $\text{CuS}$ forms and  ${\text{[Cu}}^{\text{2 + }}\text{]}$drops to $1\times {10}^{-6}$ . Find the cell voltage.

Electrodes used in electrocardiography are disposable, and many incorporate silvers. The metal is deposited in a thin layer on a small plastic “button,” and then some is converted to $\text{AgCl}$: 

${\text{Ag(s) + Cl}}^{\text{ - }}\text{(aq)}\rightleftharpoons {\text{AgCl(s) + e}}^{\text{ - }}$

(a) If the surface area of the button is ${\text{2.0 cm}}^{\text{2}}$ and the thickness of the silver layer is $7.5\times {10}^{-6}m$, calculate the volume (in ${\text{cm}}^{\text{3}}$) of  used in one electrode. 

(b) The density of silver metal is ${\text{10.5 g/cm}}^{\text{3}}$. How many grams of silver are used per electrode? 

(c) If  is plated on the button from an ${\text{Ag}}^{\text{ + }}$ solution with a current of $\text{12.0 mA}$ , how many minutes does the plating take? 

(d) If bulk silver costs  $\text{\$ 13.00}$per troy ounce ($\text{31.10 g}$), what is the cost (in cents) of the silver in one disposable electrode?

A thin circular-disk earring  $\text{4.00 cm}$in diameter is plated with a coating of gold  $\text{0.25 mm}$thick from an ${\text{Au}}^{\text{3 + }}$bath. 

(a) How many days does it take to deposit the gold on one side of one earring if the current is $\text{0.013 A}$ ( d of gold ${\text{ = 19.3 g/cm}}^{\text{3}}$)? 

(b) How many days does it take to deposit the gold on both sides of the pair of earrings? 

(c) If the price of gold is $\text{\$ 320}$ per troy ounce ($\text{31.10 g}$), what is the total cost of the gold plating?

A silver button battery used in a watch contains $\text{0.75 g}$ of zinc and can run until $\text{80\% }$ of the zinc is consumed. 

(a) How many days can the battery run at a current $\text{0.85}$ of  microamps (${\text{10}}^{\text{ - 6}}\text{ amps}$)? 

(b) When the battery dies, $\text{95\% }$ of the ${\text{Ag}}_{\text{2}}\text{O}$ has been consumed. How many grams of  was used to make the battery? 

(c) If  Ag costs $\text{\$ 13.00}$ per troy ounce ($\text{31.10 g}$), what is the cost of the  consumed each day the watch runs?

Commercial electrolytic cells for producing aluminium operate at $\text{5.0 V}$ and $\text{100,000 A}$. 

(a) How long does it take to produce exactly 1  metric ton ($\text{1000 kg}$) of aluminium? 

(b) How much electrical power (in kilowatt-hours,$\text{kW}·\text{h}$ ) is used [$\text{1 W = 1 J/s;1 kW}·{\text{h = 3.6×10}}^{\text{3}}\text{KJ}$]? 

(c) If electricity costs $\text{0.90}$ per $\text{kW}·\text{h}$ and cell efficiency is $\text{90\% }$ , what is the cost of electricity to produce exactly $\text{1 lb}$ of aluminium?

Trains powered by electricity, including subways, use direct current. One conductor is the overhead wire (or “third rail” for subways), and the other is the rails upon which the wheels run. The rails are supported on supports in contact with the ground. To minimize corrosion, should the overhead wire or the rails be connected to the positive terminal? Explain

Like any piece of apparatus, an electrolytic cell operates at less than $\text{100\% }$efficiency. A cell depositing $\text{Cu}$ from a  ${\text{Cu}}^{\text{2 + }}$bath operates for $\text{10 h}$ with an average current of $\text{5.8 A}$. If $\text{53.4 g}$  of copper is deposited, at what efficiency is the cell operating?

Commercial aluminium production is done by the electrolysis of a bath containing  ${\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}$dissolved in molten ${\text{Na}}_{\text{3}}{\text{AlF}}_{\text{6}}$salt. Why isn’t it done by electrolysis of an aqueous ${\text{AlCl}}_{\text{3}}$ solution?