Additional Exercises
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Oxidation was once defined as chemically adding oxygen to a substance. Use this reaction to argue that this definition is consistent with the modern definition of oxidation.
2 Mg + O2 → 2 MgO
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Reduction was once defined as chemically adding hydrogen to a substance. Use this reaction to argue that this definition is consistent with the modern definition of reduction.
C2H2 + 2 H2 → C2H6
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Assign oxidation numbers to the atoms in each substance.
a) Kr (krypton)
b) krypton tetrafluoride (KrF4)
c) dioxygen difluoride (O2F2)
4. Assign oxidation numbers to the atoms in each substance.
a) lithium hydride (LiH)
b) potassium peroxide (K2O2)
c) potassium fluoride (KF)
5. N atoms can have a wide range of oxidation numbers. Assign oxidation numbers for the N atom in each compound, all of which are known compounds.
a) N2O5
b) N2O4
c) NO2
d) NO
e) N2H4
f) NH3
6. Cr atoms can have a wide range of oxidation numbers. Assign oxidation numbers for the Cr atom in each compound, all of which are known compounds.
a) Na2CrO4
b) Na2Cr2O7
c) CrF5
d) CrCl3
e) CrCl2
7. Balance this redox reaction by inspection.
S8 + O2 → SO2
8. Balance this redox reaction by inspection.
C18H38 + O2 → CO2 + H2O
9. Balance this redox reaction by the half reaction method by assuming an acidic solution.
Cr2O72− + Fe → Cr3+ + Fe3+
10. Balance the redox reaction in Exercise 9 by the half reaction method by assuming a basic solution.
11. The uranyl ion (UO22+) is a fairly stable ion of uranium that requires strong reducers to reduce the oxidation number of uranium further. Balance this redox reaction using the half reaction method by assuming an acidic solution.
UO22+ + HN3 → U + N2
12. Balance the redox reaction in Exercise 11 by the half reaction method by assuming a basic solution.
13. Zinc metal can be dissolved by acid, which contains H+ ions. Demonstrate that this is consistent with the fact that this reaction has a spontaneous voltage:
Zn + 2 H+ → Zn2+ + H2
14. Copper metal cannot be dissolved by acid, which contains H+ ions. Demonstrate that this is consistent with the fact that this reaction has a nonspontaneous voltage:
Cu + 2 H+ → Cu2+ + H2
15. A disproportionation reaction occurs when a single reactant is both oxidized and reduced. Balance and determine the voltage of this disproportionation reaction. Use the data in Table 14.1 “Standard Reduction Potentials of Half Reactions”.
Cr2+ → Cr + Cr3+
16. A disproportionation reaction occurs when a single reactant is both oxidized and reduced. Balance and determine the voltage of this disproportionation reaction. Use the data in Table 14.1 “Standard Reduction Potentials of Half Reactions”.
Fe2+ → Fe + Fe3+
17. What would be overall reaction for a fuel cell that uses CH4 as the fuel?
18. What would be overall reaction for a fuel cell that uses gasoline (general formula C8H18) as the fuel?
19. When NaCl undergoes electrolysis, sodium appears at the cathode. Is the definition of cathode the same for an electrolytic cell as it is for a voltaic cell?
20. When NaCl undergoes electrolysis, chlorine appears at the anode. Is the definition of anode the same for an electrolytic cell as it is for a voltaic cell?
21. An award is being plated with pure gold before it is presented to a recipient. If the area of the award is 55.0 cm2 and will be plated with 3.00 µm of Au, what mass of Au will be plated on the award? The density of Au is 19.3 g/cm3.
22. The unit of electrical charge is called the coulomb (C). It takes 96,500 coulombs of charge to reduce 27.0 g of Al from Al3+ to Al metal. At 1,040 cm3, how many coulombs of charge were needed to reduce the aluminum in the cap of the Washington monument, assuming the cap is pure Al? The density of Al is 2.70 g/cm3.
Answers
1.
As oxygen is added to magnesium, it is being oxidized. In modern terms, the Mg atoms are losing electrons and being oxidized, while the electrons are going to the O atoms.
3.
a) Kr: 0
b) Kr: +4; F: −1
c) O: +1; F: −1
5.
a) +5
b) +4
c) +4
d) +2
e) −2
f) −3
7.
S8 + 8 O2 → 8 SO29.
14 H+ + Cr2O72− + 2 Fe → 2 Cr3+ + 7 H2O + 2 Fe3+11.
6 HN3 + UO22+ → U + 2 H2O + 9 N2 + 2 H+13.
The voltage of the reaction is +0.76 V, which implies a spontaneous reaction.
15.
3 Cr2+ → Cr + 2 Cr3+; −0.50 V
17.
CH4 + 2 O2 → CO2 + 2 H2O
19.
yes because reduction occurs at the cathode
21.
0.318 g
