CLAS – Chem 1B – Chapter 11
1. Which is the strongest oxidizing agent?
a. Mn2+ b. Br- c. Br2 d. Ag+
2. Which is the strongest reducing agent?
a. Na+ b. Al c. Zn2+ d. F- e. Mn
3. True or false:
a. Galvanic cells spontaneously produce a current under standard conditions
b. Electrons flow from the anode to cathode in a voltaic cell
c. Oxidation occurs at the cathode
d. Corrosion of a metal occurs at the anode
4. Using reduction potentials answer the following:
a. Is Cl2 able to reduce Cr3+?
b. Is Pb2+ able to oxidize Ni?
c. Will Au dissolve in an acidic solution?
d. Will Zn dissolve in an acidic solution?
e. What can oxidize Al but not Zn?
f. What can reduce Br2but not I2?
5. What is produced at the cathode if an aqueous solution of CaBr2 undergoes electrolysis?
a. Ca b. Ca2+ c. Br- d. Br2 e. H2 and OH-
6. Use the following data to calculate the Ksp value at 25 °C for PbSO4 (s)
PbO2 + 4 H+ + SO42- + 2 e-→ PbSO4 + 2 H2O E° = 1.69 V
PbO2 + 4 H+ + 2 e-→Pb2+ + 2H2O E° = 1.46 V
7. Consider the following reaction for a voltaic cell at 75 °C.
Pb2+(aq) + 2 Cr2+(aq) → Pb(s) + 2 Cr3+(aq)
a. Calculate the initial cell voltage for the above reaction if the initial concentrations are [Pb2+] = 0.25 M, [Cr2+] = 0.20M and [Cr3+] = 0.005 M.
b. Will the initial potential increase, decrease or remain the same if the solid lead is cut in half?
c. Will the initial potential increase, decrease or remain the same if you add NaCl causing PbCl2 to precipitate out of solution?
d. Will the initial potential increase, decrease or remain the same if the volume of electrodes is doubled by adding water?
8. Consider the following cell:
Cu (s) | Cu2+(aq)(0.001 M) || Fe 3+(aq)(0.02M) , Fe2+(aq) (? M) | Pt(s)
Determine the concentration of the Fe2+ if the measured cell voltage at 27°C is 0.5 V.
9. Consider the following cell: Al(s) | Al3+ (1.0 M) || Pb2+(1.0 M) | Pb(s)
Calculate the cell potential after the reaction has operated long enough for the [Al3+] to have changed by 0.66 M at 25 °C.
10. Consider the Galvanic cell at 25 °C described as follows:
X | X2+ || Y3+ | Y
Where X and Y are unknown metals. Given the standard reduction potential for Y3+ is 1.5 V and that K for the overall reaction in this Galvanic cell is 1.2 x 1020 what is the standard reduction potential of X2+?
11. Consider two electrodes connected by a wire. Calculate the potential at 25 °Cif one side has 0.0001 M Fe2+/Fe (s) and the other side has 10 M Fe2+/Fe(s). The standard reduction potential for Fe2+ is -0.44 V.
12. What mass of Co forms from a solution of Co2+ when a current of 15 amps is applied for 1.15 hours?
13. How long will it take (in min) to plate out 10.0 g of Bi from a solution of BiO+ using a current of 25.0 A?
14. It takes 24 min to plate out 9.8 g of an unknown metal (M) from a solution of MCl3when a current of 10 amps is applied. Identify the metal.
15. Hydrogen and oxygen gas can be obtained by the electrolysis of water. What current would have to be applied to water for 15 min in order to produce 11.3 L of gas at STP?
2 H2O (l) → 2 H2(g) + O2(g)
Standard Reduction Potentials at 25 °C
F2 + 2e– → 2F–2.87 VFe3+ + 3e– → Fe –0.036 V
Au3+ + 3e– → Au1.50 VPb2+ + 2e– → Pb–0.13 V
Cl2 + 2e– → 2Cl–1.36 VNi2+ + 2e– → Ni–0.23 V
O2 + 4H+ + 4e–→ 2H2O1.23 VCr3++ e– → Cr2+–0.50 V
Br2 + 2e– → 2Br–1.09 VCr3+ + 3e– → Cr–0.73 V
Ag+ +e– → Ag0.80 V2H2O + 2e– → H2+ 2OH––0.83 V
Fe3+ + e– → Fe2+0.77VZn2+ + 2e– → Zn–0.76 V
I2 + 2e– → 2I–0.54 VMn2+ + 2e– → Mn–1.18 V
Cu2+ + 2e– → Cu0.34 VAl3+ + 3e– → Al–1.66 V
2H+ + 2e– → H20.00 VNa+ + e– → Na–2.71 V
Ca2+ + 2e– → Ca–2.76 V
Equations
E = E°– E°= ΔG = –nFE ΔG° = –nFE° I =
Rules and Hierarchy for Assigning Oxidation Numbers/States
1. What type of substance is being analyzed?
a. Element ⇒The oxidation state for atoms in their elemental form are 0
b. Molecular Compound ⇒The sum of the oxidation numbers in a compound equals 0
c. Ionic Compound / Ion ⇒ Analyze the cation and anion separately
- Mono-atomic ions ⇒The oxidation state for mono-atomic ions is the charge on the ion
- Polyatomic ions ⇒The sum of the oxidation numbers in a polyatomic ion equals the net charge on the ion
2. The hierarchy tells which elements must get assigned first:
a. Hydrogen in a molecular compound or polyatomic ion = +1
b. Fluorine in a molecular compound = -1
c. Oxygen in a molecular compound or polyatomic ion = -2
d. Chlorine, bromine or iodine in a molecular compound = -1
e. Nitrogen in a molecular compound or polyatomic ion = -3
f. Sulfur in a molecular compound = -2
Electrochemical Cells
- Galvanic or Voltaic – spontaneously produce current (E > 0)
- Electrolytic or Electrolysis – require a current to run a non-spontaneous redox (E < 0)
- Anode – where oxidation occurs – if the electrode is an active metal it will corrode as the cell discharges – anions in the salt bridge flow to this side
- Cathode – where reduction occurs – if a metal ion is reduced to it’s ground state it will plate out on the electrode – cations in the salt bridge flow to the side
- For all cells electrons flow from anode to cathode or the current flows from cathode to anode
- Concentration cell – a galvanic cell where the potential is strictly due to a concentration gradient