Practice Problems Chapter 23, 24, 25
Chpt. 23
1. Which of the following statements is always true for a spontaneous process?
(a)
Ssurroundings > Ssystem
b)
Ssurroundings > 0
(c)
Ssystem > - Ssurroundings
(d)
Ssystem > 0
2. For which process is S negative?
(a)
The evaporation of 1 mol of CCl4 (l).
(b)
Mixing 5 mL of ethanol with 25 mL of water.
(c)
Compressing 1 mol of Ne at constant temperature from 1.5 L to 0.5 L.
(d)
Raising the temperature of 100 g of Cu from 275 K to 295 K.
3. A reaction occurs at 451 K. The associated entropy change for the surroundings is
- 326 J/K. What quantity of heat is absorbed by the system?
(a)
326 kJ/mol
(b)
24.2 kJ/mol
(c)
12.1 kJ/mol
(d)
147 kJ/mol
4. For the reaction
A (g) + B (g)
>
C (g) + D (g)
H0 = 40 kJ and S0 = 50 J/K. Which of the following statements is true?
(a)
At standard conditions the reaction is spontaneous at temperatures less
than 10 K.
(b)
At standard conditions the reaction is spontaneous at temperatures greater
than 800 K.
(c)
At standard conditions the reaction is spontaneous only at temperatures
between 10 K and 800 K.
(d)
At standard conditions the reaction is spontaneous at all temperatures.
5. Consider the following reaction,
2 F (g) → F2 (g)
and the following data.
F2 (g)
F (g)
HfO (kJ/mol)
0
78.99
SO (J/mol K)
202.67
158.64
(a) Is the reaction spontaneous at standard conditions of temperature and pressure?
(b) Under what conditions of temperature will the reaction be spontaneous if both
gases have a partial pressure of 1.0 atm?
6. The normal boiling point of CS2 is 48 OC. Given that the standard enthalpy of
vaporization of CS2 is 27.66 kJ/mol and that the standard entropy of CS2 (g) is 237.7
J/mol K, calculate the standard entropy of CS2 (l)
7. In which case will the equilibrium constant (K) be less than 1 at 298 K?
(a)
H0 = 4.55 kJ/mol ,
S0 = 28.0 J/mol K
(b)
H0 = - 4.55 kJ/mol , S0 = 28.0 J/mol K
(c)
H0 = 4.55 kJ/mol ,
(d)
both (a) and (b)
S0 = - 28.0 J/mol K
8. The standard molar free energies of formation of NO2 (g) and N2O4 (g) at 250C are
51.84 kJ/mol and 98.28 kJ/mol respectively. What is the value of Kp for the
following reaction at 250C?
2 NO2 (g)
(a)
1.37 x 108
(b)
1.17 x 104
(c)
8.84
(d)
0.113
>
N2O4 (g)
Problems 9 and 10 are related to the following reaction at 699 K.
H2 (g) + I2 (g)
>
2 HI (g)
9. If G0 = -23.25 kJ/mol for this reaction, what is G for this reaction (in kJ/mol) if the
partial pressure of H2 and I2 are each 1.0 x 10-3 atm, the partial pressure of HI is
1.0 x 103 atm, and the temperature is 699 K.
10. Under the conditions specified in problem 9, will the partial pressure of HI increase,
decrease or remain the same?
11. A maximum of 1.89 mg of AgCl will dissolve in 1.00 L of solution at 25.00C. A
maximum of 5.26 mg of AgCl will dissolve in 1.00 L of solution at 50.00C. Use this
information to calculate H0 and S0 for the following reaction.
AgCl (s)
>
Ag+ (aq) + Cl- (aq)
Assume that H and S are independent of temperature.
12.
A 0.100 M weak acid (HA) solution is found to have a pH = 2.376. Given the
following
G0f ( H+ (aq)) = 0.0 kJ/mol
G0f ( HA (aq)) = -372.3 kJ/mol
calculate G0f for A- (aq).
13.
Consider the reaction
A (g) + B (g)
>
2 C (g)
Where,
H0 = -15.62 kJ/mol
S0 = -10.20 J/mol K
(a)
Suppose that a tank is filled with A (g) and B (g) such that PA = 1.00 atm
and PB = 1.00 atm at a temperature of 25.00C. Calculate the pressure of all
species after the system has reached equilibrium assuming that the
temperature is 25.00C.
(b)
If the temperature is decreased to 00C (at constant total pressure) what is
the effect on the equilibrium pressure of C (g)? Calculate the equilibrium
pressure of C (g) at 00C.
14.
The equilibrium constant, Kp, is 5.04 x 1017 at 250C for the following reaction.
C2H4 (g) + H2 (g)
>
C2H6 (g)
(a)
Calculate G0 for this reaction.
(b)
Determine whether this reaction is exothermic or endothermic.
Chpt. 24
15.
How many mol of electrons are transferred per mol of reaction in the following
reaction?
2 ClO3- (aq) + 12 H+ (aq) + 10 I- (aq) > 5 I2 (aq) + Cl2 (aq) + 6 H2O (l)
(a)
12
(b)
5
(c)
2
(d)
10
Chpt. 25
For the following problems you may need to consult the table of standard
reduction potentials found under the link to useful information.
16.
17.
Answer each of the following assuming that all species are at standard conditions.
(a)
Is H+ (aq) capable of oxidizing Cu (s) to Cu2+ (aq) ?
(b)
Is Fe3+ (aq) capable of oxidizing I- (aq) ?
(c)
Is H2 (g) capable of reducing Ag+ (aq) ?
Consider the following species at standard conditions.
Na+ (aq), Cl- (aq), Ag+ (aq), Ag (s), Zn2+ (aq), Zn (s), Pb (s)
18.
(a)
Which is the strongest oxidizing agent?
(b)
Which is the strongest reducing agent?
(c)
Which species can be oxidized by SO42- (aq) under acidic conditions?
(d)
Which species can be reduced by Al (s)?
Use the table of standard reduction potentials to choose a reagent capable of each
of the following at standard conditions under acidic conditions where [H+] = 1M.
(a)
oxidize Br- (aq) to Br2 (aq) but not oxidize Cl- (aq) to Cl2 (aq)
(b)
oxidize Mn (s) to Mn2+ (aq) but not oxidize Ni (s) to Ni2+ (aq)
19.
The following two half-reactions are involved in a galvanic cell. At standard
conditions what species are produced at each electrode?
Ag+ (aq) + eNi2+ (aq) + 2e-
20.
→
→
EO = 0.80 V
Ag (s)
EO = -0.23 V
Ni (s)
(a)
Ag (s) is produced at the cathode and Ni (s) is produced at the anode.
(b)
Ag (s) is produced at the cathode and Ni2+ (aq) is produced at the anode.
(c)
Ag+ (aq) is produced at the anode and Ni (s) is produced at the cathode.
(d)
Ag+ (aq) is produced at the anode and Ni2+ (aq) is produced at the cathode.
Consider a galvanic cell based on the following half-reactions.
Au3+ (aq) + 3e- → Au (s)
EO = 1.50 V
Tl+ (aq) + e- → Tl (s)
EO = -0.34 V
(a)
Determine the overall cell reaction and calculate EOcell.
(b)
Calculate GO and K for the overall cell reaction (at 25OC) .
(c)
Calculate Ecell at 25OC given the following.
[Au3+] = 1.0 x 10-2 M
[Tl+] = 1.0 x 10-4 M
21.
A galvanic cell is constructed by immersing a tin electrode in a 0.10 M Sn(NO3)2
solution and immersing a lead electrode in a 1.0 x 10-3 M Pb(NO3)2 solution. The
relevant standard reduction potentials are listed below.
Pb2+ (aq) + 2e-
→
Pb (s)
EO = -0.13 V
Sn2+ (aq) + 2e-
→
Sn (s)
EO = -0.14 V
Identify the half-reactions at the anode and at the cathode when the galvanic cell
is spontaneously discharged.
22.
A galvanic cell consists of a standard hydrogen electrode and a copper electrode.
2 H+ (aq) → H2 (g)
Cu2+ (aq) + 2e-
→
EO = 0.00 V
Cu (s)
EO = 0.34 V
Suppose the copper electrode is placed in a solution of unknown copper (II)
concentration. The cell potential is measured to be 0.195 V. Determine [Cu2+].
23.
A galvanic cell consists of a standard hydrogen electrode and a copper electrode.
Suppose that the copper electrode is immersed in a solution that is 0.100 M in
NaOH and that is saturated with Cu(OH)2 . Determine the cell potential. The
relevant reduction potentials are given in problem 22.
Ksp = 1.6 x 10-19 for Cu(OH)2
24.
Calculate Ksp for iron (II) sulfide given the following data.
FeS (s) + 2e- → Fe (s) + S2- (aq)
EO = -1.01 V
Fe2+ (aq) + 2e- → Fe (s)
EO = -0.44 V
25.
A galvanic cell is based on the following half-reactions.
Fe2+ (aq) + 2e- → Fe (s)
EO = -0.440 V
2 H+ (aq) + 2e- → H2 (g)
EO = 0.000 V
The iron compartment contains an iron electrode and [Fe2+] = 1.00 x 10-3 M.
The hydrogen compartment contains a platinum electrode , PH2 = 1.00 atm, and a
1.00 M solution of a weak acid (HA). If the observed cell potential is 0.333 V
determine Ka for the weak acid.
Electrolytic Cells
26.
Consider an electrolytic cell where silver metal is plated (reduced) from an
aqueous silver nitrate solution. If a constant current of 2.00 A is applied, how
long will it take to plate 50.0 g of silver?
27.
Which reaction will take place at the cathode in the electrolysis of a 1.0 M
aqueous KCl solution?
28.
29.
(a)
2 H2O (l) + 2e- → 2 H2 (g) + 2 OH- (aq)
(b)
K+ (aq) + e- → K (s)
(c)
Cl- (aq) + e- → Cl2- (aq)
(d)
2 H2O (l) → O2 (g) + 4 H+ (aq) + 4e-
Which reaction will take place at the anode in the electrolysis of a molten
(pure liquid) KCl ?
(a)
K → K+ + e-
(b)
2 Cl- → Cl2 + 2e-
(c)
K+ + e- → K
(d)
Cl- + e- → Cl2-
What volumes of H2 (g) and O2 (g) at STP are produced from the electrolysis of
water by a current of 2.50 A in 15.0 minutes?
30.
An unknown metal M is reduced by electrolysis from a solution of M(NO3)3. It
took 74.1 s for a current of 2.00 A to plate out 0.107 g of the metal. Identify the
metal.
31.
An aqueous solution contains a mixture of cations at the following concentrations.
[Cd2+] = 1.0 M
[Fe2+] = 0.10 M
[Ni2+] = 1.0 x 10-6 M
In which order will these metal cations be reduced if an applied voltage to the
solution is gradually increased?
32.
Consider the electrolysis of a solution that is 1.0 x 10-3 M in Ag2SO4 and
1.0 x 10-1 M in ZnSO4 with pH = 2.000.
(a)
Calculate the minimum voltage needed to plate silver metal at the cathode.
(b)
Will it be possible to plate zinc metal at the cathode once the
concentration of Ag+ has been reduced to a sufficiently low value?