A) Indicate the Oxidation State of Each Element, And
Chapter 18
1. In each of the following reactions,
a) indicate the oxidation state of each element, and
b) identify which substance is oxidized and which is reduced.
1) BaSO4 (s) + 4C (s) à BaS (s) + 4CO (g)
2) 2Cu2+ (aq) + 2H2O (l) à 2Cu (s) + O2 (g) + 4H+ (aq)
3) 5H2SO3 (aq) + 2MnO4 (aq) à 5SO42– (aq) + 2Mn2+ (aq) + 4H+ (aq) + 3H2O (l)
2. Complete and balance the following equations:
a) Pb(OH)42– (aq) + ClO– (aq) à PbO2 (s) + Cl– (aq) (basic solution)
b) Tl2O3 (s) + NH2OH (aq) à TlOH (s) + N2 (g) (basic solution)
c) Cr2O72– (aq) + CH3OH (aq) à HCO2H (aq) + Cr3+ (aq) (acidic solution)
3. Sketch the voltaic cell based on the following reaction:
2Cr (s) + 3HSO4 (aq) + 9H+ (aq) à 3H2SO3 (aq) + 2Cr3+ (aq) + 3H2O (l)
Show a) the composition and charge of each electrode,
b) the composition of the solution in each compartment,
c) the direction of electron flow, and
d) calculate the value of Eocell (using reduction potentials from the chapter).
4. Calculate the emf at 25oC of a cell that uses the reaction
Co (s) + I2 (s) à Co2+ (aq) + 2I– (aq)
when [Co2+ (aq)] = 0.016 M and [I– (aq)] = 0.0060 M (use the standard reduction
potentials in Chapter 19).
5. Given, Ni2+ (aq) + 2e– à Ni (s) Eored = –0.28 V
Cu2+ (aq) + 2e– à Cu (s) Eored = +0.34 V
which metal will oxidize more readily, copper or nickel?
6. A current of 5.0 amp is passed through molten Al2O3 for 24 hr. How many grams of
aluminum metal will form at the cathode of this electrolytic cell after this time?
The gain of electrons in a reaction is called
a) reduction b) oxidation
7. What is the oxidation state of P in the phosphate ion, PO43– ?
a) +2 b) +3 c) +5 d) +6
8. What substance is oxidized in the following reaction?
Cr2O7 2– + 3 SO3 2– + 8 H+ à 2 Cr3+ + 3 SO4 2– + 4 H2O
a) Cr2O7 2– b) SO3 2– c) H+ d) SO4 2–
9. What is the coefficient of water when the following equation is balanced in acidic aqueous solution?
Cr2O7 2– + Br– à Cr 3+ + Br2
a) 5 b) 6 c) 7 d) 8
10. The electrode at which oxidation occurs is called the
a) anode b) cathode c) salt bridge
11. The half-reaction occurring at the cathode in the voltaic cell reaction
3 MnO4- (aq) + 24 H+ (aq) + 5 Fe (s) à 3 Mn2+ (aq) + 5 Fe3+ (aq) + 12 H2O (l)
is
a) MnO4– (aq) + 8H+ (aq) + 5e– à Mn2+ (aq) + 4 H2O (l)
b) Fe (s) à Fe3 +(aq) + 3 e–
c) Fe (s) à Fe2+ (aq) + 2 e–
d) Fe2+ (aq) à Fe3+ (aq) + e–
12. Given the following standard reduction potentials:
Pb2+ (aq) + 2 e– à Pb(s) E° = –0.126 V
Cu2+ (aq) + 2 e– à Cu(s) E° = +0.337 V
calculate E°cell for a voltaic cell based on these half-reactions.
a) 0.463 V b) 0.211 V c) 1.95 V d) -0.211 V
13. Eocell for the following reaction is +0.13 V. What is the value of DG° (in kJ) for the reaction?
Pb (s) + 2 H+ (aq) à H2 (g) + Pb2+ (aq)
a) –25 kJ b) 15 kJ c) –52 kJ d) 62 kJ
14. The reaction O2 (g) + 4 H+ (aq) + 4 Fe2+ (aq) à 4 Fe3+ (aq) + 2 H2O (l) has E° = +0.46 V. What is
the value of the equilibrium constant for this reaction?
a) 1.05 X 109 b) 2.67 X 1018 c) 4.64 X 10-22 d) 1.27 X 1031
15. One of the differences between a voltaic cell and an electrolytic cell is that in a voltaic cell
a) an electric current is produced by a chemical reaction.
b) electrons flow toward the anode.
c) a nonspontaneous reaction is forced to occur using an electric current from an external source.
d) reduction occurs at the anode.
16. If the value of Ecell for the following reaction is +0.95 V at 25°C, what is the concentration of Cu2+ ion
if the concentration of Zn2+ is 1.0 M?
Zn (s) + Cu2+ (aq) à Cu (s) + Zn2+ (aq) E°cell = +1.10 V
a) 1.5 X 10–5 M b) 8.6 X 10–6 M c) 4.4 X 10–5 M d) 2.7 X 10–4 M
17. How many grams of copper metal can be obtained by passing a current of 1.50 amp through a
solution of Cu2+ (aq) for 45 minutes?
(Show calculation)