The Foundations of Chemistry

Exercises 899

plating silver onto jewelry. (c) A jeweler purchases highly

purified silver to use as the anode in an electroplating

operation. Is this a wise purchase? Why?

*115.The same quantity of electric charge that deposited 0.612 g

of silver was passed through a solution of a gold salt, and

0.373 g of gold was deposited. What is the oxidation state

of gold in this salt?

CONCEPTUAL EXERCISES

*116.Household electricity is an alternating current of 60 cycles

per second. Can a redox reaction be used to provide an

alternating current? Explain.

*117.Electroplating is performed by using a source of direct

current. (a) Why can’t an alternating current be used?

(b) What would happen if an alternating current were

used for electroplating?

*118.Figure 21-5 is a schematic diagram of the electrolytic cell

used to separate copper from the impurities, that is, zinc,

iron, silver, gold, and platinum. Can this process be used

to separate the metals left after the copper has been

removed? Explain.

*119.A zinc–copper cell like that shown in Figure 21-7 is con-

structed, except that an inert platinum wire is used

instead of the salt bridge. Will the cell still produce a

potential?

BUILDING YOUR KNOWLEDGE

*120.An electrochemical cell was needed in which hydrogen

and oxygen would react to form water. (a) Using the fol-

lowing standard reduction potentials for the couples

given, determine which combination of half-reactions

gives the maximum output potential:

E^0 0.828 V for H 2 O(
)/H 2 (g), OH

E^0 0.0000 V for H/H 2 (g)

E^0 1.229 V for O 2 (g), H/H 2 O(
)

E^0 0.401 V for O 2 (g), H 2 O(
)/OH

(b) Write the balanced equation for the overall reaction

in (a).

*121.(a) Given the following E^0 values at 25°C, calculate Ksp

for cadmium sulfide, CdS.

Cd^2 (aq) 2 e88nCd(s) E^0 0.403 V

CdS(s) 2 e88nCd(s)S^2 (aq) E^0 1.21 V 0

(b) Evaluate 
G^0 at 25°C for the process

CdS(s) 34 Cd^2 (aq)S^2 (aq)

*122.Refer to tabulated reduction potentials. (a) Calculate Ksp

for AgBr(s). (b) Calculate 
G^0 for the reaction

AgBr(s) 34 Ag(aq)Br(aq)

*123.Under standard-state conditions, the following reaction

is not spontaneous:

Br2MnO 4 H 2 O(
)88n

BrO 3 2MnO 2 (s)2OH E^0 0.022 V

The reaction conditions are adjusted so that E0.120 V

by making [Br][MnO 4 ]1.60 mol/L and [BrO 3 ]

0.60 mol/L. (a) What is the concentration of hydrox-

ide ions in this cell? (b) What is the pH of the solution

in the cell?

*124.Show by calculation that E^0 1.662 V for the reduc-

tion of Al^3 to Al(s), regardless of whether the equation

for the reaction is written

(i)^13 Al^3 e88n^13 Al(s) 
G^0 160.4 kJ/mol

or

(ii) Al^3  3 e88nAl(s) 
G^0 481.2 kJ/mol

*125.We wish to fill a balloon with H 2 at a pressure of 1.05 atm

and a temperature of 25°C. The volume of the balloon,

when filled, is 750 mL. How long must a current of 2.25 A

be passed through the cell in order to produce this amount

of H 2 by electrolysis of water?