Fundamentals of Electrochemistry - W.H. Freeman

1504T_ch14_270-297 1/23/06 11:35 Page 297
14-40. Do not ignore activity coefficients in this problem. If the
voltage for the following cell is 0.512 V, find K sp for Cu(IO 3 ) 2 .
Neglect any ion pairing.
Ni(s) 0 NiSO 4 (0.002 5 M) KIO 3 (0.10 M) 0 Cu(IO 3 ) 2 (s) 0 Cu(s)
Biochemists Use E
14-41. Explain what E°¿ is and why it is preferred over E° in
biochemistry.
14-42. We are going to find E°¿ for the reaction C 2 H 2 (g) 2H
2e T C 2 H 4 (g).
(a) Write the Nernst equation for the half-reaction, using E° from
Appendix H.
(b) Rearrange the Nernst equation to the form
E E° other terms
(c) The quantity (E° other terms) is E°¿. Evaluate E°¿ for
pH 7.00.
14-43. Evaluate E°¿ for the half-reaction (CN) 2 (g) 2H 2e T
2HCN(aq).
14-44. Calculate E°¿ for the reaction
H 2 C 2 O 4 2H 2e T 2HCO 2 H E° 0.204 V
Oxalic acid
Formic acid
14-45. HOx is a monoprotic acid with K and H 2 Red
a 1.4 10 5
is a diprotic acid with K and K 2 8.1 10 8 1 3.6 10 4
. Find
E° for the reaction
HOx e T H 2 Red
0.059 16
2
log a P C 2 H 4
P C2 H 2
b
E°¿ 0.062 V
14-46. Given the following information, find for nitrous acid,
HNO 2 .
NO 3 3H 2e T HNO 2 H 2 O E° 0.940 V
E°¿ 0.433 V
14-47. Using the reaction
HPO 2 4 2H 2e T HPO 2 3 H 2 O E° 0.234 V
and acid dissociation constants from Appendix G, calculate E° for
the reaction
H 2 PO 4 H 2e T HPO 2 3 H 2 O
14-48. This problem requires knowledge of Beer’s law from
Chapter 18. The oxidized form (Ox) of a flavoprotein that functions
as a one-electron reducing agent has a molar absorptivity
() of 1.12 10 4 M 1 cm 1 at 457 nm at pH 7.00. For the
reduced form (Red), 3.82 10 3 M 1 cm 1 at 457 nm at
pH 7.00.
Ox e T Red E°¿ 0.128 V
The substrate (S) is the molecule reduced by the protein.
Red S T Ox S
Both S and S are colorless. A solution at pH 7.00 was prepared by
mixing enough protein plus substrate (Red S) to produce initial
concentrations [Red] [S] 5.70 10 5 M. The absorbance at
457 nm was 0.500 in a 1.00-cm cell.
(a) Calculate the concentrations of Ox and Red from the absorbance
data.
(b) Calculate the concentrations of S and S .
(c) Calculate the value of E°¿ for the reaction S e T S .
K a
Problems 297