CMY 127
Self-Assessment Exercise 8
Subject:
Reference:
Note:
Pace:
Tutorials:
Exercises:
Theme 8: Electrochemistry
Textbook Chapter 20 Exercises p. 937 - 944
The answers to the odd-numbered questions are available in Appendix R of the textbook.
You should complete these exercises before 8 September.
The focus during the tutorial sessions which run from 9 to 12 September will be on this theme.
15
19
24
27
29
31
41
44
46
63
64
67
77
79
86
93
Answers Even-numbered questions:
24. (a) I¯ (b) Br¯ and I¯
44. 1.4 g Ag
46. 58 min
64. 1.5  104 Kwh
86. (a) 0.028 V (b) Ag+(aq) + Fe2+(aq)  Ag(s) + Fe3+(aq) (c) Reduction at Ag and oxidation at Pt
(d) 0.031 V; the cell reaction has reversed.
Additional Exercises
1. An electrolytic cell is set up with Cd(s) in Cd(NO3)2(aq) and Zn(s) in Zn(NO3)2(aq). Initially both electrodes weigh 5.00 g.
a) After running the cell for a time the electrode in the one half-cell weighs 4.75 g. Identify this electrode.
[Cd]
b) Calculate the mass of the other electrode.
[5.15 g]
+
−3
2. A galvanic cell consists of a hydrogen electrode with [H ] = 0.0125 mol.dm , and a metal electrode which is partly immersed in
–3
a 0.15 mol.dm solution of the metal’s nitrate salt. The identity of this metal is unknown, but it is known that the metal is
2+
divalent, thus forming cations of the type M (aq). The metal functions as anode in this cell. These two half-cells are connected
with a salt bridge and an external conductor, which includes a voltmeter. The temperature is 25.0°C and the partial pressure of
the hydrogen gas is 1.00 atm. When the circuit is closed, the reading on the voltmeter is 0.672 V. Identify the metal.
[zinc]
3. Consider the following standard reduction potentials at 25C:
E
CℓO4¯(aq) + 2H+(aq) + 2e¯ ⇌ CℓO3¯(aq) + H2O(ℓ) +1.19 V
CℓO3¯(aq) + 3H+(aq) + 2e¯ ⇌ HCℓO2(aq) + H2O(ℓ) +1.21 V
+
+1.65 V
HCℓO2(aq) + 2H (aq) + 2e¯ ⇌ HCℓO(aq) + H2O(ℓ)
2HCℓO(aq) + 2H+(aq) + 2e¯ ⇌ Cℓ2(g) + 2H2O(ℓ)
+1.49 V
Which one of the reagents above is the strongest oxidizing agent?
a) Which one of the reagents above is the weakest reducing agent?
b) Calculate E for the following reaction at 25C:
2HCℓO2 (aq) ⇌ HCℓO(aq) + CℓO3¯(aq) + H+(aq)
[+0.44 V]
c) Calculate E for the reaction in (c) at 80C when the concentrations and pH are as follows:
[HCℓO2] = 0.010 M
[HCℓO] = 0.10 M
[CℓO3¯] = 0.10 M
pH = 5.00
[+0.53 V]
d) Calculate G and K for the reaction in (c) at 25C.
[84.9 kJ/mol-rxn; 7.31014]
4. You have the following standard half-cell ready to be used: S4O62(aq) | S2O32
This half-cell must be combined with a Cr3+|Cr2+ half-cell in such a way that the voltaic cell delivers 0.70 V at 25C. The
following salts are available: CrCO3; Cr(NO3)3; CrSO4; Cr(OH)3; KNO3; CaSO4
a) Which two salts will you use to make up the solution for the Cr3+|Cr2+ half-cell?
[Choose soluble salts]
b) Write the cell reaction and calculate what the ratio of [Cr3+] to [Cr2+] must be in order for this cell to produce 0.70 V. [0.3113]
c) From your answer in (b), calculate suitable concentrations for [Cr3+] and [Cr2+] and then write the cell notation for this cell.
2
Given: S4O62(aq) + 2e¯ ⇌ 2S2O3 (aq)
=+0.17
5. a) Calculate the maximum electrical work that can be done by the following cell at 25 :
Tℓ(s) | Tℓ+(aq, 1 M) || Pb2+(aq, 1 M) | Pb(s)
[40.41 kJ/mol]
b) Determine the standard reduction potential for Tℓ+(aq) + e¯ ⇌ Tℓ(s)
[0.334 V]
c) What is the equilibrium constant for this reaction?
[1.20  107]
Given:
Species
2+
Pb (aq)
Pb(s)
+
Tℓ (aq)
Tℓ(s)
∆
° (kJ/mol)
(298.15 K)
1.7



° (J/Kmol)
(298.15 K)
10.5
64.78
64.2
∆
° (kJ/mol)
(298.15 K)
24.39



Well done!
©University of Pretoria
CMY 127
2014