cell separated from each other?
20.4 What is the function of a salt bridge in a galvanic cell?
20.8 Consider a galvanic cell consisting of a magnesium electrode in contact with 1.0 M
Mg(NO3)2 and a cadmium electrode in contact with 1.0 M Cd(NO3)2. Calculate
Eº for the cell and draw a diagram showing the cathode, the anode, and the direction
of electron flow
20.11 Predict whether Fe3+ can oxidize I- to I2 under standard-state conditions.
20.15 For each of the following redox reactions (i) write the half-reactions; (ii) write a
balanced equation for the whole reaction; (iii) determine in which direction the
reaction will proceed spontaneously under standard-state conditions.
(a) H2(g) + Ni2+(aq)
H+(aq) +Ni(s)(b) Ni2+(aq) + Cd(s)
Ni(s) = Cd2+(aq)(c) MnO4-(aq) + Cl-(aq)
Mn2+(aq) + Cl2(g) (in acid solution)(d) Ce3+(aq) + H+(aq)
Ce4+(aq) + H2(g)(e) Cr(s) + Zn2+(aq)
Cr3+(aq) + Zn(s)20.17 Consider the following half-reactions:
MnO4-(aq) + 8H+(aq) + 5e-
Mn2+(aq) + 4H2O(l)NO3-(aq) + 4H+(aq) + 3e-
NO(g) + 2H2O(l)Predict whether NO3- ions will oxidize Mn2+ to MnO4- under standard-state
conditions.
20.19 Which species in each pair is a beter reducing agent under standard-state conditions?
(a) Na or Li, (b) H2 or I2, (c) Fe2+ or Ag, (d) Br- or Co2+.
20.21 Use the standard reduction potentials to find the equilibrium constant for each of the
following reactions at 25ºC:
(a) Br2(l) + 2I-(aq)
2Br-(aq) + I2(s)(b) 2Ce4+(aq) + 2Cl-(aq)
Cl2(g) + 2Ce3+(aq)(c) 5Fe2+(aq) = MnO4-(aq) + 8H+(aq)
Mn2+(aq) + 4H2O + 5Fe3+(aq)20.25 What spontaneous reaction will occur in aqueous solution, under standard-state
conditions, among the ions Ce4+, Ce3+, Fe3+, and Fe2+? Calculate DGº and Kc
for the reaction.
20.26 Given that Eº = 0.52 V for the reduction Cu+(aq) + e- Æ Cu(s), calculate Eº, DGº,
and K for the following reaction at 25ºC:
2Cu+(aq)
Cu2+(aq) + Cu(s)20.28 What is the potential of a cell made up of Zn/Zn2+ and Cu/Cu2+ half-cells at 25ºC if
[Zn2+] = 0.25 M and [Cu2+] = 0.15 M?
20.32 Referring to Figure 20.1, calculate the [Cu2+]/Zn2+] ratio at which the following
reaction will become spontaneous at 25ºC:
Cu(s) + Zn2+(aq)
Cu2+(aq) + Zn(s)20.37 The hydrogen-oxygen fuel cell is described in Section 20.6. (a) What volume of
H2(g), stored at 25ºC at a pressure of 155 atm, would be needed to run an electric
motor drawing a current of 8.5 A for 3.0 h? (b) What volume (L) of air at 25ºC and
1.00 atm will have to pass into the cell per minute to run the motor? Assume that air
is 20 percent O2 by volume, and that all the O2 is consumed in the cell. The other
components of air do not affect the fuel-cell reactions. Assume ideal gas behavior.
20.45 The half-reaction at an electrode is
Mg2+(molten) + 2e-
Mg(s)Calculate the number of grams of magnesium that can be produced by passing
1.00 F through the electrode.
20.46 Consider the electrolysis of molten calcium chloride, CaCl2. (a) Write the electrode
reactions. (b) How many grams of calcium metal can be produced by passing 0.50
A for 30 min?
20.54 A constant electric current flows for 3.75 h through two electrolytic cells connected
in series. One contains a solution of AgNO3 and the second a solution of CuCl2.
During this time 2.00 g of silver are deposited in the first cell. (a) How many grams of
copper are deposited in the second cell? (b) What is the current flowing, in amperes?
20.57 The passge of a current of 0.750 A for 25.0 min deposited 0.369 g of copper from a
CuSO4 solution. From this information, calculate the molar mass of copper.
20.59 In a certain electrolysis experiment, 1.44 g of Ag were deposited in one cell (containing
an aqueous AgNO3 solution), while 0.120 g of an unknown metal X was deposited
in another cell (containing an aqueous XCl3 solution) in series. Calculate the molar mass
of X.
20.66 From the following information, calculate the solubility product of AgBr.
AgBr(s) + e-
Ag(s) + Br-(aq) Eº = 0.07 VAg+(aq) + e-
Ag(s) Eº = 0.80 V20.73. Calculate the emf of the following concentration cell at 25ºC:
Cu(s)
Cu2+(aq, 0.080 M)OKCl(sat'd)OCu2+(aq, 1.2 M)OCu(s)
20.88 In an electrolysis experiment a student passes the same quantity of electricity through two
electrolytic cells containing silver and gold salts, respectively. Over a certain period of time,
she finds that 2.64 g of Ag and 1.61 g of Au are deposited at the cathodes. What is the
oxidation state of gold in the gold salt?
20.94 A 300-mL solution of NaCl was electrolyzed for 6.00 min. If the pH of the final solution
was 12.24, calculate the average current used.
20.95 Industrially, copper is purified by electrolysis. The impure copper acts as the anode,
and the cathode is made of pure copper. The electrodes are immersed in a CuSO4 solution.
During electrolysis, copper at the anode enters the solution s Cu2+ while Cu2+ ions are
reduced at the cathode. (a) Write half-cell reactions and the overall reaction for the
electrolytic process. (b) Suppose the anode was contaminated with Zn and Ag. Explain
what happens to these impurities during electrolysis. (c) How many hours will it take
to obtain 1.00 kg of Cu at a current of 18.9 A?