CH 223 Problem Set #5
Complete problem set on separate pieces of paper showing all work, circling final answers, etc.
Covering: Chapter Sixteen, Chapter Seventeen and Chapter Guide Five
Important Tables and/or Constants: F = 96485 C/mol e-, R = 8.3145 J mol-1 K-1, Appendix G (page 1261,
thermodynamic values), Appendix L (page 1291, reduction potentials), "Redox Reactions" (Handout), Table of
Redox Potentials at the end of this problem set
Thermodynamic values also found here: http://mhchem.org/221/classroom/pdf/zThermoValues.pdf
1. Classify each of the reactions according to their spontaneity. Are these reactions enthalpy
and/or entropy driven?
a. C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(ℓ) ΔH° = -673 kJ ΔS° = 60.4 J/K
b. MgO(s) + C(graphite) → Mg(s) + CO(g)
ΔH° = 490.7 kJ ΔS° = 197.9 J/K
2. Use a Table to calculate ΔH° and ΔS° for the reaction of silicon(IV) oxide with carbon:
SiO2(s) + C(graphite) → Si(s) + CO2(g)
a. Is the reaction spontaneous at 298 K?
b. Is the reaction predicted to be spontaneous at higher temperatures?
3. Using values of ΔH°f and S°, calculate ΔG°rxn for each of the following reactions. Which of
these reactions is (are) predicted to be product-favored? Are the reactions enthalpy or
entropy driven?
a. Ca(s) + 2 H2O(ℓ) → Ca(OH)2(aq) + 2 H2(g)
(Note: for Ca(OH)2(aq), ΔH° = -1002.82 kJ/mol and S° = 74.5 J K-1 mol-1)
b. 6 C(graphite) + 3 H2(g) → C6H6(ℓ)
4. Using values of ΔG°f, calculate ΔG°rxn for each of the following reactions. Which are
product-favored?
a. HgS(s, red) + O2(g) → Hg(ℓ) + SO2(g)
b. 2 H2S(g) + 3 O2(g) → 2 H2O(g) + 2 SO2(g)
5. Estimate the temperature required to decompose CaSO4(s) into CaO(s) and SO3(g) using
values of ΔH°f and S°.
6. The formation of O3(g) from O2(g) has a standard free energy change, ΔG°, of +163.2 kJ/mol
at 25 °C. Calculate Kp at this temperature. Comment on the connection between the sign of
ΔG° and the magnitude of Kp.
7. Write balanced equations for the following half-reactions. Specify whether each is an
oxidation or reduction.
a. H2O2(aq) → O2(g) (in acid)
b. H2C2O4(aq) → CO2(g) (in acid)
c. NO3-1(aq) → NO(g) (in acid)
d. MnO4-1(aq) → MnO2(s) (in base)
8. Balance the following redox equations. All occur in acid solution.
a. Sn(s) + H+(aq) → Sn2+(aq) + H2(g)
b. Cr2O72-(aq) + Fe2+(aq) → Cr3+(aq) + Fe3+(aq)
c. MnO2(s) + Cl-1(aq) → Mn2+(aq) + Cl2(g)
d. CH2O(aq) + Ag+(aq) → HCO2H(aq) + Ag(s)
Problem Set #5 continues on the next page
Page II-5-1 / CH 223 Problem Set #5
Problem Set #5, Continued from previous page
9. Balance the following redox reactions. All occur in basic solution.
a. Fe(OH)3(s) + Cr(s) → Cr(OH)3(s) + Fe(OH)2(s)
b. NiO2(s) + Zn(s) → Ni(OH)2(s) + Zn(OH)2(s)
c. Fe(OH)2(s) + CrO42-(aq) → Fe(OH)3(s) + [Cr(OH)4]-1
d. N2H4(aq) + Ag2O(s) → N2(g) + Ag(s)
10. The following voltaic cell is created: Ag(s) | Ag+(aq) || Cl2(g, 1 atm) | Cl-1(aq, 1.0 M)| Pt(s)
a. Write equations for the oxidation and reduction half-reactions and for the overall
(cell) reaction.
b. Which half-reaction occurs in the anode compartment and which occurs in the
cathode compartment?
c. Complete the following sentences: Electrons in the external circuit flow from the
_____ electrode to the _____ electrode. Negative ions move in the salt bridge from
the _____ half-cell to the _____ half-cell.
11. Balance each of the following unbalanced equations, then calculate the standard reduction
potential, E°, and decide whether each is product-favored as written. All reactions occur in
acidic solution.
a. I2(s) + Br-1(aq) → I-1(aq) + Br2(l)
b. Fe2+(aq) + Cu2+(aq) → Cu(s) + Fe3+(aq)
c. Fe2+(aq) + Cr2O72-(aq) → Fe3+(aq) + Cr3+(aq)
d. MnO4-1(aq) + NO(g) → Mn2+(aq) + NO3-1(aq)
12. Consider the following half-reactions:
Half-Reaction
E° (V)
MnO4-1(aq) + 8 H+(aq) + 5 e- → Mn2+(aq) + 4 H2O(ℓ)
+1.51
BrO3-1(aq) + 6 H+(aq) + 6 e- → Br-1(aq) + 3 H2O(ℓ)
+1.47
Cr2O72-(aq) + 14 H+(aq) + 6 e- → 2 Cr3+(aq) + 7 H2O(ℓ)
+1.33
NO3-1(aq) + 4 H+(aq) + 3 e- → NO(g) + 2 H2O(ℓ)
+0.96
SO42-(aq) + 4 H+(aq) + 2 e- → SO2(g) + 2 H2O(ℓ)
+0.20
a. Choosing from among the reactants in these half-reactions, identify the strongest and
weakest oxidizing agents.
b. Which of the oxidizing agents is (are) capable of oxidizing Br-1(aq) to BrO3-1(aq) in
acid solution?
c. Write a balanced equation for the reaction of Cr2O72-(aq) with SO2(g) in acid solution.
Is this reaction product-favored or reactant-favored?
d. Write a balanced chemical equation for the reaction of Cr2O72-(aq) with Mn2+(aq). Is
this reaction product-favored or reactant-favored?
13. Calculate the potential at 298 K developed by a voltaic cell using the following reaction if all
dissolved species are 0.015 M:
2 Fe2+(aq) + H2O2(aq) + 2 H+(aq) → 2 Fe3+(aq) + 2 H2O(ℓ)
14. Calculate ΔG° and the equilibrium constant for the following reactions:
a. Zn2+(aq) + Ni(s) → Zn(s) + Ni2+(aq)
b. Cu(s) + 2 Ag+(aq) → Cu2+(aq) + 2 Ag(s)
Problem Set #5 continues on the next page
Page II-5-2 / CH 223 Problem Set #5
Problem Set #5, Continued from previous page
15. In the electrolysis of a solution containing Ag+(aq), metallic Ag(s) deposits on the cathode.
Using a current of 1.12 A for 2.40 h, what mass of silver forms?
16. Electrolysis of a solution of Zn(NO3)2(aq) to give zinc metal is carried out using a current of
2.12 A. How long should electrolysis continue to prepare 2.5 g of zinc?
Page II-5-3 / CH 223 Problem Set #5
20.4 Standard Electrochemical Potentials
Table 20.1
Standard Reduction Potentials in Aqueous Solution at 25 °C*
Reduction Half-Reaction
E° (V)
F2(g) + 2 e
-
H2O2(aq) + 2 H (aq) + 2 e
+
-
2-
+2.87
¡ 2 H2O(/)
+1.77
PbO2(s) + SO4 (aq) + 4 H (aq) + 2 e
¡ PbSO4(s) + 2 H2O(/)
+1.685
MnO4-(aq) + 8 H+(aq) + 5 e-
¡ Mn2+(aq) + 4 H2O(/)
+1.51
Au3+(aq) + 3 e-
¡ Au(s)
+1.50
Cl2(g) + 2 e
-
¡ 2 Cl (aq)
+1.36
3+
+
-
-
2-
Cr2O7 (aq) + 14 H (aq) + 6 e
¡ 2 Cr (aq) + 7 H2O (/)
+1.33
O2(g) + 4 H+(aq) + 4 e-
¡ 2 H2O(/)
+1.229
Br2(/) + 2 e
¡ 2 Br (aq)
+1.08
¡ NO(g) + 2 H2O(/)
+0.96
¡ Cl-(aq) + 2 OH-(aq)
+0.89
¡ Hg(/)
+0.855
¡ Ag(s)
+0.799
+
-
-
NO3 (aq) + 4 H (aq) + 3 e
+
-
-
OCl-(aq) + H2O(/) + 2 e2+
Hg (aq) + 2 e
Ag (aq) + e
+
-
-
2+
Hg2 (aq) + 2 e
¡ 2 Hg(/)
Fe3+(aq) + e-
¡ Fe2+(aq)
I2(s) + 2 e
¡ 2 I (aq)
-
-
-
O2(g) + 2 H2O(/) + 4 e
-
2+
¡ 4 OH (aq)
-
Cu (aq) + 2 e
¡ Cu(s)
Sn4+(aq) + 2 e-
¡ Sn2+(aq)
2 H (aq) + 2 e
-
+
-
¡ H2(g)
2+
-
¡ Sn(s)
Ni2+(aq) + 2 e-
¡ Ni(s)
Sn (aq) + 2 e
3+
Increasing strength of reducing agents
-
Increasing strength of oxidizing agents
¡ 2 F (aq)
-
2+
V (aq) + e
¡ V (aq)
-
PbSO4(s) + 2 e
+0.789
+0.771
+0.535
+0.40
+0.337
+0.15
0.00
-0.14
-0.25
-0.255
2-
¡ Pb(s) + SO4 (aq)
-0.356
-
Cd (aq) + 2 e
¡ Cd(s)
-0.40
Fe2+(aq) + 2 e-
¡ Fe(s)
-0.44
-
2+
2+
Zn (aq) + 2 e
-
2 H2O(/) + 2 e
-
3+
¡ Zn(s)
-0.763
¡ H2(g) + 2 OH (aq)
-0.8277
-
Al (aq) + 3 e
¡ Al(s)
-1.66
Mg2+(aq) + 2 e-
¡ Mg(s)
-2.37
Na (aq) + e
¡ Na(s)
-2.714
¡ K(s)
-2.925
¡ Li(s)
-3.045
-
+
K (aq) + e
+
-
-
Li (aq) + e
+
-
* In volts (V) versus the standard hydrogen electrode.
Tables of Standard Reduction Potentials
The experimental approach just described leads to lists of E° values such as seen in
Figure 20.14 and Table 20.1. Let us list some important points concerning these tables and then illustrate them in the discussion and examples that follow.
1. As in Figure 20.14 reactions are written as “oxidized form + electrons ¡ reduced form.” The species on the left side of the reaction arrow is an oxidizing
Page II-5-4 / CH 223 Problem Set #5
agent, and the species on
the right side of the reaction arrow is a reducing
agent. Therefore, all potentials are for reduction reactions.
■ E° Values
An extensive listing of E° valu
Appendix M, and still larger ta
can be found in chemistry ref
A common convention, used i
lists standard reduction poten
groups, one for acid and neut
and the other for basic solutio