Spectro and Redox
Thursday 21 November, 1996 Name ____________________________
Here is a portion of the Redox table we have been using:
Cr2O72- + 14H+ + 6e-
|
=
|
2Cr3+
+ 7H2O
|
+1.33
|
| O2
+ 4H+ + 4e-
|
=
|
2H2O
|
+1.229
|
| IO3-
+ 6H+ + 5e-
|
=
|
1/2
I2 + 3H2O
|
+1.195
|
| Cu2+
+I- + e-
|
=
|
CuI
|
+0.86
|
| Ag+
+ e-
|
=
|
Ag
|
+0.799
|
| Fe3+
+ e-
|
=
|
Fe2+
|
+0.771
|
1. Write the balanced complete reaction between 02 and Fe2+ using half reactions from the table above.
2. Write the balanced half reaction for O2 going to H2O2.
3. For the following electrochemical cell: (left and right have been identified for the orientationally challenged.)
A. Use the Nernst Equation to calculate the potential of the Left side.
B. Use the Nernst Equation to calculate the potential of the Right side.
C. Which side is the Anode?
D. If we connected a voltmeter to this cell, which side should the Red (+) lead and which side should the Black (-) lead go to have the meter read in the right direction?
E. If we connected a Volkswagen tail lamp to this cell, which way would the electrons flow?
F. With the electrons flowing here as a galvanic cell, at which electrode would reduction take place? Would this change if this were an electrolytic cell?
4. 20.0 mL of 0.0100 M Cr2O72-, 3.00 mL of 18 M H2SO4 and 30.0 mL of 0.100 M Fe2+ are mixed together. Compute the concentrations of all species after reaction, and compute the potential of a Pt electrode placed in this mixture vs. a Ag/AgCl reference electrode (0.228 V in 1 M KCl).
Balanced Reaction: ___________________________________________________
Cr2O72-
|
Cr3+
|
H+
|
Fe2+
|
Fe3+
|
Volume
| |
| Initial
millimoles
|
||||||
Final millimoles
|
||||||
Final Molarity
|
5. 0.4532 grams of an iron ore unknown were dissolved and made up to volume in a 500 mL volumetric flask ("Unknown Stock"). 50.0 mL of the Unknown Stock were made up to volume in a 250 mL volumetric flask ("Unknown Intermediate"). 14.5 mL of the Unknown Intermediate were placed in a 100 mL volumetric flask ("Final Flask"), color forming reagents were added, and the concentration of Fe in the Final Flask was determined to be 1.65 micrograms/mL. Compute the %Fe in the iron ore.