Electrochemistry Homework Problem Set

1. Balance the following chemical equations. Assume the reactions occur in acidic solutions (add H¹⁺ and/or H₂O as necessary). For each reaction identify the oxidizing and reducing agent. Identify which reactant is oxidized and which is reduced. Write the appropriate half reactions. Determine [delta]G for the reaction and E^o for the reaction (If you can find the necessary thermodynamic data).
   1. Sn²⁺(aq) + Hg²⁺ (aq) + Cl^1- (aq) --> Sn⁴⁺(aq) + Hg₂Cl₂ (s)
   2. Fe²⁺ (aq) + Cr₂O₇^2- (aq) --> Fe³⁺ (aq) + Cr³⁺ (aq)
   3. Hg²⁺ (aq) + Cu (s) --> Cu²⁺ (aq) + Hg (l)
   4. Zn(s) + HCl(aq) --> Zn₂+(aq) + H₂(g)
   5. CH₄ + O₂ --> CO₂ + H₂O
   6. V₂O₃(s) + H₂(g) --> VO(s) + H₂O(l)
   7. N₂O₄(g) + Br-(aq) --> NO₂^1-(aq) + BrO₃^1-(aq)
   8. V(s) --> HV₁₀O₂₈^5-(aq) + H₂(g)
   9. Br₂(aq) + Sn²⁺ (aq) --> Br^1- (aq) + Sn⁴⁺(aq)
   10. Cu (s) + HNO₃ (aq) --> Cu²⁺ + NO₂ (g)

2. Write a figure showing an electrochemical cell that will generate electricity based upon the oxidation of iron to iron (II), and the reduction of silver (I) to silver metal.
   1. Write a balanced chemical equation
   2. Write the cell notation for this system
   3. Write the half reactions for each cell
   4. Label the anode and the cathode
   5. Show the direction of electron motion through the external circuit
   6. Show the motion of ions in the solution
   7. Show the transfer of ions at the electrode surface
   8. Calculate the standard cell potential
   9. Is this cell spontaneous?
   10. Calculate the standard cell potential for the reverse reaction.

3. Identify the anode, the cathode, the reduced species and the oxidized species for the following electrochemical cells. Write the half reactions for the anode and cathode, the overall reaction. Calculate the standard cell potential and the cell potential.
   1. Fe (s) | Fe²⁺ (aq) (0.2 M) || Ni²⁺ (aq) (0.1 M) | Ni(s)
   2. Pt (s) | O₂ (g)(0.2 atm) | H₂O₂ (aq) (0.01 M pH=4) || Cu²⁺ (aq) (1.0 x 10^-4 M) | Cu (s)
   3. Zn (s) | Zn²⁺ (aq) (1.0 x 10^-3 M)|| Cl^1- (aq)(0.3 M) | Cl₂ (g) (100 torr) | Pt (s)
   4. Pt (s) | Fe²⁺ (aq) (0.02 M), Fe³⁺ (aq) (2.3 x 10^-3M) || H¹⁺ (aq) (pH = 3), Cr₂O₇^2- (aq)(0.03 M), Cr³⁺ (aq, 0.003 M) | Pt (s)
   5. Pb (s) | Pb²⁺ (aq) (1.0 M) || Cu²⁺ (aq) (1.0 M) | Cu (s)
   6. Pb (s) | Pb²⁺ (aq) (0.210 M) || Cu²⁺ (aq) (0.0145 M) | Cu (s)
   7. Pb (s) | Pb²⁺ (aq) (1.0 x 10^-5 M) || Cu²⁺ (aq) (1.0 x 10^-2 M) | Cu (s)

4. The following cell has a potential of 0.100 V. Calculate the pH at the anode:
   Pt (s) | H₂ (g, 1 atm) | H¹⁺ (aq, ? M)) || H¹⁺ (0.10 M) | H₂ (g 1 atm) | Pt (s)

5. On space craft, a H₂/O₂ fuel cell is used to produce electricity.
   1. The reactions
      1. What is the reaction at the anode?
      2. What is the reaction at the cathode?
      3. What is the balanced redox reaction?
      4. What is E_cell assuming P_o2 = 2*atm, and P_H2 = 2*atm.

   2. If two cylinders of H₂ and 1 cylinder of O₂ (cylinder: volume=200 liter, pressure = 3000 psi) are used.
      1. How many moles of electrons can be produced?
      2. How much energy (joules) is this?
      3. If this energy has to last for 1 week, what is the average power (watts) available?

   3. Compare the amount of energy produced by this fuel cell to the energy produced by the combustion of the same amount of H₂ and O₂ using [delta] G_rxn.

   4. The following relationships may be useful:
      1. V = kg m² s^-3 A
      2. A = C s^-1
      3. J = kg m² s^-2
      4. W = J s^-1

6. Electrochemical Analysis techniques are capable of detecting very small amounts of certain metals. In one type of analysis Cd²⁺ undergoes electrolysis. In this experiment it is possible to detect a signal from 1 pA of current, lasting only 1 ms.
   1. How many moles of Cd²⁺ does this correspond to?
   2. How many atoms is this?

Scott Van Bramer
Department of Chemistry
Widener University
Chester, PA 19013

© copyright 1996, S.E. Van Bramer
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Last Updated: Saturday, May 18, 1996